07 35 overview of research and environmental issues.pdf

Overview of Research and Environmental Issues Relevant to Development of Recommended Farming Practices

for Sugar Cane Farming in the Lower Burdekin Region

ACTFR Report No. 07/35

Prepared for Burdekin Dry Tropics NRM

Townsville

OVERVIEW OF RESEARCH AND ENVIRONMENTAL ISSUES RELEVANT TO DEVELOPMENT OF RECOMMENDED FARMING

PRACTICES FOR SUGAR CANE FARMING IN THE LOWER BURDEKIN REGION


Prepared for Burdekin Dry Tropics NRM

Townsville

Prepared by Aaron Davis

Australian Centre for Tropical Freshwater Research James Cook University Townsville Qld 4811 Phone: (07) 47814262

Fax: (07) 47815589 Email: [email protected]

Overview of Research and Environmental Issues Relevant to Development of Recommended Farming Practices for Sugar Cane Farming in the Lower Burdekin Region


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TABLE OF CONTENTS ACKNOWLEDGEMENT..……………………………………………………………...2 EXECUTIVE SUMMARY …………………………………………………………......2 1.0 INTRODUCTION …………………………………………………………………………………………….4

Overview of lower Burdekin Cane Industry Environment. 2.0 FERTILISER USE …………………………………………………………………..5

The role of Fertiliser in Lower Burdekin Cane Production……………………….5 Existing Usage of Fertiliser in Lower Burdekin…………………………………..6 Environmental Pressures associated with off-site nutrient movement……………9 Current Approaches and Research relevant to BMP for fertilizer and nutrient management in the lower Burdekin……………………………………………...10 Future Developments in Fertiliser and Nutrient Best Management……………..27 Nutrient Interactions with other aspects of farm management…………………..28

3.0 PESTICIDE USE …………………………………………………………………..30 The Role of Pesticides in Cane Production………………………………………30 Current Pesticide Usage in the Burdekin District………………………………..31 Environmental Pressures associated with off-site pesticide movement…………31 Current Approaches and Research relevant to BMP for Pesticide use in the Burdekin region………………………………………………………………….34 Pesticide Interactions with other aspects of farm management………………….36 Future Developments in Pesticide BMP…………………………………………38

4.0 CROP IRRIGATION SUPPLY AND PRACTICES……………………………………………………………………………38

The role of Irrigation in the Lower Burdekin Cane District……………………..38 Existing Water Usage within the Lower Burdekin Cane Irrigation Environment.39 Environmental pressures associated with low irrigation efficiency……………..44 Current Approaches and Research relevant to improved irrigation efficiency in the Lower Burdekin region…………………………………………………………..45 Interactions of irrigation with other aspects of farm management………………68 Future Developments in BMP for Irrigation within the Burdekin………………69

5.0 GREEN CANE TRASH BLANKETING…………………………………………………………………………70

The Role of GCTB in Cane Production…………………………………………70 Existing Utilization of GCTB in the Burdekin District………………………….71 Current Knowledge and Research relevant to BMP for GCTB in the Lower Burdekin………………………………………………………………………….72

6.0 INFORMATION GAPS AND SUMMARY……………………………………………………………………………..78 7.0REFERENCE………………………………………………………………………..80

Overview of Research and Environmental Issues Relevant to Development of Recommended Farming Practices for Sugar Cane Farming in the Lower Burdekin Region ACTFR Report No. 07/35

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ACKNOWLEDGEMENTS The author wishes to acknowledge the efforts and input from all parties concerned in developing this review of farm research focused on the lower Burdekin catchment. Firstly the local Water Quality Focus Group members (representing CANEGROWERS, BSES, CSIRO and DPI agencies) for their advice and guidance on a multitude of topics underpinning the content of this publication. Special thanks in particular go to the four anonymous reviewers who took the time to assess and provide valuable feedback on what became a lengthy document. The author also wishes to acknowledge the efforts of various organizations and individuals such as BSES, SRDC and Gary Ham for locating and providing an array of hard to find research material for perusal. Similarly, thanks are expressed to Zoe Bainbridge (ACTFR) for the timely supply of data and graphs from local wet season monitoring events.


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EXECUTIVE SUMMARY

The aim of this review is to provide the Burdekin Coastal Catchments Initiative (CCI) with a rigorous and comprehensive technical basis to act as a starting point for development of locally relevant and justifiable guidelines to benefit off-site water quality. As it currently stands the review is basically a repository of primarily technical information on local farming practices rather than explicit definition of recommended farming practices. This aspect of available technical information needs to be integrated with the pragmatic ‘know how’ and familiarity with district farming systems of local growers in order to develop feasible, practical and relevant farming principles that meet social, economic and environmental aspirations. A related parallel process has therefore been initiated to document local growers’ judgments, knowledge and ambitions with regard to recommended local farm practices that meet these ‘triple bottom line’ requirements. Outputs from this ongoing grower-driven process, the technical review as well as additional consultation with industry science experts will be combined at a later date to formulate palatable and locally relevant guidelines for farm management. This technical review is therefore envisioned as becoming a living document, subject to addition and reappraisal as more information and knowledge emerges with regard to local farming practices and associated environmental issues. A number of issues of particular significance from a technical perspective have emerged from this review:

• Wet season monitoring data from local creek systems suggests a distinctive water quality signature associated with fertilizer nutrient movement similar to that seen in other Queensland sugar growing regions is evident in the lower Burdekin system.

• The almost total reliance on supplemental irrigation in the lower Burdekin adds another layer of risk to chemical management (fertilizer and pesticides) for local growers. The interrelationships between furrow irrigation and off-site chemical movement (leaching and run-off) can be complex and are largely unknown at present.

• While data and practices pertaining specifically to the linkage between irrigation and chemical management is sparse, research results targeting on-farm water use efficiency is relatively abundant. Optimizing on-farm irrigation efficiency represents one avenue of minimizing off-site water quality effects in the absence of more specific irrigation-chemical usage guidelines.

• The pesticide usage behavior (chemical types, application rates etc.) of Burdekin cane growers is largely unknown. Similarly information on off-site pesticide movements from Burdekin cane farms is only recently beginning to emerge. These knowledge gaps coupled with potential upcoming changes in specific pesticide registrations makes pesticides an issue deserving greater future focus in the lower Burdekin.

• The history of Burdekin cane industry development and expansion has produced a number of local farming systems distinctly different in features such as scale, soil properties and relevant environmental issues (i.e. BRIA versus Delta farms). This variation will have significant implications for development of recommended farming practices on a local level.

• Green cane trash blanketing, a farming system promoted as a recommended practice in many other cane districts has been adopted by only a small percentage of growers across the Burdekin district. Issues related to the large size of typical Burdekin cane crops, particularly associated harvesting and irrigation difficulties have long posed a considerable impediment to the more widespread acceptance of this practice.

• Development of informed guidelines for water quality issues related to fallow


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management are necessary. Considerable uncertainty exists over the relative benefits and drawbacks of bare fallows versus fallow crops (legumes etc.) and how to appropriately manage fallow systems.

• Recent industry shifts toward controlled traffic systems hold potential, but as yet undefined benefits from an environmental perspective. While opportunities exist for improved water use efficiency as well as enhanced fertilizer and pesticide management under controlled traffic systems, these issues are yet to be investigated to any great degree.


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1.0 INTRODUCTION. The ongoing or impending delivery of a number of environmental sustainability programs such as the Rural Water Use Efficiency (RWUE) initiative, QDRM’s Land and Water Management Plan (LWMP) process, Farm Management Systems (FMS) and the Great Barrier Reef Water Quality Protection Plan have highlighted the need for a more coherent approach to the issue of Best Management Practice (BMP) on farms. While improved farming practices from an environmental perspective have received some degree of research attention, it has typically been sporadic at best, lacking strategic coordination and with detailed results often not widely disseminated or reported. The often marked differences in topography, climate, on-farm cultural practices and the history of industry development between regions also results in considerable variability in the environmental issues relevant to specific districts, and even within certain areas of the same cane growing region. As a result, what may be construed as ‘BMP’ in one cane growing district may offer minimal or even counter-productive environmental or productivity benefits in another region (even to other farms within the same district). The need for development of district specific, ‘customized’ BMP information within the cane industry cannot be understated. With this in mind, the aim of the following review is to develop a compilation of all available ‘BMP’ research directly applicable to cane farming and off-site water quality effects in the lower Burdekin district. Given the range of farming practices with some level of linkage to water quality, the topics covered will be diverse, touching upon issues such as irrigation, fertilizer, pesticide, cultivation and fallow management. A brief overview of locally relevant environmental issues linked to existing land management activities within the district will also provide contextual relevance to the need for a more focused approach to BMP identification and extension. This consolidated research appraisal can serve as a starting point for developing justifiable guidelines to feed into a number of the previously mentioned NRM initiatives as well as provide strategic direction to future research and environmental sustainability endeavors. One of the key catalysts for the recent industry shift toward more sustainable farming practices is the mounting evidence of catchment based impacts from an array of land uses on receiving water quality of the Great Barrier Reef, and subsequent impacts on marine ecosystem processes. The topic of off-site water quality entering marine environments, while no doubt significant and currently receiving considerable publicity is unfortunately one that cannot be considered in isolation from other environmental concerns. It would in fact be irresponsible to consider the specific issue of water quality in flood run-off reaching marine waters as somehow isolated from other environmental issues such as groundwater quality or the ambient water quality conditions of freshwater ecosystems in canelands. There may actually be significant linkages between these processes and water quality in the GBR lagoon that have received minimal attention in the past. It should be recognized that practices which may specifically benefit off-site surface water quality during a flood or irrigation event from caneland can also potentially have negative implications with regard to other issues of environmental significance such as on-farm water use efficiency and/or groundwater sustainability. Therefore it is a fundamental requirement that the issue of off-site water quality is managed in a holistic ‘whole of ecosystem’ approach to environmental sustainability, rather than with individual components addressed on an ad hoc basis.



Overview of lower Burdekin Cane Industry Environment. The Burdekin River in north Queensland drains through one of Australia’s largest floodplain delta environments (ca. 1,250 km2). The Burdekin River delta and floodplain supports one of Australia’s most intensively cultivated and productive agricultural areas with over 80,000 ha of irrigated crops, sugarcane being by far the predominant crop type (see figure 1). The Burdekin cane industry produces some of the highest quality and quantity crop yields (average productivity has varied between 115 and 121 t/Ha over the past 5 years) in Australia, if not the world. A substantial unconfined freshwater aquifer system (one with no overlying impermeable sediments) underlying the delta is a natural resource that has greatly supported agricultural, pastoral and domestic endeavours in the area for over 100 years. The aquifer typically provides in excess of 450, 000 ML of groundwater per annum to sugarcane irrigation. The aquifer is in contact with the sea and an artificial recharge program overseen by the North and South Burdekin Water Boards since the 1960’s attempts to maintain sufficient aquifer potential to control seawater intrusion into coastal groundwater reserves. Seawater intrusion is most pronounced during years of low rainfall (low aquifer recharge) combined with corresponding increased demands in groundwater extraction during such periods. The Burdekin Delta and floodplain are drained by the Burdekin River, Haughton River, Barratta Creek and a number of additional natural and artificial drainage channels. A notable hydrologic characteristic of the region is that due to local topography north of the main Burdekin River channel, the majority of the drainage between the Burdekin River and the Haughton River flows into the Barratta Creek system. A similar situation occurs to the south of the main Burdekin River channel where much drainage occurs through smaller creek systems other than the main river channel itself. Therefore ambient tailwater inputs from farms or flood flows associated with wet season rainfall events throughout the district actually flow through these systems into coastal wetland complexes (i.e. Bowling Green Bay) rather than discharged directly from the river mouth. 2.0 Fertilizer Use. The role of Fertiliser in Lower Burdekin Cane Production. Fourteen plant nutrients are recognized as being essential to produce a healthy and well nourished sugar cane crop. For convenience this suite of nutrients is broadly divided into two main groups indicative of the relative requirement by plants: macro and micro nutrients. The six macro nutrients, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) tend to be those most commonly applied to crops to any substantial degree by growers on an industry wide basis. The eight micro nutrients, copper (Cu), zinc (Zn), iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), chloride (Cl) and silicon (Si) while equally important, tend to be applied in much smaller quantities, if they are required at all. The sustainability of cane-growing can be



undermined by either over-zealous application or under application of fertilizers. Long-term under application of fertilisers (or ‘mining the soil’) results in loss of soil fertility and overall soil degradation, whereas over-application of nutrients can lead to significant issues with productivity declines as well as off-site environmental effects (Bruce, 2002). Existing Usage of Fertiliser in Lower Burdekin. The primary focus of this review will be the improved management of nitrogen and phosphorous by Burdekin farmers. These particular elements have been identified as two key ‘limiting nutrients’ in marine and freshwater environments (see Hunter, 1992), and similarly constitute the two key agricultural nutrients of most concern from an off-site environmental perspective. Other commonly applied or relevant nutrients will be addressed briefly as required, bearing in mind many of the best management considerations for phosphorous/nitrogen are just as pertinent to most other nutrients applied to cane farmers. Nitrogen. The main forms of nitrogenous fertilizer used across the Australian cane industry are urea, ammonium sulphate, ammonium nitrate and calcium ammonium nitrate. Urea is the dominant nitrogenous fertilizer currently utilised in sugarcane production in Australia, including the Burdekin district as the cost per unit nitrogen and transportation expenses are less than alternative products. On behalf of the Fertiliser Industry Federation of Australia (FIFA), Incitec Pivot, one of Australia’s leading manufacturers and suppliers of agricultural fertilizers recently compiled yearly estimates of nutrient use in many of Australia’s main sugarcane districts, based broadly upon factors such as sales, proportionate market share and areas under cultivation. While admittedly subject to some uncertainty due to their coarse derivation, these figures offer some of the best contemporary insights into spatial and temporal fertilizer usage patterns by Queensland cane farmers. The average nitrogen application rates in the Burdekin district for the years 2000-2003 were ca. 220kg/ha, significantly above the state average (see Table 2.0 below). A number of factors characterising the Burdekin cane industry do however contribute to this statistical outcome. A reliable supply of irrigation water and the resultant high yield potential for the crops that typify the Burdekin district are a significant factor in the high rates of nutrient input associated with the Burdekin region. The relatively low rainfall and dry-tropical climate of the Burdekin region additionally results in naturally low soil organic matter compared to the soils of most other cane growing regions across Australia. Soils in the Burdekin in fact have the lowest organic carbon content of any of Queensland’s sugarcane districts (Gary Kuhn, Incitec Pivot pers. comm.). With the main reserve of nitrogen in soil being organic matter, the nitrogen depauperate soils in the Burdekin region necessitates relatively high fertilizer application rates in comparison to other regions. Additional cultural practices in the Burdekin also produce considerable nitrogen losses relative to standard practices in other cane growing regions. The predominant practice across the district of crop burning prior to harvest combined with



minimal trash blanketing releases considerable nitrogen otherwise retained in other regions (see Mitchell et al., 2000). In accordance with at least some of the factors described above current BSES recommended nitrogen requirements for the Burdekin as a whole are somewhat above the state average and stand at 135-150 kg/ha for plant cane and 210-230 kg/ha for ratoons. In line with increasing cognizance of sustainability issues there has been recent scaling down of recommendation application rates (such as the suggestion of 270 kg N/ha when sugar price > A$300/t cited in Calcino, 1994). There is however some inconsistency in these figures with the upper limit for ratoon cane application rates in the Burdekin sometimes cited as 250kg (i.e. the current Code of Practice Handbook) which is probably unnecessary and excessive in light of more recent revised recommendations. As well as the potential negative environmental ramifications associated with over-application of fertilizer there also exist considerable productivity drawbacks linked to such practices. Excessive N application can result in reduced sucrose concentrations and sugar quality (Muchow et al., 1996), as well as constituting an unnecessary excess cost for growers. Table 2.0. Trends in Nitrogen application rates (kg/ha N) across sugarcane growing regions (Data courtesy of Garry Kuhn, Incitec Pivot)

Region 1996 1997 1998 1999 2000 2001 2002 2003 Wet Tropics 169 151 138 144 151 149 147 137 Herbert 213 198 209 204 183 201 205 191 Burdekin 272 246 247 269 233 229 234 219 Central 225 232 214 233 176 175 166 171 South Qld 161 155 155 150 148 148 120 121 NSW 164 166 164 159 173 155 150 148 Average 206 197 190 199 179 177 171 166

Historically there has been a tendency for canegrowers to err on the high side with application rates, a strategy which during past periods of higher sugar prices was often not cost prohibitive and more than offset by past high prices. The more contemporary situation of variable sugar prices, a more competitive international market, adverse growing conditions in some regions and greater environmental awareness has driven an industry shift toward reduction in overall fertilizer application rates in recent times. The general decline in nitrogenous fertilizer application rates seen across the cane industry as a whole in recent years is similarly reflected in the steady lowering trend for application rates within Burdekin district in recent times (see table 2.0). Cane industry fertilizer use surveys have revealed many cane growers have in the past (and currently) adopted their own approaches to fertilizer management, often applying nutrients in excess of recommended rates. Recent Queensland industry surveys outlined in Schroeder et al. (2002) reflect this:

• On average 80% of growers surveyed apply nitrogen fertilizer on fallow plant cane in excess of BSES recommendations, with the majority applying more than 50kgN/ha above recommended rates.



• 45% apply rates of nitrogen on replant cane in excess of recommendations. • 44% apply nitrogen on ratoon cane in excess of recommendations.

These percentages apparently did not vary much between most districts and while these figures may be industry wide averages, there is little doubt they bear significant relevance to the fertilizer management strategies employed by many lower Burdekin cane farmers. The survey results of Schroeder et al. (2002) are reflected in many ways in Incitec Pivot survey results with average yearly application rates in the Burdekin district generally at or in excess of the upper limit of BSES recommendations for ratoon cane (230 kg N/ha). Phosphorus. Phosphorus budgets from a regional cane district perspective outlined in Bloesch et al. (1997) revealed rates of P application in the Queensland sugar industry generally exceeded removal rates in all areas except the Burdekin. With the exception of the neutral P budget in the Burdekin districts, over-application of P in many areas suggested an average positive annual P balance of ca. 8.2kg P/ha in many sugar producing soils. P inputs in the lower Burdekin approximately balanced P outputs, with no apparent resultant phosphorous build-up in Burdekin soils. Opportunities to reduce P applications were therefore evident in all Queensland regions apart from the Burdekin. The naturally high P fertility soils of the intensively farmed Burdekin delta region were identified as one of the primary drivers of this effect. Older farms in close proximity to mills (which tend to be found predominantly in the Delta area) also often have a long history of mill mud application, which consequently may also play a minor role in maintaining higher relative soil phosphorus levels. It was noted however that this situation was likely to change in the near future as sugarcane production expanded into the more P deficient areas of the BRIA. Available Incitec Pivot data for the period 2000 – 2003 suggest average phosphorus application rates in the Burdekin region still tend to be below the state average at ca. 18-19 kgP/ha (see table 2.1). Similar to the situation with nitrogenous fertilizers there has also been a steady general decline in phosphatic fertilizer application within the Burdekin and across the industry as a whole in recent years. These phosphorous application rates however are regional averages, with no consideration of variable application of P fertilizers or mill by-products across a particular district. Phosphorous application rates on the higher fertility soils of the Burdekin Delta area for example tend to average < 10kg P/yr, while BRIA growers generally apply 25 – 40 kg P/yr (Evan Shannon, BSES pers. comm.). While the data at hand suggests over-application of phosphatic fertilisers may not be a huge issue in the lower Burdekin area, some scope may exist for refinement of P application rates at an individual farm scale. Table 2.1. Trends in Phosphorus application rates (kg/ha P) across sugarcane growing regions (Data courtesy of Garry Kuhn, Incitec Pivot)



Region 1996 1997 1998 1999 2000 2001 2002 2003 Wet Tropics 28 25 22 19 23 24 20 21 Herbert 28 26 25 21 21 30 26 24 Burdekin 26 23 23 22 21 19 17 19 Central 28 30 26 24 18 20 14 15 South Qld 24 27 26 21 24 27 19 21 NSW na na na na na na na na Average 27 27 24 21 21 23 19 19

Sugar Mill By-Products. It is also important to note, particularly with respect to Incitec Pivot figures on direct application rates of N and P fertiliser that substantial amounts of mill by-products such as filter/mill mud, boiler ash and to a smaller extent dunder are also applied to some cane fields in the Burdekin district. The re-use of these by-products as a cheap nutrient source, and also soil ameliorant in some cases provides mutual benefit to both the milling sector and growers from a sustainability perspective. Given the high transport costs (mill mud contains up to 75% moisture) associated with usage, use of sugar-mill by-products tends to be limited to farms in close geographic proximity to mills. Some older farms in the Burdekin district (Delta farms tending to be historically developed close to mills) have a history of by-products usage exceeding 100 years. As noted by Barry et al., (2002) the status of these products as low grade fertilizers sees often quite high applications rates employed, with little thought given to crop nutrient requirements. Mill mud for example is often applied at rates from 150 to 250 wet tones/ha. A standard application of 150 wet tonnes/ha of mill mud adds around 560kg N/ha and 340 kg P/ ha, assuming a 75% moisture content (Barry et al., 2002), far in excess of recommended fertilizer application rates. It should be noted that not all this nutrient in immediately available due to it’s organic form and that residual nutrient benefits are associated with a one-off application. Nevertheless, there are indications over-fertilization of cane soils associated with by-product usage is occurring. As well as nutrient loss issues, long-term soil accumulation of heavy metals such as cadmium and zinc through mill by-product and biosolid recycling is a sustainability issues the industry is having to grapple with (Barry et al., 2002). Environmental Pressures associated with off-site fertilizer and nutrient movement. The effect of elevated nutrient levels in marine environments is one of the better studied aspects of catchment-based pollution in marine environments. Increased inputs of nutrients ("eutrophication") often have critical impacts on marine ecosystems, especially tropical systems such as coral reefs. Elevated nitrogen and phosphorous can have direct but variable effects on coral growth and calcification, fertilization rates and embryo formation. Elevated nutrients can also have indirect effects by promoting phytoplankton



or macroalgal growth, thus adversely shading corals or supporting increased numbers of filter feeding organisms which also compete with coral for space. . Microscopic observation of coral reef organisms exposed to nutrient and sediment enriched coastal waters suggests detrimental or lethal effects after only a few hours of exposure (Fabricius & Wolanski 2000). This preliminary research suggests nutrient enhanced biological flocculation has the capacity to rapid smother reef organisms and suggests a very plausible mechanism for the apparent run-off induced degradation at a number of localities along the GBR (Wolanski & Spagnol, 2000). Some more locally relevant data pertaining to off-site movement of nutrients and associated impacts on groundwater and surfacewater quality will be elaborated upon in further sections. While the environmental risks associated with direct fertilizer applications of nutrients such as N and P tend to generate most interest, additional off-site impacts associated with nutrient movement also warrant management attention. Concerns over periodic spates of fish kills in the coastal waterways of Queensland’s sugarcane growing areas prompted assessment of the water oxygen depletion potential associated with storm or irrigation run-off from canefields. Rayment (1999) concluded cane juice (mostly in the form of water soluble sucrose) lost during mechanical harvesting as the most likely potential candidate for depressing in-stream DO in the event of farm run-off entering waterways. Bohl et al., 2002 subsequently demonstrated high biological oxygen demands (with sugars identified as the primary contributor) from the first post-harvest irrigations leaving both burnt and GCTB systems in the Burdekin area, highlighting a considerable additional potential hazard associated with off-site water movements from farms. Similarly, concentrations of mineral forms of N and P and particularly the high BOD associated with leachate from stockpiled mill mud and mill ash may pose considerable off-site environmental risk if not managed carefully (Bloesch et al., 2003). An environmental topic related to nutrient application is the heavy metal accumulation that has occurred in some Australian sugarcane soils due to use of superphosphate, some trace element fertilizers and recycled waste products such as mill mud which contain heavy metal impurities (Wood, 2000, Barry et al., 2002). Cadmium (Cd) for example is not a requisite plant nutrient for cane cropping that is commonly applied to cane fields as a contaminant of phosphatic (and some other) fertilizers. Long-term application of phosphatic fertilizers can increase soil cadmium levels. While many of these heavy metals are not taken up by cane crops, their accumulation in soils negates commercial growing of crops such as peanuts on these soils as peanuts do actively accumulate cadmium. The potential off-site movement and environmental implications of heavy metal accumulation in cane soils remains largely unknown. The persistence and toxicity of many heavy metals could have significant off-site environmental implications. Current Approaches and Research relevant to BMP for fertilizer and nutrient management in the lower Burdekin. The concept of crop nutrient cycles and the inter-relationships between nutrient inputs, outputs and movement pathways is well established in the Australian sugar industry (see



Reghenzani & Armour, 2002). A sound comprehension of the concept of nutrient budgets and the ways in which various components can be influenced by management activities is fundamental to enhancing nutrient management by farmers. Crop Uptake. As noted by Reghenzani & Armour (2000), a well grown crop of cane is the best avenue to effectively capturing nutrients from applied fertilizer and soil reserves. While cane is apparently inefficient at recovering fertilizer N, usually ca. 20-40% of the total applied (Vallis & Keating, 1994), crop uptake is the greatest single output component of cane nutrient balance and represents the one process most amenable to effective management by growers. (although the combined loss of N through other pathways can constitute a greater relative output). Crop uptake therefore needs to be appreciated as a major method of minimizing nutrient loss from fertilizer as well as nutrients available from additional pathways such as soil mineralization and crop residue. Reghenzani & Armour (2000) highlight that the common, perhaps industry wide perception that fertilizer nutrient input is the major determinant in crop yield needs to be replaced by the concept of crop fertilizer nutrient demand. The aim of fertilizer application should be to match, not exceed crop demand. Crop demand cannot be forced by luxury nutrient supply (i.e. excessive fertilizer application). Although removal of harvested N can provide the largest single specific output component of the nitrogen budget, combined loss of N via gaseous losses (denitrification and volatilization), leaching and run-off can constitute the major output component of a crop nitrogen balance. The relative importance of these various loss pathways varies with type of fertilizer used, prevailing climatic and pedological characteristics as well as management strategies. To maximize crop nutrient uptake (from Reghenzani & Armour, 2000):

• Fix soil constraints to growth prior to planting (poor drainage, sodicity etc.) • Develop balanced fertiliser program based on soil and leaf analysis • Appropriate varietal selection • Use planting techniques to ensure good plant germination (well prepared seed

bed, adequate soil moisture, sterile equipment and planter correctly set up for fungicide spray, fertilizer placement and coverage

• Appropriate pest management • Encourage crop uptake by placing fertilizer in close proximity to the cane row and

root system (not inter-space) • Apply fertiliser when crop is actively growing • Reduce crop stress (use appropriate irrigation scheduling).

Volatisation. Urea (the most commonly used industry N fertilizer) when placed in soil or on sugar cane trash is rapidly hydrolysed to ammonium compounds by the naturally occurring enzyme, urease (Wood & Kingston, 2000). If urea is on the soil or trash surface it can convert to ammonia gas, which is then lost to the atmosphere, a process known as volatilization. Losses of N through volatilization have perhaps surprisingly received considerably more



dedicated research attention (particularly on GCTB systems) than have other processes such as leaching, denitrification or run-off. The broader environmental implications of ammonia gas volatisation remain an rarely considered unknown in this process. Wood & Kingston (2000) provide an overview of some of the more salient findings to emerge from accumulated research into volatilization losses on GCTB systems:

• Urea is dissolved and hydrolysed by water from rainfall or dew formations. Upon evaporation this water releases ammonia, a daily pattern of which can form related to wetting and drying of trash.

• N losses can amount to 30-40% of applied N if no heavy rain is received. • Rainfall of 20mm or more is sufficient to wash much of the applied urea through

the trash blanket and into the soil surface, where volatilization losses are greatly reduced.

• High pH conditions exacerbates ammonia loss, some fertilizer contain urea/muriate of potash mixes which lose more N than urea alone (due to high pH of potash)

Urea is obviously susceptible to significant volatisation losses, particularly when surface applied, but these losses can be substantially reduced by appropriate management practices as follows (from Reghenzani & Armour, 2000, Wood & Kingston, 2000):

• Use of appropriate N fertilizer application rates • Sub-soil placement of urea to trap any urea losses (for GCTB place urea beneath

trash using coulters) • N fertiliser application delayed until crop is 50cm in height if surface applied.

This promotes canopy absorption of ammonia as well as minimizes dew formation on trash as well as reduction of air mixing/flow over soil surface, all factors reducing rate of loss.

• Apply urea to coincide with moderate irrigation/rainfall and rapid root growth, thereby promoting N entry into soil as well as root uptake.

• Consider the use of alternative nitrogenous fertilizers with lower volatilization risks during high risk periods (i.e. ammonium nitrate, sulphate of ammonia, calcium ammonium nitrate, urease inhibitors or polymer coating of urea granules).

• Use mixtures of ammonium suplhate and urea so that the acidity of ammonium sulphate neutralizes the alkalinity of urea

Once urea has been hydrolysed to ammonium ions held in the soil, the transformation products of urea behave in much the same way as other nitrogenous fertilizers, and are therefore subject to loss via leaching, denitrification or run-off. Denitrification. Denitrification is the loss of nitrogen as a gas from the soil and occurs through the reduction of nitrates to nitrous oxide and nitrogen, both of which can escape to the atmosphere. The denitrification process usually occurs under conditions of high nitrates, anaerobic/waterlogged conditions and the presence of an organic carbon source readily decomposable by soil microorganisms. Trash blanket retention is one practice that can significantly facilitate denitrification by providing a ready carbon source as well as



keeping soils relatively moist. Reghenzani & Armour (2000) outline some of the key techniques to reduce denitrification losses that target these specific factors promoting loss:

• Improved surface drainage to avoid pondage, particularly under GCTB systems • Avoid excessive fertilizer application to avoid high nitrate concentrations • Place fertilizer on well drained raised rows, not low, poorly drained interspaces • Use cover crops over the wet or fallow season to trap residual nitrogen for

subsequent plant cane crops Denitrification losses tend to be more significant on heavier textured soils in comparison to lighter, well drained soils. Leaching. Over-application of nitrogenous fertilizers can in some cases lead to substantial deep leaching of nitrogen from crop root zones to groundwater aquifers. Nitrate (NO3

-) contamination of the groundwaters underlying major agricultural regions is a reasonably common circumstance on a global scale. Somewhat surprisingly perhaps, direct quantification of nitrogenous deep drainage losses from farms apparently not been assessed to any great degree in the Burdekin district, despite a number of groundwater quality studies suggesting the issue deserves more focused attention. Brodie et al., (1984) first documented elevated nitrate levels (up to 38 mg/L) in a number of lower Burdekin bores during the period 1976-1977. The high nitrate concentration correlation that emerged with areas south of Home Hill was attributed to the low water table in that particular region. Weier et al. (1999) documented nitrate concentrations in excess of 50mg/L (NHMRC standard) in 5% of bores in the Burdekin region, with a further 7.3% falling within the 25-50mg/L bracket. While examination of nitrogen isotope ratios could not conclusively identify the origin of this nitrate, isotopic ratios of the high concentration samples (those exceeding 50 mg/L) revealed at least 45% of the nitrate was of inorganic origin (probably N fertilizers). The residual 55% had isotopic values of indeterminate origin, and could represent N mineralized from cultivated fields or possibly mixing of organic and inorganic sources. In a further evolution of the broad-scale study of Weier et al., (1999), Biggs et al. (2001) monitored trends through time in 56 high nitrate concentration bores (≥ 20mg/L) in the Burdekin region between October 1998 and June 2000. The 56 monitored bores were mainly concentrated within five kilometers of the Burdekin River near the towns of Home Hill and Clare. There were some additional isolated bores located near the Haughton River, some south of Clare and south-west of Brandon. 90% of these bores demonstrated negligible trends in nitrate concentration through time over the sampling period. 6 % of bores demonstrates decreasing trends while only two of the sampled bores showed increasing trends in nitrate concentration. The relatively short observation period of this study does however undermine the capacity for detection of genuine temporal trends in nitrate levels. A recent assessment of the denitrification potential of the lower Burdekin aquifer documented nitrate concentrations ranging from <0.1 to 15 mg/L NO3

- N in 57 QDNRM



bores (and a few farm production bores) throughout the Burdekin (Thayalakumaran et al., 2004). Nitrate distribution with depth revealed variable trends between sampling periods in this study. Concentrations were significantly higher in shallow bores during the January 2004 sampling period in comparison to September/October 2003 sampling, suggesting a quick response to fertilization, irrigation and recharge through the soil. In a similar outcome to previous studies of the spatial distribution of groundwater quality, bores with elevated nitrate were primarily observed in the Airville-Home Hill area. Recent CSIRO analysis of the groundwater database data maintained by the Queensland Department of Natural Resources and Mines provides one of the more comprehensive spatio-temporal assessments of groundwater nitrate trends in the lower Burdekin region (see Barnes et al., 2005). This study documented 13% of Burdekin Bores (non-private bores with > 5 water quality measurements) had elevated (>20 mg/l) average nitrate levels and 21% of all bores (152 of 714) have statistically significant rising nitrate levels. Since 1990, 11 individual nitrate readings were above the Australian drinking water standard (100 mg/L). If nitrate levels continue to increase at a rate of 0.25 mg/L/year (the 5% trimmed mean trend rate) then Barnes et al. predicted 132 bores to have elevated nitrate concentration (>20 mg/L) by 2006. Spatial plotting of data showed two apparent clusters of high nitrate level bores. These clusters also correspond to areas with highest increasing nitrate trends. These results reflected the findings of much previous research with these areas were located (i) west of the Burdekin River between Clare and Mount Kelly, and (ii) near Home Hill. Given the reliance of both farmers and other community stakeholders on the continued quality of the Burdekin groundwater resource, BMP’s to reduce nitrate leaching to groundwaters should be pursued. Locally developed data on nitrate losses under different management practices or in different areas are however unfortunately largely lacking. In one of the few studies quantifying nitrate leaching under Burdekin sugarcane crops Klok et al. (2003) outlines some examples of nitrate-nitrogen concentration in soil-water samples taken at 1.5m depth through a year’s sugar cane crop. The first example demonstrated a nitrate-nitrogen peak in soil water (ca. 26 mg/L) closely following fertilizer application with nitrate levels that gradually trailed off after the next 10 irrigations to settle at a stable level below the concentration of irrigation water. The second example from a heavier, less permeable soil with lower irrigation application (but apparently substantially higher N application rate) demonstrated a more sustained and fluctuating concentration of nitrate-nitrogen (10-25mg/L) that again tailed off to below the N concentration of applied irrigation water. The long-term fate of nitrates entering the Burdekin Delta aquifer system is currently very poorly known. Recent estimations by NRM & W estimated that freshwater discharge to the sea from the Burdekin Delta ranges from 1500 to 9000 ML/yr, equating to a potential nitrate discharge to marine environments ranging anywhere between 4.5 and 117 tonnes of nitrate per year (McMahon et al., 2002). The breadth of this range underlines the minimal knowledge regarding freshwater and seawater movement through the heterogenous Burdekin delta aquifer system. By way of comparative context, yearly nitrate discharge at the upper level of this range would be comparable to the modeled



yearly average dissolved inorganic nitrogen export of 140 tonnes/year from the local Haughton River catchment (see Brodie et al., 2003). The recent discovery of oceanic discharge of terrestrial groundwater through ‘Wonky Holes’, ancient riverine paleochannels that carved across current ocean shelves during times of lower sea levels has also identified previously unsuspected potential for terrestrial nutrient ingress to the GBR lagoon (see Stieglitz, 2005). The natural denitrifying potential of the lower Burdekin groundwater system further complicates assessment of this issue. CSIRO research has found conditions facilitating denitrification (denitrifying bacteria in the presence of dissolved organic carbon, ferrous iron or pyrite) exist in the lower Burdekin region (Thayalakumaran et al., 2004). Preliminary results suggest nitrate levels in groundwater could be attenuated to some degree prior to discharge into the GBR lagoon, although associated geochemical processes (increased weathering rates of iron and manganese hydroxides) could compromise aquifer water quality in other ways. Reghenzani & Armour (2000) can provide a number of management suggestions to reduce leaching losses of N and increase uptake by crops:

• Keep to recommended application rates. • Place nitrogen and narrow band or split stool on the crest of a hilled up row.

Hilling up reduces the amount of water leaching through the row by counteracting the natural tendency of cane to direct rain towards the row. Less water moving past a small surface area fertilizer band reduces leachate losses and promote uptake by early crop stages.

• Under irrigated cropping systems, adjust application rates to avoid deep drainage below root zones.

• Foster development of an extensive and healthy root system including reduction of soil compaction, increasing soil organic matter, crop rotation and appropriate root pest and disease control.

Klok et al. (2003) highlighted the need for further research to better quantify the relative contributions of various irrigation and fertilizer management strategies to deep drainage and other avenues of nitrate loss. While the generalized strategies of Reghenzani & Armour (2000) no doubt have some relevance, again locally developed guidelines outlining management strategies for different irrigation systems and soil types are required. Leaching losses tend to be greater in plant crops compared to ratoon crops due to the more extensive root system development found under ratoon crops (Reghenzani et al. 1996), as well as the more intensive cultivation (ripping) events traditionally conducted prior to planting.. Run-off. Nutrients lost in drainage water through irrigation and particularly rainfall generated flood run-off tends to be the most prominent environmental issue from the perspective of off-site effects of farm practices. Monitoring data at both farm and catchment scales is only just beginning to become available in formats that provide meaningful insights into off-site effects of Burdekin farming systems. Recent wet season monitoring of flood events in the Barratta Creek complex draining much of the lower Burdekin region for example allows some characterization of local water quality during floods (see



Bainbridge et al., 2006 for more detailed discussion). Figures 1 to 9 outline flood hydrographs and their relationship to a number of key water quality parameters recorded during a January monsoonal event for the 2004-2005 wet season from three lower Burdekin monitoring sites; Upper Barratta (Clare Road), East and West Barratta Creek (both at the Bruce Highway). The upstream site on Barratta Creek was originally regarded as a potential reference site due to the relatively smaller areas of upstream catchment converted to cane land, but results suggest even a low proportion of catchment under cane can still influence water quality to a significant degree. Samples from East and West Barratta Creek are collected considerably further downstream after the system has received considerable drainage from cane growing land. Figures 1 - 3 depicting total suspended solids (TSS) dynamics in Barratta Creek indicates that even without substantial green cane trash blanketing being practiced in the area suspended solid values tend to be quite low during flood events. No significant increases in TSS entrainment are apparent between upstream and downstream sites. This is not a particularly surprising outcome with the extremely flat topography of the farming area probably not conducive to significant sediment movement during flood events. It is notable from the perspective of comparisons to other cane farming areas such as the Wet Tropics where significant historic topsoil losses during rainfall events was a significant driver of the widespread adoption of green cane trash blanketing systems in these areas (see Prove et al., 1995). Figure 1. Upper Barratta Creek: total suspended solid (TSS) values, 2004-2005 Wet Season



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Figures 4 to 6 outline nitrogen species dynamics in the Barratta system floodwaters during the 2004-2005 wet season. A number of interesting outcomes pertaining to local land use emerge from nitrogen concentration dynamics. Particulate nitrogen (where nutrient particles are bound to sediment and transported through erosional processes) tends to be the primary nitrogen species in floodwaters throughout the rest of the Burdekin catchment, particularly in rangelands environments. Most nitrogen in the Barratta system however, even at the upstream site, takes the form of inorganic nitrate. The high proportionate fraction of nitrate at even the upstream site indicates an agricultural fertilizer influence throughout the catchment. The overall nitrate values and proportionate contribution of nitrate to total nitrogen values tends to increase from upstream to downstream sites in the Barratta system during flood events, particularly during the ‘first flush’ periods of hydrographs. This increase in inorganic nitrogen proceeding downstream also strongly suggests an increased degree of fertiliser based nutrient input from local canelands as water progresses downstream. Results for phosphorous species also illustrate a similar effect. While particulate phosphorus tends to dominate phosphorous fractions throughout the main Burdekin catchment, inorganic filterable reactive phosphorus (FRP), referred to as phosphate, constitutes the dominant phosphorus species throughout the Barratta system (Figures 7 to 9). Again, while actual values are not alarmingly high, the predominance of inorganic phosphorus species is strongly indicative of fertilizer influence from adjacent farmlands. The relative dominance of inorganic rather than particulate (sediment bound) forms of both nitrogen and phosphorus reflects the low TSS values recorded in the Barratta system discussed previously. Most nutrients in Barratta Creek floodwaters move in a dissolved form rather than attached to sediment particles. These patterns in water quality parameters are similar to those emerging from water



quality studies in other Queensland sugar cane areas such as the Mackay-Whitsunday region (Rohde et al., 2006). Sugar growing areas, despite local differences in farming systems, hydrology and topography generally exhibit higher nutrient loadings dominated by dissolved inorganic nutrients in floodwaters compared to equivalent non-cane catchments. This ‘fertiliser signal’ is a concern from an environmental perspective due to the bioavailability of these nutrients and consequent uptake by ecological communities. Figure 4. Upper Barratta Creek: Nitrogen species values, 2004-2005 Wet Season. (TN: total nitrogen; PN: particulate nitrogen; DON: Dissolved organic nitrogen; NOx: Nitrate; NH3 : Ammonia).


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total phosphorous; PP: particulate phosphorus; DOP: dissolved organic phosphorus; FRP: filterable reactive phosphorus).


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Ham (2002) provides one of the few detailed measurements of nutrient loss in surface drainage water from irrigated sugar farms in the Burdekin. Nine sites were monitored for an array of variables (pesticides and nutrients) for one full irrigation season with four sites being monitored for a further two seasons. Ammonium-N commonly increased in irrigation tailwater run-off during the post planting periods (April to May) and then again during peak fertilizer application periods (September to November). These peaks varied in size often reach 3-6 mg/L, but were typically of short duration. Smaller, sharper peaks (<2 mg/L) also occurred during the January to mid-April periods and seemed to be associated with rainfall events or temporary water-logging (Usually in heavier textured soils). Much of the time, levels were near zero. The nitrate values in tailwater collected by Ham (2002) fluctuated widely in concentration levels. Similar to ammonium most losses were associated with the post-planting and peak fertilizer application periods. High values (>5 mg/L) were generally found as sharp, short duration peaks during such times. Rainfall events soon following sub-surface, split stool fertilizer application and irrigation resulted in peak run-off concentrations of 10.8 mg/l. Other high loss events could sometimes be linked to particular on-farm events such as surface application of fertiliser. Recommended Farm Practices for minimizing nutrient losses. An array of management options and on-farm practices associated with these nutrient movement pathways have been suggested to optimize crop nutrient usage and minimize nutrient export from cane crops. The actual effectiveness of these practices varies



depending upon the specific nutrient, underlying climatic and topographical conditions, soil properties and the nutrient loss mechanism specifically being targeted. Not all recommended management practices can be or need to be comprehensively implemented en masse, nor are all practices necessarily compatible with one another. Therefore some flexibility in development of a nutrient export minimization strategy is always warranted. Growers may need judicious selection of the array of available techniques, but should target as many locally relevant practices as possible to address the various nutrient loss pathways relevant to their particular farming and socio-economic circumstances. The information presented in the following review is that based on currently available research which in many cases is less than comprehensive. It merely outlines on the basis of current knowledge what may constitute BMP for many farming situations in the Burdekin, and many practices will no doubt change or be refined as better information comes to hand. Optimisation of Nitrogen inputs in line with BSES recommended levels. Results from an array of industry farm management surveys has provided mounting evidence that rates of fertilizer application (in particular nitrogenous fertilizers) are being applied by many growers in excess of BSES recommended rates. Reasons include the return from an extra tonne of cane far outweighing the cost of extra applied nitrogen and extra fertilizer also providing a cheap form of crop insurance. The BSES have therefore recently conducted a number of district specific trials to demonstrate how bringing nitrogen rates back within industry guidelines would not only save production costs, but also improve CCS and overall profitability for growers (Shannon, 2002). Reductions in nitrogen application back to within recommended range would not only improve profitability and promote ‘Best Management Practice’, but also display positive promotion of improved agronomic practices to the broader community. A series of nitrogen strip trials across Queensland were developed where tonnage and CCS (commercial sugar content) obtained were related back to N application rates and net dollar return/hectare calculated for each rate applied. Results of the Burdekin district 1 year nitrogen strip trials are outlined in table 2.2 below. Table 2.2. Nitrogen trial results Burdekin district (from Shannon, 2002). 3rd ratoon - Jardine Rate kg/ha T cane/ha Ccs T sugar/ha Net $ / ha 195 110 14.07 15.5 2662 240 117 13.63 15.9 2640 300 115 13.54 15.6 2505 Fallow Plant – Mona Park Rate kg/ha T cane/ha Ccs T sugar/ha Net $ / ha 130 165 13.06 21.5 3615 230 161 12.9 20.8 3353 Due to an array of constraints the trials from the Burdekin district were unfortunately



restricted in their temporal extent, somewhat limiting the conclusions that could be drawn from results. Available results did however provide evidence that more nitrogen does not necessarily mean more profits to the grower, but that a balance between nitrogen application rate and tones/CCS obtained is required. Interestingly, in the more comprehensive data sets available from other Queensland cane districts (usually three years data), not once did a result occur where the most profitable rate exceeded current BSES recommendations for ratoon crops. In cases where excess nitrogen was applied as a cheap form of crop production insurance, trial work also demonstrated that over successive seasons, some potential profitability is actually lost with this practice. Adherence to BSES recommendations was therefore demonstrated to maximize chances of profitability (given appropriate climatic conditions etc.). Site (Soil) Specific fertilizer Management Guidelines. With the partial exception of nitrogen, Australian nutrient management on cane farms have historically (and often currently) been based upon a set of generalized recommendations for use on an industry wide basis (see Calcino, 1994, CANEGROWERS, 1998 etc.). These guidelines were primarily developed on the results of yield calibration trials that were averaged across districts and soils, despite the often large differences in regional yield responses that emerged during trials. Minimal specific guidelines exist accounting for different regions, soil types, climatic regimes, nutrient soil reactions or nutrient use efficiency. Even nitrogen recommendations, for which the Burdekin region has a higher relative recommended application rate than the rest of the state, presents a somewhat coarse-scale approach to the issue. There has accordingly been little systematic utilization of soil classification or pedological properties to guide nutrient management in the Australian sugar industry, despite the concept assuming major importance in overall nutrient management on farms in South African sugar industry for example (see Schroeder & Wood, 2001). The application of a single set of nutrient recommendations for ‘average soils’ on an industry wide basis, with minimal differentiation according to climate regime and soil type has resulted in both under and over supply of nutrients in certain circumstances (Wood et al., 2003). Re-evaluation of some of the original BSES trial data (which had useable soil property information) used to develop recommendations demonstrated the capacity to group different responses to N according the general soil type (colour & texture) in various cane growing regions (Schroeder et al. 1998). The significant variability existing between many soils types based upon easily mineralisable aerobic N (the ability of soils to release N from organic matter, usually via microbial activity) in particular is one soil property attracting considerable research interest in developing a more targeted approach to future fertilizer recommendation development. Soils with higher organic matter mineralize more N and accordingly require lower fertilizer inputs. Schroeder & Wood (2001) categorized soil types in the Herbert and Bundaberg area according to their ability to mineralize N from organic matter to produce an N mineralization index. The use of the N mineralization index in conjunction with district yield potential and the N requirement of a particular crop size were used to generate district specific N guidelines. This approach produced recommendations considerably lower than current BSES guidelines in all



circumstances except for large crops (>120t) grown on soils with low organic matter. The fact that the N mineralization index is based on soil organic carbon allows for guidelines to be easily utilized throughout the industry. As well as fine-tuning N application rates according to site specific soil characteristics, recent research into site specific nutrient management recommendations has focused on phosphorus and potassium, two additional nutrients cane requires in greatest quantity. The separation of soils into P sorption classes with higher rates of P fertilizer recommended on high P sorbing soils has been advocated (Wood et al. 2003). Re-evaluation of data from past BSES potassium fertilizer trials has similarly demonstrated that soils can be separated into at least two broad groups according to responsiveness to applied K fertiliser. Soil specific nutrient management guidelines have been recently developed in the Herbert district (particularly to assist growers who rarely take soil or leaf samples). Since the 1980’s a detailed soil survey program(1:5000) has progressively gathered data on the distribution, properties and fertility characteristics of 24 different soils, covering 80% of cane area and soil sampling from over 1200 locations (see Wood & Bramley, 1996). Obviously a system based on 24 different soil types was not pragmatic, statistical analysis however revealed grouping soils into 5 different clusters sharing similar physical and chemical properties was possible. The resultant soil analytical database has been used to compile reference booklets for growers outlining soil formation and field appearance, appropriate nutrients, tillage and water management practices and environmental risk management strategies for each soil type (Wood et al., 2003). As such, much recent research advocates recognition of the variability in soil properties is fundamental in determining appropriate nutrient management practices for specific circumstances on individual farms (Bruce, 2002, Wood et al., 2003). Best practice nutrient management is now regarded as using a detailed understanding of different soils and their properties to develop soil specific fertilizer management to ‘fine-tune’ nutrient applications to the specific requirements of the crop on a particular soil type (Wood et al., 2003). Soil Testing and Leaf Analysis. Soil tests of cane paddocks can be useful condition indicators at a block scale. Soil fertility tests across districts can help identify if nutrient management advice for particular elements are being followed. While soil testing can identify areas with increased potential for off-site movement, tests do not ascertain if this movement is actually occurring.While cane farmers are encouraged to undertake soil testing after every crop cycle, it is evident that few growers actually base their fertilizer applications on regular soil or leaf tissue sampling. In a similar situation to soil testing, while leaf (foliar) analysis has become a useful and widely used diagnostic and advisory implement in many other Australian and overseas agricultural enterprises, it that has not been embraced to any great degree by the Australian sugar industry. Leaf analysis’ most common current use is in the ad hoc



diagnosis of micronutrient (trace element) deficiencies in poor growth areas or problematic crops, with little in the way of systematic use. Leaf analysis can also have considerable utility in overall fertilizer recommendations (Schroeder et al., 1999) if used in conjunction with various other tools such as soil testing to ensure balanced nutrition in crops. The concept of ‘whole of crop cycle’ fertilizer recommendations using soil and leaf analysis as tactical tools as part of a more strategic, longer-term approach to nutrient management is now being actively promoted within the industry (Wood et al., 2003). This allows farmers to develop a much more informed decision-making approach, with the identification is blocks requiring special management. Integrated ‘whole of system’ approaches to managing fertilizer inputs. The need to take into account all sources of N available to a crop, not just that applied as direct fertilizer is fundamental to a responsible whole of system approach to managing nutrient inputs (Schroeder et al., 2005). In planning a fertilizer program under irrigation a number of points have to be considered including yield expectation, available irrigation supplies. A perhaps neglected consideration is making allowance for all nutrient inputs (not just those artificially applied) such as soil mineralization, nitrogen accumulation from fallow legume crops and nutrients contained in irrigation water, mill by-products returned to fielded and nutrient return from practices such as GCTB. Consequent fertilizer applications should be discounted for all of these inputs (although there may be some uncertainty about the amount and timing of N release associated with many of these factors). Best Management Practice for Fertiliser Management in the Burdekin. In line with the issues elaborated in preceding sections, BMP for nutrient management in the Burdekin should consider: Application Rates. Nutrient application rates should align in most respects with BSES recommendations with regard to different crop stages. Further refinement should involve regular soil and leaf testing to tailor rates to a paddock scale. An important component of this process is that in accordance with ‘whole of system’ approaches to fertilizer management growers should recognise and discount all nutrient inputs available to crop in a particular paddock including: Nutrients ‘hidden’ in other products. Some products such as gypsum have considerable amounts of other elements in their composition which negates the need for specific application of sulphur for example. Trash Blanketing. While available for other districts, there is unfortunately minimal data on nutrient returns from GCTB breakdown in the Burdekin area. Fallow Legume and small vegetable crops. The amount of nitrogen available to subsequent sugar cane crops from fallow legumes



depends on the specific type of legume, the quality of the crop and whether the grain was harvested. The amount of nitrogen being returned to the soil following a legume crop can be used to adjust the amount of nitrogen fertilizer required for different soils following different legume fallows (Schroeder et al., 2005). Soil testing for mineral nitrogen (ammonium and nitrate) is more recommended when assessing residual nitrogen in a block following harvest of a rotational small vegetable crop. Residual nitrogen can be particularly high in many cases due to the often substantial fertilizer applications used for small vegetable crops and the under utilization of fertilizer nitrogen. Mill by-products. Nitrogen application rates should also be adjusted where mill by-products such as mill mud and mud/ash mixtures have been used (Barry et al., 2002). The amount of N applied to a paddock needs to be discounted for up to 3 years after the application of mill by-product. (Schroeder et al., (2005) outlined N application modifications appropriate for the Bundaberg district following mill mud or a mud/ash mixture application to paddocks. Similar fertilizer application modifications appropriate for the Burdekin district are not yet available. Nutrient availability in irrigation water. Irrigation water in many cases can constitute a valuable and often under-utilised source of nutrients such as sulphur, nitrogen and potassium. Nitrogen occurs in both nitrate and ammonium form in Burdekin district aquifer supplies ranging from 0-384.5 kg N/10ML (60-120kgN/10ML reasonably common). Biggs et al. (2001) noted that in the Burdekin area where irrigation application to crops is commonly around 15ML/ha, irrigation water from bores with 50 mg NO3

-/L sees approximately 170kg/ha of nitrogen supplied to crops simply in irrigation water. Other nutrients such as sulphur, potassium and zinc can also be present in Burdekin irrigation water at levels that negate supplementary application to crops. There is a need however to recognize that high levels of some nutrients can hinder uptake of others. Excessive amount of sulphur in water may prevent uptake of other nutrients. Therefore it is recommended to not consider soil/leaf tests in isolation when developing fertilizer application recommendations or strategies, use water tests as well to help refine application rate recommendations Fertilisation Application and Irrigation Timing. For reasons associated with maximizing crop uptake and reducing nutrient losses (discussed previously), fertilizer application below the surface is generally regarded as ‘best practice’. Recent BSES surveys indicate Burdekin growers are amongst Queensland’s leading regions in recognizing the importance of sub-surface application of fertilizer, with close to 100% of growers using this method in line with Code of Practice recommendations (O’Grady & Christiansen, 2001). Fertiliser application beside the stool is the most common method of applying fertilizer below the surface across Queensland, although split stool is another fairly common approach in the Burdekin district.



Fertiliser placement recommendations (especially urea based fertilizers) for the Burdekin district are as follows (Gary Ham pers. comm..): Plant cane- part banded during planting with balance applied sub-soil during later during the cultivation/hilling up operation. Ratoon Cane (furrow irrigation): buried 75-100mm (GCTB or burnt), stool split application by coulter or either side of stool by coulter or tynes. Overhead irrigation: GCTB/Burnt – buried as above

GCTB only: surface applied followed by not < 20mm irrigation Where possible fertilizer should be applied into moist soil with subsequent irrigation delayed until the schedule time, thereby maximizing opportunity for plant uptake. Post-fertilisation irrigations should be sufficient to replace RAW, but avoid leaching or excess run-off. Heavy irrigation following fertilization should definitely be avoided Future Developments in Fertiliser and Nutrient Best Management. BMP development is obviously a dynamic and evolving process as new approaches, knowledge and technologies come to light. A number of seminal BMP practices are on the horizon in the cane industry, that while still in their inception and likely at least several years away from widespread adoption hold considerable potential for refining nutrient management across the industry. The Nitrogen Replacement Strategy. An alternative approach to N fertiliser application recommendations that has been recently developed (and is currently being assessed in the Burdekin) is that N application should be aiming to replace the amount of N removed by the previously harvested crop (Thorburn et al., 2003). The basis of this approach is that while in the long-term without fertilizer inputs, natural processes (mineralization etc.) cannot maintain high cane yield and in fact reduce soil organic matter and impinge negatively upon soil health. However, for a single crop, there is considerable evidence that applying little or no N has minimal negative impacts upon yield. This effect is due to either residual large quantities of mineral N in the soil profile (from existing fertilizer application) or the mineralizing capacity of the soils was sufficient to meet crop needs in the short term. Thorburn et al. (2003) noted that risk and uncertainty are major motivating factors underlying individual growers’ decision-making agendas with regard to fertilizer management. Grower’s profit margins tend to be influenced primarily by productivity rather than fertilizer costs. Given the difficulty in predicting upcoming crop yields it is typically seen as a rational short-term tactic to over apply N fertilizer and minimize risks of low production and profits. However, growers assume long-term average N response curves apply to each individual year and that reducing N application in any year will dramatically reduce yields, despite compelling evidence to the contrary. Given that N soil reserves or N mineralization capacity are usually sufficient to adequately supply a single crop, a change in philosophy from fertilizer application to ensure yield in a forthcoming crop to instead replace the N removed by the previous crop could be a more risk averse



approach to reducing yearly fertilizer application rates. For individual cane blocks the off-take of N each year can be estimated by measured cane yield in combination with N concentrations in cane, or direct measurements made at mills. Long-term modeling simulations of different N fertilizer management strategies: N replacement, current BSES fertilizer management and current industry average management suggest gross margins under the N replacement system was higher than the other two approaches with no difference in simulated soil organic matter characteristics. Thorburn et al. (2003) did acknowledge the concept is yet to be applied to other soil types and growing situations as well as considerable uncertainty as to how to account for other N inputs (other than fertilizer) and losses (other than harvested cane). Nutrient Interactions with other aspects of farm management. Irrigation. The in many ways overriding nature of interaction between cane irrigation and nutrient movement is underlined by the run-off data outlined in Ham (2002) and mounting evidence of nitrate contamination of Burdekin groundwaters. Given the strength of the association between water use efficiency and movement of sensitive compounds, optimization of irrigation application and subsequent WUE are a core component of carefully consider fertilizer and nutrient management. As well as issues of basic productivity, the concept of WUE is closely linked to minimization of off-site water quality impacts in the wider environment. Considering the general paucity of research information dealing specifically with nutrient dynamics under the various current farming practices and the consequent lack of BMP for fertilizer management under furrow irrigation, examination of irrigation practices and their links to solute movement is one avenue for improved fertilizer management in the Burdekin. Irrigation management strategies associated with fertilizer application are of fundamental importance to reducing N loss via various pathways such as run-off, deep drainage, volatilisation before crop uptake. Management strategies promoting irrigation BMP are dealt with in detail in later sections of this review. The interactions and management implications associated with water application and nutrient movement is an issue deserving substantial future research focus. Fallow Management. Substantial benefits can be accrued by farmers in the breaking of a sugarcane monoculture through the use of well managed fallow crops (such as legumes) in sugarcane farming systems (Garside et al., 2001). The need to account for the nitrogen contribution of fallow crops to overall development of nitrogen application rates has already been touched upon. Productivity and nitrogen budget benefits aside, the quality of fallow management is also thought to have considerable implication for water quality. Reghanzani & Armour (2000) suggest fallow fields and plant crops are most at risk of significant movement of soil and nutrients via run-off in rainfall events. For this reason the use of cover crops during fallow periods or spray-out of previous crop with stubble retention to protect soils from rain drop impact during wet season rainfall events is a



recommended practice. According to a recent industry audit a significant proportion of farmers in the Burdekin currently utilize conventional cultivated bare fallows (Wrigley, 2005), often as a nematode control measure. The have been substantial recent shifts away from this practice however, toward increasing adoption of managed crop fallows. The actual benefits of cover crops (legumes etc.) versus bare fallows or variations thereof to water quality leaving farms has not however been tested or validated as yet in a rigorous way. Reduced sediment movement would seem a logical outcome of a cover crop during the fallow cycle. Issues such as the nitrogen uptake/contribution of fallow crops and subsequent movement during rainfall/irrigation events are yet to be fully investigated. There is now some thought that actually harvesting legume crops rather than simply ploughing fallow in is required due to the large amounts of N generated by a fallow that will not be utilized by a subsequent cane crop and is according vulnerable to loss. Constructed Wetlands. In many ways the development of constructed wetlands is an evolution of the concept of tailwater recycling on farms. As well as retaining water leaving paddocks, the wetland ecosystem should theoretically provide additional filtering and assimilation capacity of many entrained materials found in farm run-off. The potential value of using constructed wetlands to improve irrigation drainage water quality has been assessed to some degree in the BRIA by Hunter & Lukacs (2000). Here performance trials in 1994/95 and 1999 (4 bays, 60m long, 6m wide and ca. 1m depth) and were undertaken to quantify changes in concentrations and loads of suspended solids, phosphorous, nitrogen and some commonly applied herbicides (diuron and atrazine) between constructed wetland inlets and outlets. Results of the 1999 study period demonstrated the constructed wetlands (planted with local wetland species) removed 60-70% of suspended solid load and concentrations at outlets were significantly lower than at inlets (similar results were found in 1994/95). The control bay (no macrophytes) removed 16-49% of suspended solid load. There was however a net increase in total phosphorous loads at outlets of vegetated bays (ranging from 0.4 to 67%) despite the marked decreases in suspended solid loads, and concentrations were significantly higher than at inlets. Interestingly, P loads in the un-vegetated control bay decreased on both monitored occasions. There were some changes in P composition of water passing through the wetlands consistent with uptake and cycling by flora and fauna, but the exact processes responsible for observed results were not clear. Earlier monitoring of these wetlands in 1994/95 suggested a much greater capacity for these wetlands to remove phosphorous from tailwater. Changed wetland composition/condition (the systems were five years old during the 1999 monitoring period) and different phosphorous composition of inlet water were suggested as possible causal factors behind the relatively diminished capacity of wetlands to remove phosphorous. On both the 1994/95 and 1999 sampling occasions, changes in nitrogen loads were variable and relatively small and concentrations at inlets were generally not significantly



different from outlets. The results of this trial underlined considerable research into the processes underpinning constructed wetland dynamics is required to better elucidate their potential value. The fact that results differed between the two monitoring stages of this project highlighted that research into how to maintain wetland effectiveness through time is a particularly important research need. 3.0 Pesticides. The Role of Pesticides in Cane Production. Like many agricultural industries, pesticide usage in the Australian sugar industry is a significant part of the overall farming system for many growers in order to remain productive and competitive. Herbicides are the primary pesticide category used in Australian sugar production with chlorpyrifos the most common insecticide utilized on an industry wide basis (Cavanagh, 2003). The recent moves by the sugar industry toward adoption of practices such as reduced/zero tillage and a corresponding increase in pesticide use (particularly herbicide control of weeds in ratoon crops) makes a more focused approach to pesticide management essential for genuinely sustainable agricultural practice. Current Pesticide Usage in the Burdekin District. A comprehensive recent assessment of pesticide usage behaviors by Burdekin cane growers is unfortunately unavailable. Some insights into contemporary insecticide usage patterns in the Burdekin district can be gleaned from the research of Cavanagh (2003) regarding historical usage patterns of the now banned (but highly effective) organochlorine pesticides. Since the ban on agricultural organochlorine usage in 1987, chlorpyrifos (trade name SuSCon®)has become the primary insecticide used throughout the cane industry. Despite effective control of greyback cane beetle Dermolepida alborhirtum (the most significant insect pest of sugarcane) and other insect pests in most areas, chlorpyrifos has had minimal effect in controlling greyback grub in the Burdekin district. Alkaline soil hydrolysis and microbial degradation were suggested as likely causes of this failure. Microbial degradation may also be a causal factor in the diminishing effectiveness through time of other insecticides such as heptachlor observed in the Burdekin district (Cavanagh, 2003). The recent expansion of sugarcane growing area associated with the BRIA development is notable from the perspective that as a consequence of the main soil types in this area, BRIA cane is generally not susceptible to cane beetle damage and insecticides are rarely used (Cavanagh, 2003). Recent sales figures from farm chemical suppliers in the lower Burdekin nevertheless indicate chlorpyrifos (as Suscon, Chlorpyrimax, Fortune and Lorsban) is the dominant insecticide sold in the district (unpublished data). This data is somewhat misleading however, given some significant recent shifts in pesticide product usage across the



district. Imidacloprid is now the dominant cane grub control chemical utilized across the Burdekin. Additional insecticides such as fipronil (as Regent etc.) and bifenthrin (as Talstar etc.) are also used for control of minor pests such as wireworms and crickets. Documentation of specific herbicidal usage practices from an on-farm perspective in the Burdekin is virtually nil. Again chemical sales figures can provide a proxy indicator for basic relative application rates. Data suggests 2,4-D (Amine), atrazine (Gesaprim, Atramax, Atradex, Nutrazine, Gesapax combi etc.), diuron, glyphosate (Roundup, Weedmaster duo) and paraquat (Boa, Gramoxone, Nuquat, Shirquat) constitute the primary herbicides utilized in the Burdekin at least with regard to total sales (unpublished sales data). Environmental Pressures associated with off-site fertilizer and nutrient movement. By virtue of the often prohibitive costs of intensive analysis of pesticides, the presence, dynamics and environmental effects of off-farm pesticide movement has been only sporadically investigated in Queensland. The limited results available do however suggest the issue of pesticide movement from farms and other human industry warrants further attention. Broadscale surveys of sediment herbicide concentrations in the nearshore GBR lagoon (primarily in the Wet Tropics region) during 1998 and 1999 detected the presence of both atrazine and diuron, two commonly applied agricultural herbicides (Haynes et al., 2000a). Detection of these levels of diuron contamination are of concern as laboratory trials have indicated that diuron concentrations of less than 1 µg L-1 significantly reduce photosynthetic rates in seagrass commonly found along the Queensland coast (Haynes et al., 2000b). Diuron has also been implicated (although not conclusively linked) to dieback of mangrove communities in the Pioneer River estuary on the central Queensland coast (see Wake, 2006 for an overview). The specific eco-toxicological effects of pesticides within natural environments however remain largely unknown. A distinctive water quality signal associated with pesticide use in the cane industry is emerging as a characteristic indicator of catchment land use during flood events (see Rohde et al., 2006). Data pertaining specifically to pesticide dynamics both on and off -farm in the lower Burdekin region is largely lacking, a situation not dissimilar to most Queensland catchments, whatever their dominant landuse. A CCI project targeting pesticide behaviors on-farm as well as within the broader environment is currently underway to address some of these knowledge gaps. Locally relevant pesticide data has also be generated from a small array of research efforts through time and are discussed in more detail below. Pesticides in Burdekin Surfacewater.



The study results of Hunter et al. (1998) outlines pesticide survey results from several drains, streams and lagoon in the lower Burdekin region carried out on three occasions between November 1995 and August 1997. Although sampling frequency was limited, this study found evidence of pesticide residues in water, sediments or biota across a range of BRIA and Burdekin Delta sample sites. Atrazine and 2,4-D were the most commonly detected herbicides. Atrazine was detected in 30% of water samples analysed (range 0.2 to 40 μg/L), while 2,4-D was detected in 19% of water samples over the study period (range 0.3 to 27 μg/L). Research efforts targeting pesticide dynamics in the surface water directly draining irrigated cane farms in the Lower Burdekin region has been minimal, although the work of Ham (2002) is an exception. During this study samples of inflow and outflow water were collected at each irrigation and rainfall event on nine farms over the course of one or two seasons. Pesticide analysis included most of the commonly applied pesticides in the district including chlorpyrifos, diuron, grammoxone, 2.4-D acid, MCPA, glyphosate, atrazine, ametryn, ioxynil and 2,4-D methyl ester. Herbicides were the most frequently recorded pesticide with atrazine the most commonly recorded chemical in run-off, followed by diuron, ametryn and 2,4-D acid. The most commonly applied insecticide in the district chlorpyrifos was found in only a small number of samples, with a maximum concentration of 0.111 μg/L recorded. This outcome is not surprising given the relative immobility of chlorpyrifos bought about by its propensity for binding to soil particles. No glyphosate or gramoxone was found above the limits set (0.2 μg/L detection limit). Peaks of concentration of individual herbicides were typically less than 20 μg/L and of 2-3 days duration, although more pronounced spikes were detected in a number of cases. Atrazine had a sharp peak of 70.2 μg/L, ametryn at 45.2 μg/L, 2,4-D acid at 91 μg/L, all with a duration of under 24 hours. Diuron had with a maximum concentration of 62 μg/L, but generally in the range of 3-25 μg/L (Ham, 2002). These results probably reflect in large part the relative potential of these various pesticides for movement in solution (although local usage intensity could be another contributing factor). The period of September to mid-January accounted for almost the entirety of pesticide loss, not surprisingly corresponding to the period of most intensive pesticide application. An important note is that while the measured levels of some pesticides in tailwater run-off exceeded the trigger ANZECC values for aquatic ecosystem protection, there was no account of dilution effects during flow to or through stream systems. Pesticide Residues in Burdekin Groundwater. The issue of pesticide contamination of the Burdekin aquifer has been a topic of sporadic research attention through the years. Brodie et al. (1984) recorded low concentrations of Heptachlor and ỹCH (the dominant insecticides of the day) from an array of bores in the Ayr and Home Hill and Delta farming season through 1976 – 1977. These were with few exceptions found to be in the 0-3 ng dm -3 range, well below drinking water guidelines. In one of the more spatially comprehensive surveys Keating et al. (1996) and Bauld et al. (1996) outline the results of a nitrate/pesticide study in the early 1990’s sampling forty



bores distributed between the Burdekin Delta and the north-eastern sector of the BRIA. While screening for approximately 80 pesticides, only atrazine and its degradation product deethylatrazine (DEA) were detected (the ratio of DEA to atrazine in groundwaters is indicative of the rate at which atrazine has move into an aquifer). The frequency of atrazine positive samples ranged from ca. 76% (in the BRD in 1992) to ca. 70% (in the Delta and BRIA in 1993). Atrazine concentrations were generally very low with one-half to three quarters of atrazine positive samples containing less than 100 ng/L, but some notable exceptions included a Delta site returning markedly higher pesticide levels (i.e. 1450 ng/L in 1992 and 1300 ng/L in 1993). Interestingly, because of marked changes in usage patterns, the pesticides previously found in Delta groundwater by Brodie et al. (1984) were not detected during this study. DEA was commonly present in atrazine positive samples, but never found not associated with the parent compound. Most DEA to atrazine ratio values (DAR) were low (80% had DAR < 1.0 and 65% were < 0.5), consistent with rapid leaching to the water table and the probable occurrence of preferential flow processes through macropores or cracks. A recent pilot study by Klok & Ham (2004) assessed the active ingredient levels of a range of the most commonly used pesticides in the groundwaters and soil profile at six farm sites across the Burdekin Delta (incl. MCPA, 2,4-D, diuron, atrazine, amtryn, chlorpyrifos, pendimethalin and hexazinone). Analysis of irrigation samples from groundwater bores taken from fluming during irrigation events suggested the groundwater itself was relatively uncontaminated. 28% of the 67 soil water samples (1.5m depth) returned a measureable level of pesticide. Hexazinone and MCPA were found at a small number of sites, all below ANZECC trigger values for aquatic ecosystem health. Atrazine was found in four samples, with two above the ANZECC trigger value of 0.7 μg/L. Diuron was detected in 14 of the 67 samples, with all values well above the trigger value of 0.2 μg/L (maximum level was 0.9 μg/L). 2,4-D was detected in eight samples with all results well below the trigger value of 140μg/L. Soil profile samples (taken down to 1.5m depth) returned small amounts (<0.023mg/kg) of diuron and atrazine adsorbed to soils particles at a number of sites. These results prompted the authors to suggest water quality of the aquifer was not adversely affected by pesticide leaching at the time, but there was substantial potential for pesticides such as diuron and atrazine to leach through the soil profile and ultimately into groundwater sources. Current Approaches and Research relevant to BMP for Pesticide use in the Burdekin region. Pesticide Characteristics. It has been well established that following application a number of chemical, physical and microbial processes all contribute to breakdown or persistence of the pesticide including volatilization, photolysis, chemical hydrolysis, oxidation or biological and microbial degradation (Simpson et al., 2002). In addition to these processes, pesticides can also be lost from a system through surface run-off or deep leaching during rainfall and irrigation events. As Simpson et al. (2000) illustrates, while there may be some



limitations on controlling chemical and microbially mediated pesticide dynamics (particularly from a farmers perspective), understanding physical behaviour, specifically transport processes, can lead to improved on-farm management of pesticides. Understanding the physical properties of various pesticide types (solubility, soil-binding characteristics, persistence etc.) is fundamental to managing the risk of excessive off-site movement. Not all pesticides behave in the same manner and differences in application, persistence and mobility will strongly affect movement dynamics after application. Literature reference values for solubility and mobility are easily obtainable for most commonly used pesticides in the Australian sugar industry (see table 3.0 below) and provide a base overview of some environmentally relevant pesticide properties. While site specific values on specific soil types and at different soil depths would be more reliable and informative, literature values provide at least a useful guide to likely environmental mobility risk of applied pesticides. Table 3.0. Literature values of key physical properties of some pesticides used in sugar production (from Simpson et al., 2001). Common name Trade Name

(examples) Solubility in water (mg/L)

Half-life in soil (t½) days

Soil/water (KOC)

Atrazine Atradex 0.033 60 100 Diuron Diurex 0.042 90 480 2,4 D-amine Amicide >1000 10 20 Glyphosate Roundup 1000 47 24000 Ametryn Gesapax combi 0.185 60 300 Chlorpyrifos SuSCon® Blue 0.0004 30 6070 Paraquat Gramoxone 620 000 1000 1 000 000 Triafluralin Treflan 0.0003 60 8000 As well as recognizing the likelihood of variability in pesticide behaviour across different climatic and pedological regimes, care must be also taken in interpretation of reference values. Rather than relying solely on solubility values to gauge risk of off-site movement, data on soil-water partition co-efficients (i.e. KOD or KOC values) provide important additional context as to how a pesticide will likely behave after application. KOC values reflect the relative affinity of a compound for adsorption to soil particles with consequent ramification for likely avenues of off-site loss. In combination this data can be useful in predicting the relative distribution of a pesticide between water and sediment bound phases in a soil/water mix (i.e. storm run-off down a furrow). Some highly soluble pesticides because of ionic composition bind tightly to soil particles and therefore minimal risk of groundwater contamination (i.e. paraquat). In contrast, other less soluble species (atrazine, ametryn and diuron) are less tightly bound to soil particles and are susceptible to off-site movement in both groundwater and run-off. These sort of properties also indicate why soil and sediment retention will have a significant positive effect on off-site movement of chemicals such as trifluralin, paraquat, chlorpyrifos and glyphosate which predominantly bind tightly to soil particles. Such



management strategies will have far less effect on managing losses of chemical with lower KOC values such as atrazine, ametryn and diuron which tend to be both water soluble and move in solution while binding minimally to soil particles (although see below). Studies by Hargreaves et al. (1999) and Simpson et al. (2001) provide some of the first Australian data on the persistence and dissipation of a number of widely used pesticides under sugar production, While not specifically relevant to the Burdekin region, with the field data obtained from sites in the Bundaberg/Childers area, the data is useful in highlighting the key periods for pesticide movement. Simpson et al. (2001) for example documented pesticide dissipation rates (DT50) through previously mentioned ‘on-site’ mechanisms (volatilization, hydrolysis, oxidation tec.) tends to be quite rapid for most compounds in most situations (see table 3.1 below). Diuron was the one prominent exception on the red ferrosol soil type where it appeared highly persistent, perhaps due to incorporation into the soil matrix to the extent it was somewhat protected from many normal breakdown processes. The general message however is that higher risk periods tend to be associated with the time period shortly after pesticide application. The potential risk levels associated with losses via surface run-off or leaching will be proportionate to the amount available to be removed at any moment in time. As such, concentrations in run-off or leachate can be high if rainfall or irrigation events occur at inopportune times relative to application. Table 3.1 Field dissipation rates (DT50) for pesticides in Bundaberg soils (0-2.5cm), from Simpson et al., 2001).

DT50 (days) Soil pH % OC % Clay Atrazine Ametryn Diuron 2,4-D

Grey kandosol 7.2 0.80 3 3-13 3.5 13 - Redoxic hydrosol 7.1 0.72 8 2.5-27.5 2 15.5 1.8

Red ferrosol 6.0 1.23 63 1-7 14.5 >250 12 Yellow chromosol 5.1 0.95 6 2.5 4 - - As an added layer of complexity to the pesticide management issue Simpson et al. (2002) also highlighted some changes in pesticide adsorption properties after application . Atrazine for example is not tightly bound to soil (5% on sediment) initially after application, a not unexpected outcome for a pesticide with a relatively low KOC value. Over time however this increases to 75% sediment bound after ca. 25 days. Therefore in the case of atrazine, the primary avenue for off-site loss in the immediate post application period will be in solution, with a shift to sediments over time. Diuron, a pesticide for which losses through solution and sediment equally contribute to off-site movement in initial stages after application, shifts to soil bound entirely through time. Despite this added detail, the underlying message from a farm management perspective is that the major risk periods for off-site movement tend to be confined to periods immediately after application. Knowledge of these risk windows is fundamental to managing the temporal dimension of responsible pesticide management (Simpson et al. 2000). Irrigation or significant rainfall soon after pesticide application generates



significant potential for pesticide movement in solution. Available data suggests that a short time after application however, the level of pesticide likely to move in solution is drastically reduced. Therefore, allowing time for applied pesticides to adsorb to soil (or bind to foliage) will greatly improve management of off-site run-off losses and also significantly reduce losses to groundwater through leaching. This reduction in the capacity of pesticides to move in the water (solution) phase is however accompanied often by increased potential for movement via sediment. Any management strategies minimizing sediment losses (GCTB, minimum tillage etc.) should alleviate some of this risk. With knowledge of the approximate time period posing potential for highest off-site losses, appropriate strategies can be then developed to avoid or minimize chances of significant run-off or leaching during these periods. Pesticide Interactions with other aspects of farm management. Irrigation and Pesticide Management. The linkages between pesticide movement and irrigation is a primary consideration for farm management in the lower Burdekin region given the virtual total reliance of the district on fully supplemented irrigation. Simpson & Ruddle (2002) consider the addition of irrigation imposes an added risk for off-site losses, particularly if there is a limited understanding of links between persistence and the mobility of pesticides being utilized. Some pre-emergent herbicides such as atrazine or diuron often rely on some form of incorporation, either through cultivation or ‘watering in’ by rainfall or irrigation. Such a process is obviously fraught with significant risk of off-site movement if not managed appropriately. While it is virtually impossible to control amounts and timing of rainfall relative to pesticide application, the decision to irrigates triggers a number of processes in addition to the increasing of soil moisture. As discussed previously, both the timing and intensity of particular irrigation (or rainfall) events relative to pesticide application can exert significant influence on amounts of pesticide moving off-site. However, with an awareness of pesticide properties, efficient irrigation should not directly produce unacceptable off-site losses. Simpson & Ruddle (2002) noted that efficient irrigation which keeps water near the root zone should also minimize pesticide losses through either leaching or run-off. Perhaps the one drawback of irrigated cane and relative pesticide movement is that higher soil moisture levels resulting under irrigation could produce higher run-off or leaching in the event of unexpected rainfall (compared to the same non-irrigated crop situation). Pesticide Best Management Practice. As the somewhat mixed results regarding pesticides residues in the Burdekin aquifer environment suggest, coherent documentation of pesticide dynamics both on-farm and in the natural environment is still sketchy at best (let alone development of locally relevant pesticide management guidelines). Most researchers highlight the need for more detailed spatial and temporal studies to better define pesticide characteristics of the region. Current best management practice for pesticide use in the Burdekin is probably best based around the pesticide management considerations outlined by Simpson et al. (2002):



• Do not apply pesticide immediately before irrigation or in the likelihood of heavy rain

• Irrigation scheduling should avoid high risk periods (particularly where furrow irrigation or heavy overhead irrigation is used)

• Reduce soil and sediment loss from surface run-off. Significant reductions in pesticide transport in run-off can be achieved, particularly pesticides such as paraquat, trifluralin and chlorpyrifos, which have high adsorption to soil particles.

• Off-site pesticide movement risk can be reduced by not treating large areas at the one time (splitting applications). Avoiding large areas of maximum pesticide residue concentrations reduced amounts available for movement during rainfall or irrigation

• Understand that some herbicides such as atrazine, ametryn or hexazinone are particularly mobile and can move off-site rapidly (leaching or run-off), particularly in association with irrigation or significant rainfall shortly after application.

• Excessive irrigation can carry some pesticides (atrazine) well below root zones and outside areas of effective weed control leading to groundwater contamination

• Freshly applied pesticides are often more mobile (less bound) than those which have had time to bind to soil or foliage

• Irrigation tailwater can contain high pesticide residues. Tailwater recycling and avoiding excessive irrigation after pesticide applications will reduce off-site pesticide losses.

• Vegetated wetlands can be much more effective than basic tailwater retention dams in removing herbicides such as atrazine and diuron, removing up to 40% of incoming pesticide (see Hunter & Lukacs, 2000).

• Additional care should be exercised where storm or irrigation run-off discharges near streams or sensitive habitats. Good water management is central to effective pesticide management (recycle pits where pragmatic for example).

• In highly porous soils or in areas with shallow water tables, less mobile alternatives should be considered to minimize potential contamination of groundwater or stream baseflow.

A computer software application (SafeGauge) based on much of this previously discussed knowledge was recently developed to aid cane farmer’s risk management of pesticide application at a block scale. Probably for a variety of reasons (including ‘user friedliness’) the product has not yet been embraced to any great degree by growers in the lower Burdekin. Future Developments in Pesticide BMP. Issues such as recent development of new chlorpyrifos formulations that counteract high pH soils could cause significant changes in usage patterns across the Burdekin in the not too distant future. Of even greater potential significance are the findings of recent Australian Pesticides and Veterinary Medicines Authority reviews of approvals for active pesticide ingredients such as diuron and atrazine (see NRA, 2002, APVMA, 2005). Recommendations from these reviews (still in preliminary draft form), if approved could



see significant changes in the permissible uses and application rates for these particular herbicides. Such an outcome would greatly affect existing pesticide usage behaviors by cane growers with shifts to other products seemingly inevitable. Exactly how such usage readjustment would develop can only be speculated upon at this point in time. 4.0 Crop Irrigation Supply and Practices. The role of Irrigation in the Lower Burdekin Cane District. Sugar cane is a crop with high water demands to maintain cell turgor for growth and to sustain the plant transpiration stream, enabling nutrient uptake in roots and gas exchange in leaves. While some sugar areas rely solely on rainfall to meet crop water requirements, many sugar growing districts in the Queensland sugar industry use irrigation to some degree. Cane growers utilize irrigation to increase profit and productivty, enhance germination, maintain stool viability and expand management options such as herbicide and fertilizer application (Tilley & Chapman, 1999). The climatic vagaries of the Burdekin regions’ dry-tropical climate, with it’s low and variable rainfall has seen a reliance on supplemented water supplies for cane cropping since floodplain lagoons were first used to complement local rainfall in the 1880’s. The existing irrigation water supply environment in the lower Burdekin is somewhat more complex, with water currently managed by three separate organizations whose authorities relate to geographically distinct sections of the region. Existing Water Usage within the Lower Burdekin Cane Irrigation Environment. The Burdekin Delta region has traditionally been heavily reliant on groundwater extraction for the majority of irrigation water supply, although there has been a recent shift toward increased surface water usage. Following issues arising from seawater intrusion along the coastal fringe due to over-exploitation of the aquifer, the North and South Burdekin Water Boards (NBWB and SBWB) were created in the mid-1960’s to manage the Burdekin Delta aquifer. The North and South Burdekin Water Boards are currently constituted under the Water Act (2000) to use portions of the flow of the Burdekin River to replenish sub-artesian groundwater aquifers of the Delta area in order to increase the quantity and quality of water supply for irrigation, domestic, stock and industrial purposes. In the North Burdekin Water Board section, distribution of extracted water is via surface water flow in channels. The South Burdekin Water Board has a groundwater based distribution mechanism involving pumping extracted Burdekin River surfacewater into recharge pits, which supplements the natural groundwater aquifer. The Burdekin Delta irrigation area as a whole is composed of predominantly porous, sandy, high permeability soils. As previously mentioned, groundwater extraction for irrigation water supply is relied



upon heavily in the Delta region, with over 1,800 groundwater pumps in operation in the lower Burdekin. Recent management efforts are however underway to encourage Delta farmers to utilize more surface-water supplies in irrigation efforts. In the past, seepage of applied irrigation groundwater past crop root zones as well as irrigation application of surface water from the Burdekin River too turbid for use in recharge pits (water spreading) has been an integral part of the aquifer replenishment and management. A substantial expansion of the Burdekin cane industry occurred with the increased security of surface water supplies afforded by the completion of the Burdekin Falls Dam in 1986, the largest water storage in Queensland (1,860,000 ML). This expansion mainly took the form of the Burdekin River Irrigation Area (BRIA), the largest land and water conservation initiative undertaken in Queensland. The BRIA is managed by Sunwater, a State Government owned corporation. Since the first new land was auctioned in 1988 a further 50000 ha of new farm land has been developed. Irrigated water is supplied to farms across the BRIA through a surface channel network. The soils of the BRIA are substantially different from that of the Delta area, and are predominantly composed of cracking clay, sodic duplex and non-sodic duplex soils. Sugar cane crop irrigation requirements vary considerably over the Australian sugar industry as a whole. Irrigation inputs in the Burdekin region are substantial in relation to most other cane growing regions in Australia, representing up to 250% of effective rainfall (Kingston et al., 2000). The Burdekin region, along with the Mareeba/Dimbulah and Ord (Western Australia) cane districts are the only Australian cane regions totally reliant on full irrigation of crops (Holden et al., 1998). Just as considerable variability exists across districts in the degree of reliance on supplemented irrigation inputs, substantial variability likewise exists regarding the actual irrigation systems employed to deliver water to crops. In areas such as the Burdekin and Tablelands where full irrigation is required, the favoured irrigation system is flood/furrow irrigation. Over 95% of growers in the lower Burdekin currently utilize furrow irrigation for their crops, with a small proportion utilizing drip or overhead irrigation alternatives (Tilley & Chapman, 1999). Furrow Irrigation. Furrow irrigation is one of the oldest as well as most widespread forms of irrigation worldwide. Raine & Bakker (1996) noted that due to the lack of irrigation design guidelines, growers have typically opted for the ‘least cost’ design, with little consideration of water use efficiency or long-term viability. Irrigation efficiency under furrow systems can vary markedly (discussed in more detail below), highlighting the need for carefully considered management to maximize water application efficiency. There are a number of reasons behind the popularity of furrow irrigation in many environments. From Sutherland (2002), the perceived strengths of furrow irrigation can be considered as:

• Low pumping costs as a result of low pump pressure requirements • Low labour requirements and costs on well designed and managed systems • Low technical requirements, with low infrastructure and capital investment with



initial system establishment • Fields generally receive adequate water.

There are however an array of purported weaknesses attributable to furrow irrigation (Sutherland, 2002), namely:

• significant potential for inefficiency through processes such as deep drainage losses and tailwater run-off.

• Poor water distribution uniformity • Built to a price, not specific criteria, therefore lacking any formal design • Considerable skill or ‘craft experience’ is required for efficient management of

furrow irrigation operations. • High labour component associated with poor design • Minimal capacity to supply small volumes of water • Limited control of water distribution uniformity, which tends to be very soil

dependent. Water Use Efficiency (WUE) in the lower Burdekin Cane District. Given the recent industry shift toward improved environmental performance, comparative assessment of WUE on cane farms in the Burdekin has been carried out from scales at the whole of state level down to the relative performance of individual farms. In a comparison of water use efficiency across Queensland’s sugar regions Tilley & Chapman (1999) compiled a review outlining crop water index (CWI) and subsequent efficiency ratios for individual cane growing districts throughout the state. CWI is defined as the ration of yield to the total water available to a crop (effective rainfall plus irrigation). The work of Kingston (1994) demonstrating a response of 12.2 tonnes of cane per hectare for each megalitre of transpiration was used as the benchmark CWI for subsequent calculation of efficiency ratios. Tilley & Chapman’s review revealed relatively low efficiency ratio (ER%) of 41% for Burdekin delta farms (see table 4.0 below). The Burdekin BRIA ER was similar to that of many of Queensland’s other cane growing districts (ca. 70%). Table 4.0 Area irrigated, actual CWI, and efficiency ratios based on published benchmark equation and probable benchmark for areas (adapted from Tilley & Chapman, 1999). Region Area irrigated

(ha) Actual CWI (tonnes cane/ML)

Efficiency RatioA (CWI = 12.2)%

Efficiency RatioB (CWI = 13.8)%

Tableland 4000 9.2 75 67 Herbert 4586 8.5 70 62 Burdekin BRIA 30000 8.5 70 62 Burdekin Delta 39000 5.0 41 36 Proserpine 17963 8.9 73 64 Mackay 54446 8.6 70 62



Sarina 10771 8.5 70 62 Bundaberg 36829 9.1 75 66 Childers 11653 9.8 80 71 Maryborough 4776 9.2 75 67 Total (weighted)

214024 8.1 67 59

Efficiency ratioA based on published benchmark equation. Efficiency ratioB based on possible CWI caculated from furrow irrigation results from a Bundaberg experiment. Considerable quantitative research also exists regarding the efficacy of furrow irrigation on individual farms across the lower Burdekin region. In some of the earlier WUE research in the district Raine & Shannon (1996a) documented seasonal water application efficiencies (defined as proportion of applied water available to crop) for furrow irrigation ranging between 31 and 62% on farms lacking tailwater recycling facilities (see table 4.1 below). The application efficiency of individual irrigations revealed even more marked variance, ranging between 14 and 90%. This low water application efficiency was generally associated with deep drainage losses through highly permeable soils. Table 4.1. Irrigation efficiencies for commercial sugar cane production in the Burdekin region (from Raine & Shannon, 1996a). Site Soil Average Volume

applied (ML/ha/irrigation)

Average Application efficiencyA

Mulgrave A Cracking clay 1.5 62 Leichardt Non-sodic duplex 2.1 34 Jardine Non-sodic/sodic duplex 1.5 40 Jarvisfield alluvial 1.4 42 Rita Island alluvial 1.6 38 Home Hill alluvial 2.0 30 Clare alluvial 1.0 62 Mulgrave B Sodic duplex 0.9 49 A without tailwater recycling Holden & Mallon (1997) similarly noted widely varying outcomes in the average irrigation application efficiencies of individual Burdekin Delta area farms through the course of a season. Table 4.2 below outlines average irrigation efficiencies for 13 irrigation monitoring sites across a range of Delta soil types (plant cane) over the course of the 1995-1996 growing season. The authors noted in particular the wide range of efficiencies emerging with little consistent pattern in terms of soil type, furrow length or irrigation water quality. This suggested that in addition to issues such as basic soil type, the varying management practices at play on each farm (tillage practices, inflow rates etc.) may also have had considerable bearing on irrigation efficiency. Table 4.2 Irrigation efficiencies of Delta Plant cane (1995/96).



Soil type Average

volume (ML/ha/irrigation

RAW (ML/ha)

Deep drainage losses (ML/ha/irrigation*

Irrigation application efficiency (%)

Drill length (m)

Water EC (dS/m)

Sandy loam 0.6 0.4 0.1 68 335 0.2 Sandy clay loam 0.7 0.5 0.1 67 630 1.22 Silty clay loam 0.8 0.5 0.2 66 380 1.89 Clay loam 1.2 0.7 0.2 58 370 0.35 Sandy loam 0.9 0.5 0.3 54 650 0.49 Sand 0.7 0.3 0.3 46 700 1.22 Sandy clay loam 1.7 0.5 1.1 29 760 0.33/1.45 Clay loam 2.4 0.7 1.6 29 390 0.35 Loam 2.3 0.6 1.6 27 150 1.05 Sandy loam 2.1 0.5 1.5 24 225 2.43 Sandy clay loam 1.9 0.5 1.4 24 400 0.5 Clay loam 5.4 0.7 4.6 13 830 1.71 Sandy loam 4.0 0.5 3.4 13 150 0.49 Average 1.89 1.26 40 459

*Assumes 0.1 ML/ha tailwater losses RAW refers to Readily Available Water Irrigation monitoring at eight sites (4 plant, 4 1st ratoon) monitored throughout the 1996/97 season revealed irrigation application efficiencies on plant cane very similar to that of the previous year. Another point to emerge was the apparent improved irrigation efficiency on ratoon cane compared to plant crops (see table 4.3 below). This effect likely resulted from the lessened cultivation as well as increased compaction/consolidation from water and machinery associated with ratoon crops, leading to a reduction in deep drainage losses. Table 4.3 Irrigation application efficiencies on Delta plant and ratoon cane, 1996/97 (from Holden & Mallon, 1997). Soil type Average volume

(ML/ha/irrigationRAW (ML/ha) Deep drainage

losses (ML/ha/irrigation*


Plant Clay loam 1.2 0.7 0.7 58 Sandy loam 0.7 0.4 0.3 57 Clay loam 1.6 0.7 0.9 44 Sandy loam 3.2 0.4 2.8 13 Average 1.66 1.18 43 First Ratoon Sandy loam 0.6 0.4 0.2 70 Clay loam 1.2 0.7 0.5 61 Clay loam 1.5 0.7 0.8 48 Sandy loam 0.9 0.4 0.5 47 Average 1.05 0.5 57 *Assumes 0.1 ML/ha tailwater losses



RAW refers to Readily Available Water In a similar finding to that of Raine & Shannon (1996), Holden & Mallon (1997) found the main pathway of irrigation loss on these Burdekin Delta farms was deep percolation past the root zone. Tailwater losses were usually minimal if not non-existent and the 0.1 ML/ha assumed tailwater loss was considered an overestimate of actual tailwater losses. Results of more recent CSIRO/BSES studies on the water balance components and economic analysis of some Delta farm irrigation practices parallel many of these previous results with deep drainage again highlighted as the dominant water use inefficiency on many Delta farms (Charlesworth et al., 2002). High water applications were correlated with soil permeability, but management factors and field configuration were also key variables. In this case WUE from the perspective of cane tonnage per megalitre of water rather than irrigation application efficiency % was used to define efficiency outcomes. WUE ranged significantly from 3 t/ML up to 14t/ML, demonstrating again the efficiency of water application practices can be quite variable, even within the same cane growing district (table 4.4). . Table 4.4. Summary of first year crop yields and water balance components (adapted from Charlesworth et al., 2002).

Farm no. 1 2 3 4 5 6 7 8 Size (ha) 8.3 10.2 5.5 8.9 12 14.7 7.8 5.5 Soil Type Tenosol Vertosol Kandosol Vertosol Kandosol Kandosol Hydrosol Tenosol Yield (t/ha) 141 130 88 94 100 84 144 Evapo-transpiration

13 13 9 10 10 9 16

Rain 8 8 8 8 8 8 8 Irrigation 18 10 11 11 18 24 24 Run-off 3 5 2 2 3 3 1 Drainage 10 1 8 7 13 20 15 Deep Drainage (% irrigation)

40 4 44 38 50 62 49

Upflow 0 1 0 0 0 0 0 WUE (t/ML) 8 14 8

Plant 2001

9 6 3 6 Benchmarking Irrigation in the Lower Burdekin Cane Growing District. The relatively broad scale irrigation monitoring and WUE trials undertaken by Holden & Mallon (1997) led them to establish the following furrow irrigation benchmarks as achievable WUE targets for Burdekin cane growers: BRIA 1.0 ML/ha/irrigation 10-12 ML/ha/year 10 tonnes cane/ ML total water 115 tonnes cane/ha 17-18 tonnes sugar/ha



Delta 1.5 – 2.0 ML/ha/irrigation (plant crops) 1.0 – 1.5 ML/ha/irrigation (ratoon crops) 20 – 35 ML/ha/year 4 – 5 tonnes cane/ML total water 125 tonnes cane/ha 18-19 tonnes sugar/ha (average 4-5 ML effective rainfall per year)

Holden & Mallon (1997) noted the generally good efficiencies achieved in the BRIA. It was suggested growers with tailwater recycling capacity should be able to achieve 12 tonnes of cane / ML of water and irrigation efficiencies of more than 80%, and these figures should regarded as the irrigation efficiency target for BRIA growers. Average irrigation efficiency of 60% and yearly water usage not in excess of 16 ML/ha/year were the figures regarded as achievable for Delta growers. Environmental pressures associated with low irrigation efficiency. The interactions between irrigation, water movement and agro-chemicals have already been highlighted in previous nutrient and pesticide sections of this review. Additional issues associated with water use efficiency, particularly water table management represent some of the foremost environmental sustainability issues from a local perspective. Rapidly rising water tables and associated waterlogging and salinity threats are of particular relevance to the Burdekin-Haughton Water Supply where irrigation is dominated by surface water use (ABARE, 2003). In the delta area of the Lower Burdekin, where irrigation is dominated by ground water use, the main management issue is maintenance of a strong freshwater flux to maintain the freshwater/seawater interface and thereby minimise potential for saltwater encroachment. The results of ground water monitoring in the Burdekin River Irrigation Area indicate that losses from surfacewater supplies (originally though to be the extensive farm irrigation in the area) has resulted in saline ground water being less than 2 metres from the surface in some areas, resulting in a loss of agricultural production in these areas. Ground water salinity in the Burdekin River Irrigation Area has increased from less than 1000 uS/cm in the early 1970s to around 2500 uS/cm in 2000. Over the same period, the average water level in the aquifer has risen from around 15 metres below the surface to around 5 metres below the surface (ABARE, 2003). Whether these water table rises are actually solely attributable to on-farm irrigation inefficiency is currently a contentious local issue. Possible inefficiencies and losses in irrigation area surfacewater distribution networks (channels etc.) have also been suggested as a plausible contributing factor, but not yet assessed in a rigorous manner. Whatever the underlying mechanism is this is a management issue requiring urgent attention before problems become irreversible. Current Approaches and Research relevant to improved irrigation efficiency in the Lower Burdekin region.



The CANGEGROWERS Code of Practice contains a suite of fairly generalized recommendations regarding management strategies to improve furrow irrigation efficiency. This includes practices such as customizing furrow profile, reduced tillage and installation of on-farm tailwater recycling capacity (CANEGROWERS, 1998). At this point in time, most significant adoption of new WUE practices have been limited primarily to these simplest, cheapest and most easily adopted measures (i.e. reduced cultivation and changed furrow profile).While many of these suggested measures have particular relevance to the lower Burdekin region, with a number providing substantial demonstrated improvements to WUE (described below), an array of additional measures and considerations have since emerged which may have specific relevance to certain local irrigation situations. As well as altered furrow profile, reduced cultivation, and tailwater recycling, substantial improvements in WUE are possible for furrow irrigation systems through appropriate selection and implementation of modifications such as furrow length, furrow flow volumes, surge techniques. Sutherland (2002) noted that ‘no particular irrigation system is inherently more or less efficient than others’, with the qualification that well managed furrow, trickle or overhead irrigations can achieve application efficiencies of over 90%. It is perhaps debatable that this 90% level of efficiency is pragmatically achievable or as importantly economically feasible in reality for all growers across the Burdekin farming sector. The highly permeable soils that characterize much of the Burdekin Delta for example do not lend themselves to significant relative furrow irrigation efficiency. While much of the substantial variability in furrow irrigation WUE between sites and throughout the season is almost invariably attributable to soil type, irrigation design (at farm scale) and specific management practices also make a substantial contribution in most cases. Therefore, where an irrigation system already exists, there is usually potential to develop improvements in WUE with massive capital expenditure, particularly when dealing with furrow irrigation. Improving irrigation efficiency in the lower Burdekin has been the focus of a number of studies of varying spatial and temporal extent (see Raine & Bakker, 1996a, Holden et al., 1998a, Raine & Shannon, 2000, Klok et al., 2003). Following is what could be regarded as an overview for BMP for furrow irrigation in the Burdekin district. These recommendations are based upon specific Burdekin trial results where possible, but also includes recommendations or experiences drawn from other cane growing districts where relevant. As the examples outlined below illustrate, considerable scope exists for adoption of new (or at least improved) furrow irrigation practices that can drastically benefit WUE in many situations. Not all are necessarily compatible or practical in every situation, some however are almost certainly useful avenues to improve WUE under less than ideally designed irrigation schemes. Soil Types and Soil Infiltration Characteristics. While issues such as furrow profile and slope can play significant roles, soil infiltration rates and accumulated infiltration relative to inflow is the overriding issue to be considered in surface irrigation enterprises (Sutherland, 2002). Soil type and clay content



are the soil characteristics which predominantly influence infiltration rates. For this reason, consideration of uniform soils types or groupings of soil types based on comparable infiltration rates should be a fundamental requirement at the preliminary stages of farm design and irrigation planning. If achievable, such measures will greatly enhance irrigation manageability in terms of application efficiency as well as associated reductions in operating and labour requirements. In paddocks with variable soil types, water will infiltrate into different soils at different rates, with the arrival of the wetting front at paddocks’ end consequently staggered. This in turn makes efficient management and run-off minimization difficult. It may not always be a feasible option under many existing farm structures to design an irrigation system that pragmatically groups soils types in a field depending upon comparable soil infiltration characteristics. In some cases, variability in soil types down furrows can actually be turned to the irrigator’s benefit in enhancing irrigation application efficiency. The opportunity time for water infiltration in a furrow irrigation operation naturally tends to be longer at the top end of paddocks where water has been on the soils surface for a longer period of time. This can be used to advantage in situation where fields have less permeable soils in upper sections of furrows and more permeable soils in the lower sections of the field (Sutherland, 2002). For example, arranging blocks so that sodic soils are at the top of blocks promotes longer duration of wetting low infiltration soils throughout an irrigation. Similarly, if sodic (or low infiltration) soils are near the top of a block, short irrigations can be applied more effectively to the top of the block following the main irrigation (Nelson, 2001). In situations where variability in soil type may pose a challenge in managing water application efficiency, strategies such as compound slopes may offer avenues to mitigate some of the problems posed by variable soil infiltration rates (discussed in later sections). Sodic Soils. Sodic soils are a particular soil type/soil infiltration issue of considerable local relevance to the lower Burdekin cane district. Sodic soils are a soil type with low structural stability when wet, with a consequent array of production, water application efficiency and environmental considerations. Sodicity is defined as the proportion of cation exchange capacity of the soil that is taken up by sodium, generally expressed as the exchangeable sodium percentage (ESP). Soils with an ESP > 6 in the A or B horizons are typically defined as sodic and those with an ESP > 15 are generally regarded as highly sodic. Salinity and sodicity normally occur together due to the fact the dominant salt in nature is sodium chloride, saline soils are also generally sodic. In terms of distribution of sodic soils, the soils of the Burdekin Delta are generally fertile, permeable sands, loams and clays, with minimum occurrence of sodic soils. Where sodic soils do occur in the Delta, they tend to occur in low-lying areas with marine influences, and under irrigation with saline sodic waters (Nelson, 2001). The BRIA is comprised primarily of the alluvial plains of the Burdekin and Haughton rivers and local creeks and soil sodicity is the primary production limitation to growth in the area (Nelson, 2001). Of the soil surveyed BRIA region, 72% is composed of soils with a sodic B horizon (ESP>6).



Much of the research effort focused upon remedying soil sodicity effects have focused almost solely upon the cane yield and productivity improvement aspects of the issue (see McMahon et al., 1996, Ham et al., 1997). There are however many additional environmental benefits to be gained from effective management of sodic soil problems. When sodic soils ‘wet up’, soil structure becomes unstable, large soil pore collapse and small pores clog up. Water infiltration is poor and irrigation water may only penetrate 100mm into the soil. Consequently, under rainfall or irrigation, run-off is high and irrigation efficiency is low, even under low inflow conditions. The clay that disperses in sodic soils under cultivation is usually highly prone to movement in run-off. Soil permeability to air also decreases, causing anaerobic conditions. Applied water either runs off (with minimal application efficiency) or stands for long periods causing water-logging problems for crops. Soil properties from the wet stage to various stages of the drying cycle makes cultivation or any vehicular traffic difficult. Without delving too deeply into the chemistry of sodicity, Nelson (2001) outlines three main categories for management of sodic soils, stating most real-life management will involve a combination of all three options:

1. Avoid cropping sodic soils. 2. Reducing sodicity and preventing increase of problem 3. Managing soil despite a sodicity problem.

Given that sodic soils are expensive to develop and provide lower production returns, the most effective methods to manage sodic soils is to avoid bringing them into production entirely. Similarly when leveling paddocks, exposure of sodic soils should be avoided and sodic layers should be buried as deeply as possible (a non-sodic top layers of at least 300mm is preferable). Therefore non-sodic soil should be used as top dressing and sodic soils placed in non-cropped areas such as headlands and roads. Non-sodic layers may be present below sodic soils and possibly excavated and used when developing recycle pits. Current research has demonstrated different irrigation methods will not significantly improve production on sodic soils, although there were some consistent yield differences between irrigation approaches (Ham et al., 1997). The use of soil ameliorants such as gypsum or lime has however been shown to be an effective management strategy for improving sodic soils. These ameliorants provide a soluble source of calcium which replaces the exchangeable sodium in soils which is then leached down the profile. Gypsum was most effective on alkaline sodic soils, while both gypsum and lime were similarly effective on acid sodic soils (Ham et al., 1997). Fixing soil constraints to crop growth prior to planting is therefore a core component of ensuring subsequent fertilizer uptake and use by plants, as well as irrigation management strategies are optimized. Some of the specific minor aspects of irrigation management to more effectively manage sodic soils will be touched upon in subsequent sections.



Irrigation Water Quality. The water quality characteristics of available water supplies and subsequent effects on irrigation application efficiency are a sometimes vastly under-appreciated issue in effective irrigation management. The chemical constituency of irrigation water and subsequent reactions with different soil types can have a multitude of effects on long-term farming sustainability. As Ham & Nelson (1998) note, through time soils take on the chemical character of the irrigation water used on them. Utilisation of irrigation water with a quality inappropriate to soil type properties can cause an array of productivity and environmental problems such as sodicity, salt accumulation in root zones, minimal water use efficiency or excessive deep drainage. The decision as to whether water of a particular quality is suitable for use on a particular soil type should be based on what state the soil will be when it eventually comes into equilibrium with irrigation water. Water quality and the amount of leaching are the two most important factors to consider in this process. Ham & Nelson cite four main components to water quality from an irrigation perspective:

• Salinity:- total quantity of dissolved salts in water (dS/m) • Sodicity hazard:- composed of 1). sodium absorption ration (SAR), a prediction

of how water will affect the sodicity of the soil. Through time the exchangeable sodium percentage (ESP) or irrigation water will approximate the SAR of applied irrigation water; and 2). Residual or free alkali (RA): RA is another water quality parameter influencing soil ESP. RA signifies the amount of sodium carbonate and sodium bicarbonate in water. These two salts remove calcium and magnesium from soil and replace them with sodium, thereby increasing soil ESP.

• Toxic ions: excessive amounts of chloride, sodium, boron, lithium and other elements potentially toxic to crops (rarely encountered in sugarcane)

• Potential clogging or corrosive materials such as iron, clay or calcium carbonate can cause issues with trickle or overhead irrigation systems. Acidic water with a high level of chloride ions are the most corrosive with turbine pumps prime candidate for corrosion problems.

Ham & Nelson (1998) also outline seven types of irrigation water quality based primarily on electrical conductivity (salinity) and residual alkali content (see Table 4.5 below, adapted from Ham & Nelson, 1998).

Table 4.5: Summary of water quality types. Water Description Quality Corrective measures Type 1 EC = 0 – 0.6 dS/m RA = 0 – 0.2 meq/L

Low salinity water – when some light texture soils are irrigated with type 1 water, soil particles disperse, forming a slurry which impedes water penetration.

Poor on light soils Irrigation water can be mixed or treated with gypsum. Soil may also be treated with gypsum or burnt lime

Type 2 EC = 0 – 0.6 dS/m RA = 0.2 – 2.4 meq/L

Low salinity water with residual alkali – residual alkali in this water type aggravates water penetration problems on light texture soils.

Poor on light soils Type 1 and Type 2 waters are similar in their effect on water penetration and require similar remedial measures



Type 3 EC = 0.6 – 1.5 dS/m RA = 0 – 0.6 meq/L

Average salinity waters – can be used on all soil types as they do not cause water penetration problems or result in excessive build up of soluble salts in leaching occurs

Good Nil

Type 4 EC = 0.6 – 1.5 dS/m RA = 0 - 0.24 meq/L

Average salinity water with residual alkali – the moderate amount of soluble salt in this water type encourages binding of soil particles when wet and allowing adequate water penetration. If the RA content exceeds 0.6 meq/L, soil dispersion may occur, particularly if large amounts of calcium have been removed from soil (poor water penetration can then result)

Good – fair Light soils may require similar treatment to Type 1 and Type 2 waters.


High salinity water – use of type 5 water on poorly draining soils will cause salt build-up in root zone (most common on heavy soils or soils with clay subsoil). On clay soils, water with an EC> 1.5 dS/m should not be used, although saltier waters can be used on lighter soil.

Fair – poor Irrigation management is vital. Slow, heavy irrigation to leach salt from crop root zone is required to prevent salt accumulation in soil. Deep ripping soil may improve leaching to below root zone.


Very high salinity water – can be used only on free draining sandy soil to avoid serious salt accumulation in root zone. Water with in EC > 3.0 dS/m should on be used in extreme circumstances.

Very poor Frequent, heavy irrigations to leach excess salt through root zone. If salt build up is evident, allow soil to completely dry out and concentrate salt in soil solution. Soils should be wet to at least 1m depth.

Type 7 EC > 3.2 dS/m or RA > 2.4 meq/L

Waters unsuitable for irrigation Not suitable for use

Not suitable for use

Readily available irrigation water quality analysis (BSES or other private testing firms) can easily provide the information required to assess the suitability of a particular water source for crop irrigation. Improving irrigation water quality. The mixing of water from different sources with differing water quality characteristics is frequently used by Burdekin farmers to alleviate some irrigation water quality problems. Mixing low salinity water (open channel water) from a moderately saline groundwater bore is a common approach, locally referred to as ‘shandying’. Mixing of recycled tailwater with low salinity channel water may also offer an avenue to improve applied irrigation water quality. Similarly, modifying the quality of irrigation water for soils with identified water penetration problems via addition of an ameliorant such as gypsum is another avenue for farmers (see Ham, 1986). Addition of a salt such as gypsum can improve the quality of type 1 and type 2 water for irrigation use. Likewise gypsum can



improve type 4 waters by removing the residual alkali (although increasing the salinity of type 4 water should be avoided). Despite the prevalence of the practice of modifying irrigation water quality across much of the district, established guidelines for management are absent. How some of the traditionally recommended irrigation strategies for high salinity waters (i.e. frequent, heavy irrigations to leach salts through root zone etc.) mesh with current sustainability issues such as WUE, and minimization of off-site nutrient and pesticide movement is debatable. This is an information shortfall requiring some attention. Furrow Slope. Laser leveling consistent furrow slopes across a paddock is essential to maximizing uniformity of water application, water advance and therefore run-off volumes. Any significant degree of variation in slopes between furrows makes efficient management of a run-off event almost impossible. Differences in soil infiltration rates and wetting front advancement on different furrow grades makes the desired uniform advancement of the irrigation wetting front down furrows exceedingly difficult to accomplish. Recognition of soil type and water infiltration characteristics is another fundamental consideration for furrow slope leveling on a block. Water intake and subsequent application efficiency can be reduced in blocks with too much slope. When soil properties result in water penetration problems, block slope should not exceed 0.125% or 1:800 (Nelson, 2001). An un-replicated trial in Ham et al. (1997) demonstrated that reducing a natural slope from 0.49% to 0.07% increased cane yield by 24% over a crop cycle (see table 4.6 below). While irrigation data was not available, the improved yield would seem strongly indicative of improved RAW for plants and hence improved water application efficiency.? In general, where water penetration is poor, slope should not exceed 0.125% (Ham & Nelson, 1998). Table 4.6. Yield (t/ha) on Q117 on different slopes at Gaynor. Crop 0.07% slope 0.49% slope Plant First ratoon Second ratoon Third ratoon

106 64 65 34

93 50 52 21

Continual maintenance of furrow slope is a desirable outcome through multiple crop cycles. Variability in advance times in a precision leveled field can be as little as 3-5%, with later ratoons or wet harvested fields can exhibit variability of as much as 25-30% in advance times (Sutherland, 2002), an outcome hindering effective irrigation management in many circumstances. Therefore laser leveling or brushing between crop cycles was recommended practice to maintain a maximum degree of slope uniformity across a field through time. Recent farm system shifts toward controlled traffic layouts with permanent beds however means this requirement may be extended to every 2-3 crop cycles. In situation where uniform soil types exist throughout a field, the use of compound slopes



along furrows can improve water distribution uniformity. Here it may be advantageous to have steep slopes on the upper 1/3 of the paddock (thereby increasing advance rates) with progressively lower grades on the subsequent 2/3 of the furrow length. This slows advance rates in lower sections of the furrow, increasing opportunity time for water infiltration. Similarly, variations in soil types along sections of furrows can be addressed in some cases with judicious use of compound slopes down the length of the furrow. Increasing slopes across a field through sections of more permeable soil while reducing grades over less permeable soil types can help manage opportunity for water infiltration across the entire field and consequently maximize efficiency? Furrow Profile. Furrow shape/geometry has been repeatedly demonstrated as one of the more significant management variables in determining overall water infiltration and irrigation efficiency in the Burdekin region. The traditional furrow cross section commonly employed within the Burdekin sugar industry was a broad based ‘U’ shape formed using hill up boards. ‘V’-shaped furrows and higher hills (formed by simply tilting the standard hill up boards forward 10-15%) were found to substantially reduce irrigation advance periods and increase irrigation efficiency in comparison to the standard U shaped furrow in many circumstances (Raine & Bakker, 1996a, table 4.7). Soil surface cores taken prior to irrigations revealed this increased efficiency was closely associated with reduced water infiltration into the soil due to beneficial surface compaction in V shaped furrows. Raine & Bakker (1996a) documented 57-94% reductions in saturated hydraulic conductivity in V shaped furrows compared to the conventional furrow shape (table 4.8 below). Table 4.7. Effect of furrow shape on irrigation efficiency for an alluvial soil (from Raine & Bakker, 1996a).

Furrow shape

Application rates (L/s)

Furrow length (m)

Irrigation Time (hrs)

Water Applied (ML/ha)

Application efficiencyA

(%) Broad based

‘U’ Narrow

based ‘V’

1.7

1.7

470

470

13 8

1.09

0.67

46

75

A Average soil water deficit = 0.6 ML/ha Table 4.8. Saturated hydraulic conductivity in “V” and “U” shaped furrows (From Raine & Bakker, 1996a).

Saturated hydraulic conductivity (mm/hr). Irrigation Date “U” shaped furrows “V” shaped furrows

12/4/95 1.4 0.6 09/5/95 4.7 0.3 31/5/95 5.8 2.3



The resultant lower water infiltration rates in V shaped furrows sees substantially faster irrigation advance rates with reductions in irrigation periods of up to 40% a recurrent theme emerging from comparative trials (Raine & Bakker, 1996a, Raine & Shannon, 1996). This outcome has implications for not only water application efficiency, but also the time and scheduling aspects of irrigation cycles as well as reductions in pumping and supply system requirements. Raine & Shannon (1996) similarly documented substantial increases in application efficiency at Maidavale and Colevale farms in comparative trials of conventional and V shaped furrows (table 4.9 below). Here the modified furrow profile required applications of less than 40% of the water volume applied to conventional furrows, an effect that continued for subsequent irrigations throughout the season. This study also highlighted time savings in irrigation period (up to 40%), reductions in pumping and supply system requirements and enhanced capacity for irrigation scheduling. All of these advantages were accrued without any significant difference in production between conventional and modified systems. Table 4.9 Effect of changing furrow shape on water usage in post hill-up irrigation, Colevale and Maidavale sites (Raine & Shannon, 1996).

Irrigation water applied (ML/ha) Colevale Maidavale

Furrow Shape

Average Range Average Range Normal ‘U’ 8.3 8.1-8.4 7.3 4.6-12.9

Modified ‘V’ 2.9 2.8-3.4 2.9 2.8-6.5

Just as soils with relatively high water infiltration characteristics can be managed with soil compaction and furrow shape techniques, soils with low infiltration capacity can alternatively be managed with furrow profile manipulation or the use of ameliorants such as gypsum or lime to improve infiltration (depending on soil chemistry). The traditional broad-based ‘u’ shaped furrow should only be used on low infiltration soils or soils with soakage problems (surface sealing or sodic soils). Here, a broader, flat interspace with small hills maximizes water intake While still being evaluated, the use of variable furrow shapes along the row length, to address either significant slope or soil type changes down the row is another avenue showing some promise in improving WUE (Tilley & Chapman, 1999). Furrow Length. Furrow length is one of the most variable irrigation parameters to be found across cane farms in the lower Burdekin. Efforts to minimize capital investment in irrigation infrastructure have seen many farms in the recently developed BRIA with furrow lengths in excess of 1500m. Furrow lengths on some Delta farms may alternatively be only 100-200m in length (?). In general, longer drills mean reduced water application efficiency, however the effect is much more dramatic on high infiltration soils such as sands or loams which lose considerable water to deep drainage. Raine & Bakker (1996a) identified reductions in irrigation efficiency with increasing furrow length across all soil types at a



range of sites throughout the Burdekin cane growing district (see table 4.11). This relationship between increased furrow length and decreased irrigation efficiency was much more pronounced on high infiltration alluvial and non-sodic duplex soils. On these soil types increasing furrow length from 300 to 700m and 100 to 500m decreased application efficiency from 73 to 42% and 57 to 34% respectively. In these cases the majority of excess irrigation water was lost as deep drainage with minimal tailwater run-off. In such cases where furrows are overly long for the particular soil type, the top section of blocks tends to get over-watered with little water reaching the end of the furrow. Table 4.11 Furrow irrigation efficiencies with changes in furrow length for some Burdekin soils (from Raine & Bakker, 1996a).

Soil

Application rate (L/s)

Furrow Length

(m)

Irrigation time (hrs)

Water Applied (ML/ha)

Application efficiency without

recycling A

Application efficiency

with recycling A

Alluvial 2.8 300 500 700

3 7 15

0.82 0.94 1.44

73 64 42

91 70 43

Non-sodic duplex

2.5 100 300 500

2 8 18

1.23 1.56 2.09

57 45 34

62 47 35

Cracking clay

2.7 400 800 1200

7 15 23

1.19 1.22 1.23

76 74 73

91 87 85

A Average soil water deficit: alluvial = 0.6ML/ha; non-sodic duplex = 0.7ML/ha; cracking clay = 0.9ML/ha In contrast, increasing furrow lengths from 400 to 1200m on cracking clay soils produced relatively minor decreases in overall application efficiency (76 down to 73%). Even on the longer furrows, deep drainage losses were minimal with the majority of excess water lost in this case as tailwater run-off. These results highlight the substantial effect soil infiltration properties play on irrigation efficiency. Altering furrow lengths on many established farms would likely necessitate major re-organisation of farm design and irrigation infrastructure layout. Holden & Mallon (1997) noted considerable reluctance on the part of growers to adopt costly redesign measures such as shortening furrow length (or re-levelling increased furrow slopes etc.) as considerable expenditure had been committed to leveling fields or joining paddocks for ease of management. For just these reasons Raine & Shannon (1996) outlined related, but additionally cost benefit focused trials of alternative management strategies conducted on commercial, furrow irrigated farms in the Burdekin delta region (i.e. Colevale and Maidavale). These sites were all located on the highly permeable soils that typify ca. 8000ha of the Delta area. This study identified significant decreases in irrigation requirements (from 1.78 down to 1.03ML/ha/irrigation in the first irrigation after ‘hill-



up’) when furrow length was decreased from 600m to 300m. Cost benefit analyses factoring in additional water infrastructure, production losses associated with additional headland area and increased WUE identified significant positive returns with reducing furrow length from 600 to 300m. This study did stress that while substantial direct financial benefits can be gained by changing furrow length and profile, further work to fully cost alternatives under different farm conditions and better integrating farm labour, tillage and harvesting requirement into the process was warranted. Irrigation Cut-off. Raine & Bakker (1996b) noted a tendency for many farmers in the Burdekin area to continue irrigation after water has reached the end of furrows to ensure the root zone soil is completely recharged. There is no real measure of the period of time required to recharge soil water deficit, with irrigation controllers or timers rarely used and the irrigation is often continued until such time as it is convenient to be manually switched off. The lack of a measurement of time necessary to recharge soil water deficit under commercial conditions increases the potential for significant amounts of water to be lost as tailwater run-off. In a SIRMOD assessment of one specific irrigation on an alluvial soil, Raine & Bakker (1996b) demonstrated savings of up to 20% of applied water if irrigations are ceased once soil water deficit has been recharged (see table 4.12). In this case, ceasing irrigation at the appropriate time reduced not only tailwater run-off, but also produced a significant reduction in water volumes lost as deep drainage. It was suggested that more than 10% of water applied to alluvial soils would be saved by more accurate timing of irrigation cut-off. Irrigation cut-off times have similarly been identified in other furrow irrigated districts in Queensland as important potential contributors to improved water application efficiency (see Linedale et al., 2001). Table 4.12 Typical Volume balance for a 470m furrow on an alluvial soil irrigated at 3.4 L/s/furrow (from Raine & Bakker, 1996b) Treament Application

time (hours)

Applied volume (ML/ha)

Soil Water Deficit (ML/ha)

Deep Drainage (ML/ha)

Tailwater run-off (ML/ha)

Application efficiency (%)

Actual 8.5 1.44 0.6 0.56 0.28 42 Optimum cut-off

6.7 1.13 0.6 0.43 0.1 53

Furrow Inflow rates (High Flow and Surge Irrigation). The efficiency implications of furrow inflow (water application rates in litres/sec/furrow) rates are from all appearances a somewhat overlooked component of irrigation management in the Burdekin. Very few research projects in the Burdekin have dealt with the issue in any explicit or comprehensive way. The role furrow inflow rate plays on irrigation efficiency appears to be a function of soil type. Raine & Bakker (1996a) demonstrated some interesting (and perhaps in some ways counter-intuitive) results from trials varying furrow inflow rates over different soil types (see table 4.13).



Table 4.13. Effect of water application rates on furrow irrigation efficiency (from Raine & Bakker, 1996a). Soil type Furrow length

(m) Application rate (L/s/furrow)

Volume Applied (ML/ha)

Application EfficiencyA (%)

Cracking-clay 1647 1.4 2.8

1.38 1.33

65 68

Alluvial 470 1.7 2.8

0.92 1.13

65 53

A Average soil water deficit: Alluvial = 0.6 ML/ha, cracking clay = 0.9 ML/ha Changing inflow rates on cracking clay soils for example produced no discernible difference of volume of water applied. This is a not unexpected result as the majority of water infiltration these soil types occurs by filling crack volume, therefore a change in water applications rates would not be expected to alter crack volume, infiltration or corresponding irrigation efficiency. Reducing the furrow inflow rates on the higher infiltration alluvial soils resulted (perhaps surprisingly) in less infiltration and increased application efficiency. These soils have no appreciable cracks and water infiltration is a function of the soil’s saturated hydraulic conductivity. The improved efficiency evident in this example was postulated as a function of reduced wetted perimeter and surface area available for infiltration under the lower furrow inflow rates. Raine & Bakker (1996a) did highlight that further reductions in application rate may not produce comparable increases in application efficiency. At very low application rates, the infiltration intake of a furrow on alluvial soil may actually exceed application rate, resulting in irrigations which may fail to reach the end of furrows and produced reductions in overall efficiency. The identification of optimal furrow inflow rates across different soils types and system designs is one are requiring attention. Unfortunately the range of inflow rates assessed in the study of Raine & Bakker (1996a) was far from comprehensive. As the variability of water application rates expressed in examples across this review illustrate, furrow inflow rates on Burdekin farms can vary from 0.6 l/s up to 4.6 l/s (in some following surge irrigation examples). An SRDC funded monitoring project in Bundaberg identified variable and/or inappropriate furrow inflow rates as one of the primary operational contributors to the low water use efficiencies observed on a significant proportion of furrow irrigated farms in that district (Linedale, 2001, Linedale et al., 2001). More appropriately matching inflow rates to particular soil types (as well as irrigation cut-off times) produced some marked improvements in WUE on a number of Bundaberg farms. A series of guidelines for inflow rate ranges appropriate to different soil types as well as different stages of the season were also developed. High flow irrigation is for example a technique where relatively high inflow rates are used to reduce opportunity time at the head end of paddocks and reduce excessive



accumulated infiltration (end fill). Linedale et al., (2001) highlighted some furrow irrigation situations where increasing inflow rates of 3.4 L/s to 6 L/sec decreased advance time (200 down to 95 minutes) and increased overall irrigation efficiency (92% up to 99%). A potential major shortcoming of high flow water application is the heightened potential for a major run-off event with higher inflow rates. The advent of new pumping technologies such as variable flow controls on pumps and cutback strategies has alleviated these concerns to a certain extent and allowed some recent forays into high-flow irrigation practices. At a pre-determined point in the advance phase or a high flow irrigation application (say 75%), pump flow is cut back to reduce inflow and potential for a significant tailwater run-off event. Improving Inflow Rate Efficiency. One of the major drawbacks of furrow irrigation systems is they confer minimal flexibility in varying application volumes into individual furrows. Systems employing siphons from head ditches are particularly difficult to manage in this regard. Inflow rates to furrows can vary by as much as 30% if the discharge ends of siphons are not placed at a consistent level relative to the head of water in the head ditch. Variations in siphon height by 100mm can alter inflow rates by as much as 1L/sec (Sutherland, 2002). While not directly relevant to the typical Burdekin farming situation, ensuring consistency of cup height in fluming as well as level fluming layout is similarly important to ensuring irrigation efficiency. Of direct relevance to Burdekin farms Linedale et al. (2001) identified a major culprit in high inflow variability in Bundaberg were gross irregularities in the aperture size of hand cut cups inserted into roll-out fluming. Sutherland (2002) also advocated use of pre-fabricated controller cups with a moulded orifice as preferable to blank cups with the orifice cut manually by hand ‘on the back of the ute using a knife’. Differences in hole size between cups has the potential to introduce substantial variability in inflow rates along different furrows. Linedale et al. (2001) highlighted the degree of inflow variability associated with hand-cut versus moulded cups in Table 4.14. Tests over the five sites indicated inflow variability was reduced by 69% using moulded cups, resulting in much improved control of inflow rates and water advance times (a fundaemental component of application efficiency). Table 4.14. Inflow variability: hand cut versus moulded cups (from Linedale et al., 2001). Site Average inflow

(L/s) Range of deviation to average inflow (%)

Hand-cut cups Moulded cups 1 2.28

2.20 -21 to +27 -7 to +9

2 0.59 0.54

-32 to +69 -7 to +11

3 1.24 1.54

-11 to +5 -3 to +4

4 0.9 -22 to +11 -11 to +6



1.46 5 1.35

1.69 -15 to +11 -8 to +4

Average range % 45 14 The use of a range of colour coded cups of different size ranged apertures can also be useful in counteracting pressure differentials and physical limitations that may be present on some farms such as undulating headlands or progressively shortening rows. Similarly, laser leveling the headland to minimize variations in water head along the fluming length can aid in producing uniform inflow rates. Variations in furrow inflow play a substantial role in the uniformity of water advancement along furrows, and hence the final volume of run-off likely over the course of a complete irrigation. Surge (Pulse) Irrigation. Surge (pulse) irrigation is a more recent, advanced irrigation system involving water being intermittently pulsed along two different sets of furrows at increasing frequencies using an automatic butterfly or ball valve that fits into lay-flat fluming (McGuire et al., 1998). While surge irrigation has seen considerable use in other industries such as cotton, it has not yet been embraced to any great degree in the Queensland sugar industry. Past WUE studies in the Burdekin have repeatedly noted that much of the lack of interest from growers in surge irrigation may have been due in large part to the initially high price (ca. $3000) of surge valve units (Holden & Mallon, 1997, Tilley & Chapman, 1999). Recent technological improvements producing better and cheaper units with additional timer and programmable capabilities may now make the system more appealing to growers. The major efficiency benefits of the surge irrigation process are conferred via soil consolidation and fine sediment settling at the end of each irrigation pulse. Fine sediments are held in place on the soils surface by the negative pressure of water infiltration through the profile. This in turn reduces the infiltration rate and increases irrigation advance during the next irrigation pulse over the previously wetted soil. Surge irrigation may alleviate deep drainage losses by reducing water intake at the top end of the field, a common inefficiency for furrow irrigation. Careful managing, monitoring and irrigation scheduling are required for surge irrigation techniques. However, advantages such as reduced pumping costs, deep drainage losses and reduced fertilizer application rates can be obtained (Sutherland, 2002). Hardie et al. (2000) documents a 1997/98 pilot surge irrigation trial on a freely draining Delta soil in the Burdekin. The low sloping field (0.03%) and 400 metre long furrows had previously proven difficult to irrigate effectively. Five irrigations were monitored on the trial between February and June 1998 (Table 4.15 below). The surge irrigation system used significantly (p < 0.05) less water (1.31 ML/ha), at 26% higher irrigation application efficiency than the conventionally continuous volume irrigation system. The ends of furrow in both treatments were blocked, therefore savings in water use resulted from reduced drainage losses below the root zone. The surge irrigation also conferred



substantial benefits in terms of reduced irrigation time with twice as many rows irrigated under surge in the same time as one irrigation under conventional continuous water application. No differences in cane and sugar yield emerged between the two systems, with WUE of surge finishing 0.9t/ML higher than the continuous system. Table 4.15. Results from surge and continuous furrow flow irrigation of a first ratoon crop on a freely draining soil (from Hardie et al., 2000)

Water applied per irrigation (ML/ha)

Irrigation Application efficiency * (%)

Date

Continuous Surge Continuous Surge

Volume Ratio

12 February 1998 20 March 1998 6 April 1998 27 April 1998 4 June 1998

1.80 2.14 3.06 3.42 2.16

1.16 1.83 1.38 1.16 1.18

41.7 35.1 24.5 21.9 34.7

64.6 41.0 54.4 64.7 63.6

0.64 0.86 0.45 0.34 0.54

Average 2.70 1.39 31.2 57.7 0.57 * Irrigation application efficiency calculated on 0.75 ML/ha of readily available water. Volume ratio is the amount of water infiltrated under surge irrigation divided by the amount of water infiltrated under continuous low irrigation. Hardie et al. (2000) also outlines a replicated 1998/99 trip trial established on a fourth ratoon crop to validate the results documented in table 4.15 above. The trial consisted on two replicated surge irrigation (4 x 15 drill replications) and conventional (3 x 15 drill replications) treatments on another freely draining Delta soil (0.16% slope) which received 10 irrigations through the season (see table 4.16). Table 4.16. Results from second surge trial, comparing continuous and surge irrigation systems on a freely draining soil (from Hardie et al., 2000).

Water applied per irrigation (ML/ha)

Irrigation Application efficiency * (%)

Date

Continuous Surge Continuous Surge

Volume Ratio

25 September 1998 8 December 1998 21 December 1998 11 January 1999 27 January 1999 22 march 1999 11 April 1999 3 May 1999 31 May 1999 5 July 1999

1.03 1.37 0.76 1.07 1.26 0.78 1.04 1.02 1.07 1.20

1.05 0.91 0.63 1.00 0.90 0.66 0.68 0.69 0.96 1.32

73.0 54.9 98.1 70.2 59.5 95.9 72.5 73.2 69.9 62.2

71.3 82.3 118.4 74.8 83.3 114.3 110.3 109.1 77.8 56.7

1.02 0.67 0.83 0.94 0.71 0.84 0.66 0.67 0.90 1.10

Average* 1.06 0.88 70.7 85.1 0.83 Surge Continuous Difference Cane Yield* (t/ha) CCS* Sugar Yield * (t/ha)

145.5 14.95 21.7

147.9 14.85 22.1

2.4 0.1 0.4

* No Significant Difference (p > 0.05 paired T-test) * Irrigation application efficiency calculated on 57 mm of readily available water. Volume ratio is the



amount of water infiltrated under surge irrigation divided by the amount of water infiltrated under continuous flow irrigation. The surge system applied an average of 0.18 ML/ha less water per irrigation at 14 % higher application efficiency than the continuous system (a t-test revealed no significant difference however). No significant differences emerged between treatments in terms of either cane yield or CCS. A recent study by Klok et al.(2003) in the Burdekin Delta region comparing conventional farming techniques with what it generally regarded as constituting irrigation BMP revealed dramatic savings are possible in water application rates with the use of surge irrigation techniques. Three sites, where BMP’s incorporating surge irrigation were contrasted with conventional irrigation systems all produced improvements in WUE (see table 4.17, sites 3-5). Two sites in particular decreased total crop water requirements by 10 and 25 ML/ha in terms of total volumes applied, representing savings of 33% and 44% respectively (even the med-cracking clay, a not particularly free draining soil?). The sandy loam trial at site 4 produced some marginal improvements in WUE but nothing to the extent highlighted at the other two sites. Another very meaningful result of the trials was that surge irrigation BMP increased yields at all sites by 5-10t/ha, with no significant differences in CCS between irrigation strategies. Table 4.17. Outline of site details, treatments, irrigation application, crop yields and CCS for conventional (Conv) and Best Management Practice (BMP) in furrow irrigation (from Klok et al., 2003).

Site Soil Types and Cane Variety

Conventional treatment

BMP treatment

Total irrigation (ML/ha)

Yield (t/ha)

CCS

1 Sandy Loam (Q117)

U furrow, 0.75L/s

V furrow, 0.75 L/s

Conv:21.8 BMP: 21.8

Conv: 129 BMP: 129

Conv: 15.3 BMP: 15.3

2 Med-cracking clay

(Q127)

Cultivation, 2 L/s

No Cultivation,

2 L/s

Conv: 12.8 BMP: 11.4

Conv: 129 BMP: 129

Conv: 15.0 BMP: 15.0

3 Med-cracking clay

(Q183)

V furrow, 2.3L/s

V furrow, surge, 4.6L/s

Conv: 34.0 BMP: 23.7

Conv: 112 BMP: 119

Conv: 15.9 BMP: 15.6

4 Sandy loam (Q117)

V furrow, 2.6L/s, minipan

V furrow, surge, 4L/s,

minipan

Conv: 22.4 BMP: 20.9

Conv: 161 BMP: 171

Conv: 15.3 BMP: 15.3

5 Sand (Q183) V furrow, 1.2 L/s, minipan

V furrow, surge, 2L/s,

minipan

Conv: 70.8 BMP: 46.9

Conv: 147 BMP: 151

Conv: 13.6 BMP: 13.6

6 Sandy Loam (Q183)

U furrow, 2.4L/s

V furrow, 2.4 L/s, minipan

Conv: 19.9 BMP: 18.9

Conv: 162 BMP: 184

Conv: 11.8 BMP: 11.8



This overview of surge trial results reveals that considerable water savings and improved application efficiencies are achievable with surge irrigation. The exact magnitude of these benefits is largely dependant on soil types, slope, furrow length and other cultural practices. While a small degree of uncertainty exists regarding optimization of the benefits of surge irrigation, the rapidly mounting evidence suggests significant improvement in WUE are generally associated with surge irrigation in particular environments. Hardie et al. (2000) strongly advocated the more widespread adoption of surge irrigation on furrow irrigated freely draining soils in particular. Surge irrigation has also yielded some promising results from a yield perspective (rather than WUE specifically) on sodic soils and may also warrant further investigation in that regard (see Ham et al., 1997). In this case trickle and surge irrigation consistently produced higher, although non-significant yields among an array of irrigation techniques on shallow duplex sodic soils in the Burdekin. Alternate Furrow Irrigation. Alternate furrow irrigation (AFI) or ‘skip row’ irrigation is a technique where water is applied to every other furrow rather than the conventional situation of irrigation water being applied to every furrow. AFI has been successfully applied to a variety of cropping systems and climatic environments to conserve water without associated losses in production. The potential benefits of alternate furrow irrigation to the cane industry are not yet fully understood, but water savings may be possible under certain circumstances (McGuire et al. 1998). Bakker et al. (1997) provides the results of some of the limited research conducted into the value of AFI in the lower Burdekin farming environment. AFI reduced yield when crops were irrigated at the same frequency as conventionally irrigated cane (due to the lower soil moisture resulting under AFI). However, if irrigations were applied more frequently in response to crop’s evapo-transporative demand, no yield decreases were produced. In fact across most of the soil types assessed, AFI resulted in less water application and higher crop WUE if irrigations were scheduled appropriately. Specifically, a 41% increase in CWI (4.9 to 6.9t cane/ML) and a corresponding 31% reduction in water volume required emerged under alternate furrow compared to conventional every furrow irrigation techniques. It was therefore suggested AFI could be used in the sugar industry in areas or under conditions of restricted water supply. The feasability of AFI is likely to be very soil dependent. Bakker et al. (1997) noted that on self-mulching clay soils (which exhibit substantial cracking), excessive lateral water movement between furrows was seen to introduce significant inefficiency in AFI and undermine the basic intent of the concept. Although more irrigations were required under AFI, only half the number of furrows are irrigated under every irrigation and twice the area can be watered, two points presenting potential labour and time savings for growers. A greater understanding of this system and its applicability to various soil types and management strategies is still required, along with guidelines for its practical application.



Tailwater Recycling/Retention. As already highlighted in previous sections, efficiency losses on high infiltration soils tends to occur through deep drainage and little benefit is likely to be gained by recycling the small amounts of tailwater leaving these paddocks (see table 4.11, Raine & Bakker, 1996a). While minimizing deep drainage and ‘end fill’ at the top end of paddocks are the primary avenues for improved water use efficiency in permeable soils, management of tailwater run-off is the more relevant focus for relatively impermeable, fine-textured soils. In the recently developed BRIA for example, efforts to minimize capital investment in irrigation infrastructure, together with the slow infiltration rates of the sodic duplex and cracking clay soils typifying the area, furrow lengths of more than 1000m are not uncommon (with some over 1500m in length). In contrast to the deep drainage losses encountered on freely draining soils, the extended contact times and uneven irrigation advance rates across the long, relatively impermeable paddocks of the BRIA generates significant potential for tail-water run-off. Some tailwater loss inefficiencies are also likely attributable to the lifestyle issues associated with intense irrigation management on these sometimes remote farms. This includes practices such as letting water run all night thereby avoiding the social detraction of nocturnal irrigation changes. Introduction of tailwater recycling schemes can produce significant improvements in irrigation application efficiency for fine textured soils, as well as retaining sediments and chemicals on-farm. Substantial increases in efficiency were associated with the addition of tailwater recycling capacity to the cracking clay soils typifying the BRIA (see Table 4.11, Raine & Bakker, 1996a). Recent grower surveys revealed almost half of the cane growers in the lower Burdekin have tailwater dams/retention basins/artificial lagoons on their farms (O’Grady & Christiansen, 2001). The respective area of farm feeding into these systems was however variable with 16% of Burdekin respondents having all blocks draining into some form of tailwater system and a further 32% having some blocks with tailwater retention capacity. Growers may see adoption of tailwater recycling as a cheaper option than substantially changing farm layout and management practices where surface run-off is the primary pathway of irrigation water loss. Sutherland (2002) makes a valid point however that while tailwater recycling can be a useful safety mechanism for capturing excessive irrigation run-off and minimizing off-farm losses of water and entrained materials, a genuinely efficient irrigation system should be the desired outcome. The labour and operating costs of re-lifting run-off water from retention dams for re-application is not as cost-effective as an efficient initial irrigation application. The fact some growers sell water from recycling pits to adjacent farms may undermine this argument in some cases however. The detailed long-term benefits or drawbacks of tailwater recycling are yet to be fully explored. One concern tied to tailwater recycling systems is the disease transmission potential of tailwater systems in transmitting diseases of sugarcane such as Chlorotic streak or other water borne pathogens. Increased salinity of irrigation tailwater systems is another issue yet to be addressed in any depth. With regard to increased salinity



associated with tailwater retention, several beneficial effects may in fact arise from such an outcome that are beneficially relevant to the management of sodic soils. As well as increased overall water use efficiency, salinity in recycled water tends to increase with resultant water penetration improvements for sodic soils, as well as minimizing off-site movement of turbid water from sodic paddocks. Another potential concern regarding tailwater recycling facilities is the actual magnitude of water loss from the pits themselves. There are at present no explicit design criteria or guidelines for tailwater recycling dam construction or operation. Cultivation Practices. Cultivation practices play a significant role on the infiltration characteristics and corresponding irrigation efficiencies of many soils. The potential role reduced/minimum tillage practices may play in improving water use efficiency within the Burdekin region has been investigated to a certain degree, with results generally suggesting improved WUE, but overall savings can be variable. Raine & Shannon (1996a) documented some improvements in irrigation efficiency in ratoon crops on permeable alluvial soils in the Burdekin region under minimum till compared to conventional cultivation practices (see table 4.18 below). These improvements were however largely limited to the first irrigations, with little or no difference between alternative irrigation treatments after initial irrigations. Klok et al. (2003) similarly documented small savings in overall water application between conventional tillage and no tillage treatments in a paired strip trial on medium cracking clays in the Burdekin delta see Table 4.17, site 2). Table 4.18 Application efficiency of irrigations conducted immediately after cultivation is reduced (From Raine & Shannon, 1996a).

Treament A Irrigation date Volume applied ((ML/ha)

Application efficiency B

Cultivated 03/10/1994 18/10/1994 08/11/1994 15/12/1994

4.3 1.7 1.5 1.3

14 35 40 46

Minimum Till 15/19/1994 06/10/1994 21/10/1994 07/11/1994

2.0 1.6 1.5 1.4

30 38 40 43

A Successive irrigations conducted on an alluvial soils with 470m furrow length (a) cultivated = after the last cultivation following harvesting and (b) minimum till = immediately after harvesting. B Average soil water deficit = 0.6 ML/ha Irrigation efficiencies generally increase from plant crop to first ratoon crop (Holden & Mallon, 1997). This is probably due to less cultivation being associated with ratoon crops and compaction/consolidation from both water and machinery reducing deep drainage losses. A trial monitoring a plant crop on sandy loam soil saw Holden et al. (1998a) highlight substantial reductions in water use in on-farm trials comparing combinations of reduced tillage and adoption of V-shaped furrows with conventional practices. The farm



utilized in this experiment had the previous year exhibited quite low irrigation efficiency (<20%). In this case, the reduced and conventional tillage treatments were established just after crop planting. Prior to alternative furrow shape treatments being installed at the ‘hill-up’ stage of the cropping process, more than 5ML/ha of applied water (over 3 irrigations) had already been saved via reduced tillage alone. The overall combination of altered furrow and reduced cultivation treatments subsequently reduced overall water use throughout the trial by 60%, with no effects on cane yield (table 4.19). This equated to overall savings of 15 ML/ha through the season under ‘BMP’ in comparison to standard farming practice. Table 4.19. The effect of furrow shape and irrigation practices on irrigation water usage of sugarcane grown on an alluvial soil (From Holden et al. 1998a)

Tillage Reduced cultivation (ML/ha)

Conventional cultivation (ML/ha)

Total before hill-up 5.18 10.74 Furrow shape Broad U Narrow V Broad U Narrow V

Total (8 irrigations) ML/ha/irrigation

15.74 1.97

10.56 1.32

25.46 3.18

17.55 2.19

Experiment was a plant crop on sandy loam soil, with a readily available water content of 0.4 ML/ha. Inflow rate of irrigation water was 0.6 l/s. Reduced cultivation entailed one residual herbicide spray plus two cultivations after planting. Conventional cultivation was seven cultivations after planting. No yield differences were measured. In a similar tillage trial for a plant crop on a property with a less free draining alluvial clay loam soil Holden & Mallon (1997) documented some definite although not as pronounced water savings as demonstrated in the previous example. The less freely draining soil and higher irrigation inflow rates employed by the farmer in this case saw relatively reduced but still not insignificant water savings (2.5 ML/ha/year) to the previous example. Table 4.20. Effects of cultivation practices on irrigation water usage for cane grown on an alluvial soil (from Holden & Mallon, 1997). Cultivation Treatment Reduced Cultivation Conventional Cultivation Total Water applied (ML/ha/yr)

15.73 18.15

ML/ha/irrigation 1.21 1.45 Irrigation application efficiency (%)

57.8 48.3

Cane Yield (tones of sugar/ha)

25 25.27

Experiment was a plant crop on a clay-loam soil with an available water content of 0.7 ML/ha. Reduced cultivation included two blanket herbicide sprays plus three cultivations after planting. Conventional cultivation was six cultivations after planting. The main disadvantage of reduced tillage is at least one herbicide has to be applied to



control weeds (usually a mixture of atrazine and ametryn). Holden et al. (1998a) did note at the time some reluctance on the part of Burdekin delta growers in adopting minimum tillage practices. The use of residual herbicides was a relatively new concept in an area where cultivation was commonly used as a weed control strategy. The valid point was subsequently raised as to whether increased residual herbicide use on freely draining soils should be pursued. Another of the relevant environmental benefits ascribed to reduced cultivation practices is in reduction of soil erosion and subsequent suspended sediment entrained in flood water during flood events. Soil erosion from conventionally cultivated ratoon cane lands in the wet-tropics regions of north Queensland was measured in the range of 47-505 t/ha/yr, with an annual average of 148 t/ha/yr (Prove et al., 1995). Trials of alternative management strategies revealed no-tillage practices spectacularly reduced this erosion to < 15 t/ha/yr. An interesting additional point to emerge from this study was that while sediment loads in flood run-off would be substantially reduced as a result, the nutrient losses from no or minimal tillage soils may not be proportionately reduced. Physical and chemical analyses revealed only fine soil fractions are eroded from no tillage soils, and associated with this fine sediment fraction is a more active nutrient pool. The higher specific surface area of eroded sediment from no-tillage soils also has implications for duration of suspension and transport of suspended material. How these erosion reduction outcomes from the sloping, rainfall reliant canelands of the wet-tropics translate to the much flatter, furrow irrigated cane fields of the lower Burdekin region is somewhat debatable. While the issue has not been assessed to any great degree, substantial erosive soil losses seem unlikely to be a huge issue in the lower Burdekin apart from perhaps isolated processes on a landscape scale such as stream and river bank degradation that may occasionally occur. Data from other cane growing areas such as the Mackay-Whitsunday region which shares relatively similar topology (relatively flat) and dry-tropical climate to the Burdekin can provide some degree of vicarious context. Recent studies of run-off characteristics between sub-catchments dominated by different land uses suggests the level of suspended sediment from cane-growing catchments in the Mackay-Whitsunday region is not vastly different to that lost from more natural, forested catchments (Rohde et al., 2006). While not quantitatively validated, an analogous situation would conceivably be expected in the lower Burdekin. Controlled Traffic. A related topic to BMP issues such as furrow shape and strategic tillage is the recent advocacy for controlled traffic/permanent bed configurations in cane fields. As noted by McGarry et al. (1997) the cane industry has become ‘out of step’ with other cropping industries such as cotton and grains in that controlled traffic systems (where crop row spacing and equipment track width) are accepted practice. There has existed a long-standing mismatch between crop row spacing (1.5m) and equipment track width (harvester and haul-out trucks at 1.8m) throughout the Australian cane industry. This mismatch and the use of larger and heavier equipment during harvest has been associated with yield losses through soil compaction and direct damage to stools affecting crop emergence and ratooning ability of cane (Braunack & Peatey, 1999, Braunack & Hurney,



2000). Remedying the incongruence between commercial row spacing and equipment track width allows establishment of a stable row/inter-row configuration (permanent beds and laneways) where all wheels traffic specific zones on a continual basis. Recent industry shifts toward controlled traffic systems hold considerable potential for improved profitability of this alternative farming system (see Garside et al., 2004). Productivity and soil health issues aside however, tangible environmental benefits accompanying a switch to controlled traffic systems are yet to be quantified to any great degree. This is not to say substantial scope does not exist associated with adoption of controlled traffic/permanent bed systems for enhanced fertilizer and pesticide management. Controlled traffic systems have however seemingly been attributed considerable environmental merit in addition to production improvements in many recent sustainability incentives schemes for example, despite minimal documentation of relative environmental value. Research into comparative environmental sustainability benefits and BMP implications of controlled traffic/permanent beds with regard to chemical usage and water use efficiency is certainly required. Irrigation Scheduling. Irrigation scheduling is the frequency and timing at which irrigation water is delivered to a crop root zone. The optimum efficiency is primarily determined by soil type as different soils have different storage capacities of readily available water (RAW) (Holden, 1998). RAW is the amount of water stored in soil between the refill point and full point. The refill point is when the plant begins to suffer moisture stress, a circumstance significantly reducing plant growth. The full point (field capacity) is the maximum amount of water a soil can hold against gravity when excess water has drained away. Irrigation scheduling should ideally maintain soil moisture between refill and full points. The BSES has a measured range of soil RAW storage capacities for most major soil types in the Burdekin region (see table 4.20 below) Table 4.20 Measured storage capacities of Readily Available Water (RAW) in the Burdekin (from Holden, 1998). Soil type/texture Location Readily available water

(RAW) in the root zone (mm)

Cracking clay (Barratta) Clay loam and silty clay loam Loam and silty loam Sandy loam Loamy sand Sand Sodic clay (Oakey)

BRIA Delta/BRIA Delta Delta Delta Delta BRIA

90 80 70 60 50 30-40 40-90

Knowledge of soil moisture at any given moment in time as well as the rate of soil water depletion are also fundamental components of irrigation scheduling. Soil moisture loss



occurs primarily via plant uptake and transpiration as well as soil surface evaporation. Together these processes constitute crop water use. Some predictive knowledge of the rate of soil moisture depletion down to refill point can allow irrigation scheduling that minimizes crop stress. An array of irrigation scheduling techniques are currently employed across the cane industry ranging from the subjective (and unreliable) visual estimates of crop water stress through to evaporation minipans, tensiometers, neutron probes and EnviroSCAN probes. The use of evaporation minipans as a practical tool for irrigation scheduling represents one of the major WUE initiatives launched in the lower Burdekin cane growing district (see Shannon & Holden, 1996, Holden et al., 1998 for more details). In summation, after minipan deficits were produced for the main Burdekin soil types and growers calibrated minipans to their individual farm soil types, farmers were encouraged to schedule irrigations once crop growth had fallen to 50% of maximum. Initial trials of scheduling based on evaporation minipans demonstrated increased cane yields for the 1994 and 1995 harvests. Some representative responses to minpan scheduling are outlined in table 4.21 An interesting result was the higher yields evident with increasing ratoon age. Typically cane crops produce lower yields with increasing ratoon age. Minipan scheduled crops harvested in 1994 produced more cane than the unscheduled earlier ratoon crops. Table 4.21. Yield responses (t/ha) to irrigation scheduling using evaporation minipans; R = ratoon (from Shannon & Holden, 1996). Soil Variety Unscheduled

1993 (t/ha) Scheduled 1994 (t/ha)

Production increase (%)

Alluvial Q117 124 (1R) 136 (2R) 10 Non-sodic duplex Q117 107 (1R) 120 (2R) 12 Sodic duplex Q96 88 (2R) 110 (3R) 25 More substantive results of productivity data from the 1995-96 growing also demonstrated minipans were improving crop yields (see table 4.22.). In this case the cane and sugar yields from 80 Burdekin farms in the Invicta area who employed minipan scheduling were significantly improved relative to the rest of the farms in the Invicta area (Holden et al., 1998). This improvement of one tonne of sugar per hectare represented considerable economic benefit for growers. Table 4.22. The effect of scheduling irrigations on cane yield, CCS and sugar yield of 80 Invicta farms using minipans for the first time compared with the rest of the Invicta Mill area (minipan non-users) in the 1995-1996 season (from Holden et al., 1998) Minipan users Minipan non-users P (T<=t) two-tail N Tonnes of cane/ha CCS

80 121.29 (2.62) 14.47 (0.07)

173 113.42 (1.48) 14.56 (0.05)

0.01 0.3



Tonnes of sugar/ha 17.51 (0.22) 16.51 (0.35) 0.02 Standard errors in parentheses. The resultant effects of improved irrigation scheduling on overall water consumption were not assessed in great detail. A grower survey outlined in Holden et al. (1998) outlined growers in the 1994-1995 season (without minipans) used an average of 9.6 ML/ha. During the 1995-1996 season which was apparently similar in terms of rainfall events, the use of minipans to schedule irrigations saw an average water use of 9.3 ML/ha. This result was suggested as indicative of no increased water use through minipan scheduling while increasing overall WUE through increased productivity in the vicinity of 0.57 t/ML. It was noted that in more freely draining areas outside the soil types typifying much of the BRIA may see increased water usage through minipan scheduling due to more frequent irrigations. This issue could not be addressed at the time due to the lack of water usage data for areas such as the Delta where minipans were just beginning to increase in popularity. Over the three year life of the Water Check project, over 500 evaporation minipans were distributed to Burdekin cane growers. The more recent BMP research of Klok et al. (2003) provides some additional insight into water usage likely on freer draining delta soils using minipan scheduling (see site 6 Table 4.17). Scheduling at site six did not influence the amount of irrigation water applied, however it did change the frequency with which it was applied, with smaller irrigation amounts applied on a more regular basis. The conventional (CONV) treatment received 15 irrigations compared with 22 for the BMP. The resultant 12% yield increase with little difference in total irrigation application suggests higher efficiency of application, with the crop likely to have received less water stress between irrigations. Minipan scheduling is probably the most commonly applied quantitative method of irrigation scheduling thus far applied by growers over the Burdekin region. A number of other promising irrigation scheduling techniques are currently being implemented in other fully irrigated cane cropping areas such as the Ord district in Western Australia. Utilising a Water balance concept and based on crop age and known responses to water stress, daily weather, atmospheric evaporative demand (AED) and soil water status, these tools take the form of simple scheduling tables that allows growers to monitor soil water contents of blocks on a daily basis and increase application efficiency. These approaches have seen promising although limited application in the Burdekin (see Attard et al., 2003) but are yet to be fully developed as a viable scheduling alternative for local growers. O’Grady & Christiansen (2001) provide an interesting follow-up perspective on the long-term implementation of minipans (and scheduling tools in general) within the Burdekin. Their 2001 state-wide grower surveys targeting industry perceptions of the Code of Practice and actual on-farm adoption of alternative farm (BMP) practices provides an overview of farming practices within individual cane growing districts (see graph ? below). For Burdekin growers ‘knowledge of farm soil’ closely followed by ‘crop condition’ were the most commonly used methods of irrigation scheduling. The low apparent use of minipans indicated by Burdekin growers came as a surprise in light of the initial enthusiasm with which they were adopted several years prior. Holden & Mallon



(1998) reported that in 1994/95, 70 % of BRIA growers and 4% of Delta growers utilized minipans, increasing to 83% and 48 % respectively in 1996/97. This suggest that despite the apparent yield increases that drove the previous high rate of adoption, survey results indicated Burdekin growers lost considerable interest in minipans as a scheduling tool. The results of O’Grady & Christiansen demonstrate that the use of physical measurements for irrigation scheduling is quite low among Burdekin growers. A number of confounding factors may have contributed to survey results and skewed results such as patterns of response to surveys (growers from areas that have been slow to adopt minipans had the highest rate of survey return). Loss or damage of minipan drums with no subsequent replacement was touted as another possible factor. Similarly farmers using minipan scheduling for several years to re-adjust irrigation practices and then continuing on the scheduling program without referring to minipans was forwarded as another plausible explanation. However, the biggest identified cause of reduced minipan use was the cessation of the intensive extension and promotion program by BSES that accompanied Holden & Mallon’s initial minipan launch. Holden & Mallon’s 1998 survey was undertaken as the final stage of an intensive extension program promoting the use of irrigation scheduling. The need for continual ongoing ‘championing’ of minipans was identified as a particular requirement to ensure continued adoption and use of emerging BMP tools or new technologies. Interactions of irrigation with other aspects of farm management. This overview of WUE research in the Burdekin highlights the fact that significant improvements in irrigation efficiency can be achieved through the carefully considered adoption of design and management practices suitable to a particular farms’ environmental and management constraints. As Raine & Bakker (1996a) note, proper design and management of furrow irrigation is necessary to achieve efficient water use and prevent issues such as salt accumulation in root zones and rising watertables in areas irrigated from surface channels. Poor design and management may also result in productivity losses through poor germination or soil waterlogging as well as loss of fertilizers and nutrient from the root zone. The recurrent theme emerging throughout previous sections of this review is the relationship between water usage and subsequent behavior of nutrients, pesticides and sediments. Given the relative lack of locally developed BMP specific to nutrient and pesticide use, maximizing water use efficiency (WUE) represents one of the few avenues to minimize off-site water quality effects in the absence of specific water quality Improvement guidelines. Enhanced WUE can therefore act as a surrogate for locally relevant BMP for fertilizer and pesticide use until such time more locally applicable research on these particular issues come to hand. Management of contaminants, whether it be sediments, pesticides or nutrients, and resultant minimization of off-site water quality impacts (including groundwater) is fundamentally and inextricably linked to on-farm water management for Burdekin growers. Future Developments in BMP for Irrigation within the Burdekin.



Alternatives to furrow irrigation such as centre pivot and trickle/drip irrigation systems hold a number of potential advantages from a management and environment perspective. These include superior application efficiency, more flexible application rates, uniform distribution, fertigation capacity, reduced nutrient input requirements and relatively low operational costs (Qureshi et al., 2002). The associated disadvantage with these alternative systems is the relatively high initial capital costs for installation. Integrated bioeconomic modeling comparing these alternatives to existing farming systems on Burdekin delta farms already using furrow irrigation suggests changes from furrow systems are not an attractive option under current management and water pricing regimes (Qureshi et al., 2002). Major capital investments on the part of farmers in the Burdekin to adopt alternatives to furrow irrigation (i.e. overhead or trickle irrigation systems) are probably unlikely in the near future given the current uncertainty over sugar prices as well as the yet to be demonstrated cost effectiveness of adopting alternative systems. The purported environmental benefits of alternative methods such as trickle irrigation to the sugar industry (enhanced water and nutrient use efficiency) may not be as clear cut as perhaps presented (see Thorburn et al., 1998). Kingston et al. (2000) similarly noted that the little existing information on quantitative benefits of system changes and the initial capital costs of overhead irrigation and drip irrigation are a major impediment to their widespread adoption. While there is some impending commercial scale evaluation of alternatives to furrow irrigation in the lower Burdekin, these systems are yet to be rigorously proven as viable alternatives to furrow irrigation, particularly to the extent growers will change to these systems on a district wide basis. As noted by Tilley & Chapman (1999), furrow irrigation has considerable potential for irrigation application efficiency improvements. The most substantial improvements to WUE across the lower Burdekin district over the short and medium term therefore are likely only realistically achievable via optimization of existing furrow irrigation practices. 5.0 Green Cane Trash Blanketing. The Role of GCTB in Cane Production. There has been a progressive move in much of the Australian sugar industry away from tradition ‘burnt’ cane harvesting systems when cane trash is burnt before and/or after harvest. The use of green cane trash blanket (GCTB) farming systems where the crop is harvested ‘green’ and substantial crop residues are retained as a surface mulch or trash blanket has become a widely adopted method of cane production across much of Queensland. Adoption of GCTB is often seen as an important industry shift toward more environmentally sustainable production methods. An array of purported benefits have been ascribed to GCTB including reduced cultivation requirements; weed suppression; reduced soil erosion; improved soil structure and fertility, enhanced moisture conservation, yield improvement and longer ratoon capacity. One of the greatest advantages of GCTB is it confers greater operational flexibility and reduced productivity



losses through reduced risk of standing burnt cane during wet weather. This also results in reduced soil and crop damage issues caused by harvesting and haul-out equipment in wet weather. Some cane growing districts in Queensland have completely embraced the GCTB concept with 100% of farms adhering to the system. Mitchell et al. (2000) demonstrated the gross losses of organic matter and nutrients from the immediate area when burning crops or sugarcane trash (as is the standard Burdekin practice) can be significant. Recoveries of nutrients and dry matter ranged from 5-77% of initial input. The lowest recoveries were after fires in standing cane crops. Nutrient and dry matter recoveries after a standing crop was burnt in the field prior to harvest were: K (30%), dry matter (23%), P (23%), N (23%), S (18%), Mg (17%) and Ca (11%). (These figures did not account for the long-term fates of ignition products and potential net redistribution across sugar cane growing areas?). Alternative retention of cane trash, with nutrients and organic matter accumulating in the soils has prompted speculation that fertilizer application regimes could be altered to account for this additional nutrient pool. GCTB has the potential to return up to 30-60kg N/ha/yr to the soil, depending on the size of the crop (Robertson & Thorburn, 2000).There are however still substantial information gaps regarding GCTB and its effects on soil C, N cycling and water retention processes, particularly over long time periods. Long-term studies in the Ingham district confirm substantial beneficial influences on soil micro biota due to GCTB retention, also suggesting an association between higher productivity, soil organic matter turnover and nutrient storage and release under the GCTB system (Sutton et al., 2000). Other studies however illustrate the short and long-term complexity of GCTB effects on soil properties, with potential benefit dependent on interactions between soil types, climate, as well as trash and fertilizer management practices. Work by Thorburn et al. (2000) highlighted considerable site specificity in soil responses to GCTB, and suggested any adjustment in fertilizer management associated with GCTB should be conservative and carefully monitored. A number of other issues associated with GCTB such as leaching, nutrient uptake by crops and denitrification have not been widely assessed (N losses via leaching and denitrifcation are expected to be greater under GCTB than burnt farming systems). One of the foremost environmental benefits attributed to GCTB practices is in erosion mitigation. Soil erosion rates from conventionally cultivated sloping cane lands in wet-tropical north Queensland have been measured in the range of 47-505 t/ha/yr (Prove et al., 1995). In terms of altered management practices, greatest benefits in erosion mitigation tended to be derived from combinations of reduced (zero) tillage and 100% ground cover from GCTB. However, this reduction was primarily due to the effects of no-tillage practices rather than ground cover influences, as minimal differences were observed between no-tillage practices with 0, 60 and 100% groundcover (i.e. erosive rates of 15,10 and 5 t/ha/yr respectively). The relative erosive reduction benefits conferred by GCTB on the very flat cane paddocks dominating the Burdekin are yet to be assessed, but would not be expected to approach anywhere near the level evident in the wet-tropics. Existing Utilisation of GCTB in the Burdekin District.



While many sectors of the Queensland cane industry have comprehensively adopted GCTB practices, the Burdekin region has been relatively slow to implement the system on any substantial level across the district. According to Tilley & Chapman (1999) the percentage of crop harvested unburnt in the Burdekin increased through the period 1990 to 1998 from 3.8 to 12.5%. However, more recent surveys suggest very few growers in the Burdekin utilize undisturbed trash blanket as a favoured management option (4.5%), with burning cane and retaining tops the predominant management practice (O’Grady & Christiansen, 2001). The number of farms utilizing GCTB in the Burdekin may vary from year to year depending on weather and associated harvesting constraints. Past reviews of primarily productivity based comparisons between GCTB and conventional burnt cane systems in the Burdekin have produced mixed results from the perspective of advantageous outcomes for growers (see McMahon & Ham, 1996). During the 1994 season in the Dalbeg district (Invicta Mill) which contains one of the higher proportions of growers cutting green, 11 GCTB assignments and 12 conventionally burnt and cultivated assignments produced essentially similar yields. In more quantitative commercial scale trials carried out on the low sloping, long drill length sodic duplex and cracking clay soils of the BRIA some negative issues associated with GCTB emerged. Increased cane losses during harvest, higher harvesting costs as well as some depressed yields (with waterlogging under the low sloping, low permeability soils suggested as the underlying cause) saw growers reluctant to subsequently adopt the alternative farming system. Concurrent trials on steeper sloping sodic duplex and cracking clay soils saw GCTB produce cane yields equal to or superior to conventional systems, with production losses due to waterlogging on low permeability soils apparently not such an issue. Smaller scale BSES plot trials on well-drained alluvial soils produced similar results with similar or significantly superior crops produced under GCTB over 3 ratoons. Given these variable outcomes, as well as recognition of growing environmental pressures to not fire cane likely to force GCTB upon the Burdekin at some point in the future, Holden & McMahon (1997) compiled a thorough review of what is widely regarded by the industry (growers, industry support organizations, harvesting contractors etc.) as the major constraints to widespread adoption of GCTB throughout the Burdekin district. Many of the expressed sentiments echoed the drawbacks potentially associated with GCTB that were highlighted by McMahon & Ham (1996). Harvesting difficulties, particularly concerns that existing harvesters were ill-equipped to efficiently harvest green cane were seen as one of the major impediments to adoption of GCTB in the Burdekin. Crops greater than 120t/ha are reportedly difficult to cut green. With the crop average in the Burdekin 125t/ha. Issues such as continual choking problems in harvester feeding mechanisms likely associated with cutting large, tangled (lodged) crops were particularly seen as a major drawback. Increased harvesting losses are also associated with GCTB, with losses up to 10% higher in comparison to those typifying conventional burnt cane growing systems (McMahon, 1995). Slower harvesting rates, increased fuel consumption and greater machinery component wear were also seen as outcomes likely to greatly increase harvesting expense or feasibility in the event of significant adoption of the GCTB system. The slower drying under GCTB in wet years with frequent rainfall was also seen as problematic due to equipment traffic issues with associated negatives



such as soil compaction and crop damage. Waterlogging of soils under GCTB in the first post-harvest irrigation, resulting in slow ratooning of cane was also seen as a major problem, particularly on heavy, clay soils. The first irrigation application after harvesting is seen as problematic, with stubble is in a waterlogged environment often slow to ‘come away’. This is where subsequent longer-term productivity issues may emerge, or in a worst case scenario result to total ratoon failure. Early season temperatures can also be 10°C cooler under a GCTB, a situation which can adversely affect germination. Current recommended practice in the Burdekin to circumvent some of these issues is to allow cane to ratoon and emerge on its own moisture before applying first irrigations. Irrigation issues associated with managing furrow irrigation systems under GCTB were another of the overriding concerns emerging from the review of Holden & McMahon (1997). The trash blanket presents problems in interfering with water flow down furrows, to the extent that some growers had difficulty in getting water to the end of particularly long drills. The resultant, much longer irrigation application requirements certainly raise some concerns from a WUE perspective, as well as concurrent issues such as deep drainage and groundwater problems. Observation of irrigation advance under GCTB is also more difficult, posing a challenge to irrigation control and managing tailwater run-off. A valid point was raised that many of the existing farm designs in the Burdekin may not be inherently compatible to pragmatic irrigation under GCTB. Many Burdekin farms (especially in the BRIA) are furrow irrigated, have exceptionally low slopes (1:1500), as well as with very long row lengths, characteristics not conducive to easy irrigation management under GCTB. Considerable farm redesign such as re-levelling paddock slopes and shortening of drill lengths would be required in many cases to making GCTB a workable farming system in the Burdekin (at least under current furrow irrigation systems). Current Knowledge and Research relevant to BMP for GCTB in the Lower Burdekin. At the time of McMahon & Ham’s 1996 review, objective assessments of the water usage implications of GCTB were rare. McMahon & Ham (1996) did make some passing reference to WUE considerations, and like the relative productivity comparisons between conventional burnt and GCTB farm systems, results were mixed. One trial was established on a farm with inadequate water supply and consequent sub-optimal irrigation capacity. Here the farmer agreed to trial GCTB with a view to making most efficient use of available water due to it’s purported moisture conservation benefits. The perhaps surprising result of no yield increases in the GCTB treatments under such dry, sub-optimal irrigation condition did little to encourage subsequent adoption of the practice. Other trials produced more positive environmental benefits, specifically a reduced number of early irrigations in the GCTB systems due to adequate moisture being retained in the absence of a crop canopy. Waterlogging and difficulties in monitoring irrigation advance were issues that emerged however. In the previously described BSES trials moisture readings taken with each irrigation over treatments revealed GCTB used 30mm less water compared to conventional treatments over the initial irrigation, with 6mm less



water used for each subsequent irrigation. Over the entire season this equated to an efficiency improvement of approximately 1 ML/ha of water. In one of the more detailed assessments of WUE under GCTB Raine & Bakker (1996a) noted an additional 40% more water in addition to substantially decreased water advance rates for the first irrigation after harvest under a GCTB system when compared to conventional burnt treatment near Clare (see table 5.0 below). Interestingly, there were no significant differences in either irrigation efficiency or advance times between the trash retention and burnt treatments in subsequent irrigations. Table 5.0. Effect of trash retention on furrow irrigation depth of flow and advance times for selected furrow irrigations in Clare (from Raine & Bakker, 1996a).

Burnt Trash Retention Irrigation Water Application

(L/s) ISSM (%)

Depth of flow (mm)

Advance Time (mins)

ISSM (%)

Depth of flow (mm)

Advance Time (mins)

16/11/94 1.4 - 20 650 - 67 920 5/1/95 0.6 - 16 1160 - 51 1290 21/2/95 0.9 10.9 37 840 11.9 56 860 4/4/95 1 9.1 35 1070 16.3 46 1200 13/4/95 1 11.4 30 940 19.1 46 1000 9/5/95 1 14.7 34 1510 20.4 42 1380

ISSM = Initial soil moisture The proposed hydraulic processes behind this effect were interesting. The initial relative irrigation inefficiency under GCTB was suggested as being due to initial hydraulic resistance of retained trash and slow subsequent irrigation advance rates. After the preliminary irrigation loose material has either been washed off paddocks or consolidated in the furrow such that hydraulic resistance was greatly reduced for subsequent irrigations but not the extent seen in burnt treatments. However, hydraulic resistance increases in burnt treatments as the season progresses as cane lodges and natural trash levels from growing cane increases. The higher soil moisture content produced under GCTB reduces water infiltration rates which counteracts the greater hydraulic resistance under GCTB and produces irrigation advances and efficiencies similar to the burnt treatment as time goes on. Anecdotal observations and these results regarding the adoption of residue retention practices raised some concerns as to the effects of furrow irrigation efficiency possibly declining as a result of trash interfering with water flow. A number of field trials in conjunction with SIRMOD modeling were subsequently established in the Burdekin to better investigate the effect of GCTB on irrigation efficiency. Four trials were established during the 1996/97 and 1997/98 seasons to assess the efficiency and scheduling of both GCTB and conventional burnt systems of cane production (see Hardie et al., 2000).



On freely draining alluvial soils (sandy clay loam) the GCTB system used over 4 ML/ha more water than the conventional burnt system, despite a substantial reduction in the overall number of required irrigations (see table 5.1). No differences between harvested cane or sugar yields emerged between the two systems. Irrigation losses in the GCTB were reported as due to deep drainage losses associated with trash related increases in hydraulic resistance to water movement in the furrow. The resultant combination of increased opportunity time for infiltration, a larger head of water in the furrow and increased wetted perimeter contributed to these drainage losses. Table 5.1. Irrigation water used by GCTB and burnt systems on a freely draining Delta soil (250m row length, 0.0025 % slope, 1.0-1.5 l/s inflow) during the 1996/97 season (from Hardie et al., 2000). System Total

Water applied (ML/ha)

No. of irrigations

Mean water applied per irrigation (ML/ha)


Cane Yield (t/ha)

CCS* WUE** (t/ML)

GCTB Burnt

12.84 8.53

5 9

2.57 0.95

27.2 73.7

116.32 116.91

16.58 16.60

6.59 8.77

Efficiencies calculated using an average readily available water content of 0.7 ML/ha. * Commercial cane sugar percent. ** Water Use Efficiency (WUE) calculated using 4.8 ML/ha effective rainfall. Results from trials on surface sealing and sodic soils similar of slightly lower irrigation application efficiencies under GCTB treatments than burnt systems. Again this resulted from the higher irrigation application volumes and increased deep drainage associated with the GCTB system. Hardie et al. (2000) noted that as a consequence of sodic and surface sealing soils being difficult to ‘wet up’ due to their low soil infiltration, GCTB conferred significant advantage is improved water penetration and increased root zone storage in these soil types (see table 5.2 and 5.3 below). Improved water penetration resulted from the same mechanism contributing to deep drainage losses in freely draining soils, namely increased resistance to water movement down furrows. Across all sites, in both seasons, water movement along the furrow was between 14-18% slower and approximately 10 % deeper in the GCTB system. Table 5.2 Typical early season irrigation efficiencies of conventional burnt and GCTB systems, October, 1997 (from Hardie et al., 2000).

Soil type System Average water applied (ML/ha)

Runoff (ML/ha)

Deep drainage (ML/ha)

Root zone storage (ML/ha)


Surface sealing alluvial soil (sandy loam) (0.002% slope,

GCTB Burnt

0.87 0.85

0.02 0.05

0.05 0.1

0.80 0.70

92% 82%



150m, 0.2 – 1.0 l/s inflow) Surface sealing alluvial (silty clay-loam) (0.0026% slope, 650m, 0.4-0.9 l/s inflow)

GCTB Burnt

1.01 0.78

0.31 0.18

0.15 0.10

0.55 0.50

54% 64%

Sodic duplex/cracking clay (clay loam over sodic clay/heavy clay) (0.0018% slope, 1250m, 0.7 – 1.1 l/s inflow)

GCTB Burnt

1.78 1.10

0.26 0.12

0.69 0.38

0.83 0.60

47% 55%

Table 5.3. Typical late season irrigation efficiencies of conventional burnt and GCTB systems, October, 1997 (from Hardie et al., 2000).

Soil type System Average water applied (ML/ha)

Runoff (ML/ha)

Deep drainage (ML/ha)

Root zone storage (ML/ha)


Surface sealing alluvial soil (sandy loam)

GCTB Burnt (0.002% slope, 150m)

1.12 0.56

0.04 0.06

0.28 0

0.80 0.5

71% 89%

Surface sealing alluvial (silty clay-loam)


0.88 0.68

0.26 0.26

0.07 0

0.55 0.42

63% 62%

Sodic duplex/cracking clay (clay loam over sodic clay/heavy clay)


0.66 0.49

0.12 0.10

0 0

0.54 0.39

82% 80%

Progressive decomposition of the trash blanket through the season saw the irrigation application to GCTB behave similarly to conventional burnt system, an outcome reflected by the changes to Manning n (resistance to flow) over the 1997/98 cropping season (see table 5.4 below). These outcomes reflect the processes evident in Raine & Bakker’s 1996a study where hydraulic reistance in GCTB system lessens as trash consolidates or is



washed out as the season progresses, while hydraulic resistance under conventional burnt systems increases as natural trash accumulates and cane lodges. Table 5.4. Calculated values of hydraulic resistance (manning n) used in SIRMOD model simulations (from Hardie et al., 2000).

Manning n value Date GCTB Burnt

25 July 1997* 10 Sept 1997 30 Sept 1997 15 Nov 1997 24 dec 1997 16 Mar 1997 31 Mar 1997 15 Apr 1997

0.12 0.12 0.12 0.10 0.06 0.06 0.05 0.05

0.06 0.04 0.03 0.03 0.03 0.03 0.03 0.02

* First irrigation after harvest. In terms of irrigation scheduling (using deficits based upon the 50% of maximum growth rate calibration), Hardie et al. (2000) documented substantially lower irrigation frequency requirements under GCTB compared to burnt, even with full canopy closure. Over the two growing seasons during trial implementation the GCTB system required four less irrigations on the freely draining soil, two less on the 150m surface sealing soil and one less irrigation on the sodic/cracking clay soil. This water saving resulted from reduced evaporative losses in the GCTB treatment mostly before canopy closure. Soil moisture deficits were estimated by comparing ‘minipan’ evaporation (Em) to Class ‘A’ pan evaporation (EO) at the time of crop growth calibration for the two systems (Table 5.5). Table 5.5 Evaporation minipan deficits and estimated evapotranspiration for two soil types in the Burdekin in the 1997/98 season. Soil Type System Minipan Deficit

Em (mm) Estimated ET ETE (mm)

Average January Irrigation cycle (days)

Sodic duplex/cracking clay (1250m)

GCTB Burnt

83 70

58.1 49.0

11.8 10.0

Surface Sealing alluvial (650m)

GCTB Burnt

85 63

59.5 43.5

12.1 8.8

The WUE projects outlined in Hardie et al. (2000) also investigated the use of surge irrigation with GCTB management (table 5.6). The trial consisted on five non-replicated 14 drill treatments on a freely draining soil including:

• Conventional Burnt



• Alternate furrow GCTB • Surge 1 GCTB • Surge 2 GCTB • Alternate parted GCTB (pGCTB)

The parted trash treatment was effected by running a coulter over trash, followed by a broad tine to push trash aside and hold it in place with a small amount of soil. All treatments received very high flow rates of around 6.0 l/s and received seven irrigations over the 1998/99 growing season. Table 5.6. Irrigation efficiency and crop response to surge irrigation with GCTB management.

Total irrigation (ML/ha)

Average Irrigation (ML/ha)

Application efficiency % (PAWC 0.75 ML/ha)

Cane Yield (t/ha)

CCS Sugar Yield (t/ha)

WUE (t/ML)

Burnt GCTB GCTB surge 1 GCTB Surge 2 pGCTB

4.83 10.59 10.57 10.57 6.74

0.81 1.51 1.51 1.51 0.96

93 50 50 50 78

87.72 92.62 110.89 100.66 93.47

13.0 13.9 13.7 13.1 13.9

11.40 12.87 15.19 13.19 12.99

18.162 8.746 10.49 10.01 13.87

Outcomes indicated that the use of surge irrigation in conjunction with GCTB was of equivalent efficiency to standard GCTB furrow irrigation. The harvested cane yield was higher in surge irrigated treatments as a result of better wetting at the lower ends of blocks with surge irrigation. Of the three GCTB treatments, parting trash with a coulter and tine produced the highest irrigation application (78%) and overall water use efficiency in terms of tonnage per megalitre (13.87 t/ML) with no reduction in cane yield or CCS. Holden & McMahon (1997) highlighted a number of issues requiring research attention with regard to facilitating more enthusiastic industry uptake of GCTB in the Burdekin. This included research needs such as improved harvester design, varietal suitability (i.e. erect, free trash varieties), appropriate water management strategies (encompassing WUE, enhanced rationing, waterlogging and germination) and compaction minimization. The effect of trash retention on water infiltration rates, surface evaporation and irrigation efficiency across a range of soil types and farm designs obviously requires further understanding and the implications for water use efficiency under GCTB are therefore still uncertain. Relative productivity, harvesting and cost-effectiveness uncertainties aside, many of the water use efficiency and offsite (i.e. groundwater) water quality ramifications of GCTB in the Burdekin cane farming environment are also particularly undefined. 6.0 Information Gaps and Summary.



As the substantial amount of information covered in this review suggests, there is no shortage of glaring knowledge gaps impeding sound management decision making by natural resource managers in the lower Burdekin. Some of the more prominent issues that require urgent attention include:

• Irrigation practice in the Burdekin district plays a fundamental role in overall farm management, encompassing aspects of land preparation, establishment of plant and ratoon crops, pesticide and fertilizer efficacy as well as reduction of pest and disease related stresses on crops. The complexity of interactions between chemical usage, irrigation application and off-site environmental effects are significant, but poorly known. Some current research (CSIRO CSE Projects) should address some unknowns, but the associated knowledge gaps remain substantial.

• While some invaluable and informative research has been carried out on

groundwater hydrodynamics in the region, results have essentially highlighted how little is really known regarding a natural resource under pressure on an array of fronts. Issues such as nitrate leaching processes from farm to whole aquifer scale are in need of significant research effort.

• The lack of specific design criteria regarding the construction and maintenance of

tailwater recycling pits. This shortcoming is particularly pertinent given the array of financial incentive schemes targeting this particular aspect of farm management. Poorly designed recycling layouts may not provide desired sustainability outcomes and possibly contribute to local environmental problems (i.e. deep drainage).

• Detailed information of the costs and benefits of conversion to GCTB systems is

still largely absent. The uncertainties and concerns identified in grower community reviews a decade ago still exist today and little meaningful information has emerged since.

• The potential industry shift toward permanent beds and controlled traffic farming

systems opens up an array of potential unknowns as well as opportunities from an environmental as well as productivity standpoint. The implications for irrigation efficiency as well as nutrient and pesticide movement, particularly potential improvements or benefits relative to conventional bed layouts are currently unknown.

• The issues of pesticide usage patterns, off-site movement and environmental

effects of pesticides requires research attention. Much available data relevant to cane farmers has been developed in temperate rather than tropical climes and considerable benefit would be derived from locally relevant studies focusing on Burdekin farming systems.



• Recommended practices for fallow management are largely absent. How farmers should manage their fallow crop from a productivity and environmental perspective remains uncertain (harvesting a legume crop versus plough-out etc.).

If there was one ‘take home message’ from the diverse range of issues touched upon in this review it is the less than optimal WUE highlighted during this evaluation is no doubt a major driver behind many of the primary production related environmental problems discussed. By virtue of the complete reliance on supplemented irrigation characterizing the Burdekin cane industry facets of ‘on-farm’ water management such as water volumes, timing of irrigation applications, and levels of surface runoff and deep percolation have vast implications for the degree of impact of irrigated agriculture has on the broader environment. 7.0. References. Australian Bureau of Agriculture and Resource Economics (2003). Natural Resource Management in the Burdekin Catchment. Integrated Assessment of Resource Management at the catchment scale. Abare eReport 03.18. Available Online: http://abareonlineshop.com/PdfFiles/PC12597.pdf Australian Pesticides and Veterinary Medicines Authority (2005). The reconsideration of Approvals of the active constituent Diuron, Registrations of products containing Diuron and their associated labels. Preliminary Review Findings, July, 2005. APVMA, Canberra Australia.



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Thorburn, P.J., Sweeney, C.a. & Bristow, K.L. (1998). Production and environmental benefits of trickle irrigation for sugarcane: A Review. Proceedings of the Australian Society of Sugar Cane Technologists 20: 118-125. Thorburn, P.J., Park, S.E. & Biggs, I.M. (2003). Nitrogen fertilizer management in the Australian Sugar industry: Strategic opportunities for improved efficiency. Proceedings of the Australian Society of Sugar Cane Technologists 25 Vallis , I. & Keating, B.A. (1994). Uptake and loss of fertilizer and soil nitrogen in sugarcane crops. Proceedings of the Australian Society of Sugar Cane Technologists 16: 105-113. Wake, J. (2006). Mangrove health in the Pioneer Estuary. Centre for Environmental Health, Central Queensland University, Mackay. 81pp. Weier, K L and Haines, M G. (1998). The use of sugarcane in rotation with horticultural crops for removal of residual nitrogen. Proc. Australian Sugar Cane Technologists. (Ed. D M Hogarth) p.558 (Australian Society of Sugar Cane Technologists: Brisbane). Weier, K.L., Biggs, J.S., McNeil, A. & McEwan, C.W. (1999). Source and nitrate concentration in groundwater beneath sugarcane growing regions. Proceedings of the Australian Society of Sugar Cane Technologists 21, 497. Wolanski, E. & S. Spagnol (2000). Pollution by mud of Great Barrier Reef coastal waters. Journal of Coastal Research. 16(4): 1151-1156. Wood, A.W. & Bramley, R.G.V. (1996). Soil survey – a tool for better fertilizer management in the Australian sugar industry. In: Wilson, J.R., Hogarth, D.M., Campbell, J.A. & Garside, A.L. eds. Sugarcane- research towards efficienct and sustainable production. CSIRO Division of Tropical Crops and Pastures, Brisbane, p 189-193. Wood, A.W. (2000). Introduction to sustaining soil fertility and reducing heavy metal accumulation. In : Sustainable nutrient management in sugarcane production short course. Bruce, R. (ed.) CRC for Sustainable Sugar Production, Townsville. Pp.45-49 Wood, A.W., Schroeder, B.L. & Stewart, R.L. (2003). The development of site-specific nutrient management guidelines for sustainable sugarcane production. Proceedings of the Australian Society of Sugar Cane Technologists 25 Wrigley, T. (2005). The independent sugar industry environmental audit of 2004; An emphasis on farming practice. What does the industry take from it. Proceedings of the Australian Society of Sugar Cane Technologists 27: 83-95.



Inman-Bamber, N G, Robertson, M J, Muchow, R C, Wood, A W, Pace, R, and Spillman, A M F. (1999). Boosting yields with limited irrigation water. Proc. Aust. Soc. Sugar Cane Technol., 21: 203-211. Moss, A J, Rayment, G E, Reilly, N and Best E K. (1993). A preliminary assessment of Sediment and Nutrient Exports from Queensland Coastal Catchments. QDPI Environmental Technical Report No.5. (Queensland Department of Primary Industries: Brisbane). Sarich, Christopher. (1999). Effective Irrigation of Suitable Soils on Uneven Surfaces. Final Report LWRRDC Project BSE2. (Bureau of Sugar Experiment Stations, Brisbane). Simpson, B W, Hargreaves, P A, Sallaway, M, Ruddle, L J and Packett, R. (1998). Off-site movement of pesticides from sugar production systems. Proc. Australian Sugar Came Technologists. (Ed. D M Hogarth) p.559 (Australian Society of Sugar Cane Technologists:Brisbane).

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