principles of grazing animal nutrition.pdf

7/27/2019 Principles of grazing animal nutrition.pdf

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PRINCIPLES OF GRAZING ANIMAL NUTRITION

"The range resource is by far the most variable commodity that is encountered in livestock nutrition. It can vary in nutrient quality from a low comparable to sawdust to a high comparable totop quality alfalfa. It is capable of producing gains approaching 1.4 kg/day in growing classes of cattle at certain time of the year or of allowing starvation in some cattle at other times"... "Optimum

range livestock production can only be achieved through compatible livestock, wildlife and foragemanagement." (Raleigh and Lesperance 1973. In Church et. al Practical Nutrition Oregon StateUniversity Press). These authors should also have added compatible soil management to the list of practices for optimum range livestock production.

Therefore, the Range Manager needs to be continually in touch with the whole range ecosystembecause neglect of one part of the ecosystem can have serious consequences on the many otherparts.

Chemical Factors Influencing Nutrient Intake and An imal Productivity .

The major nutrients required by ruminant animals, such as cattle, sheep and goats, through theherbage consumed are:

1) Energy2) Protein (some of which can be in the form of non-protein N)3) Macro minerals - P, Ca, Mg, Na, S, K.4) Trace minerals - Cu, Se, Co, Zn, Mn, Mo, I5) Vitamins A and E6) Water

Of the first 4 categories listed above, energy (as digestible energy), protein, phosphorus andVitamin A are the most likely nutrient deficiencies encountered by ruminant animals which willrestrict intake and production. The macro minerals, other than phosphorus, such as K, Ca, andMg are rarely in deficient amounts in grazed herbage. In some cases deficiencies of Na and S havebeen reported but of more significance is the possible high NaCl and Na 2S0 4 content of underground water supplies for livestock. High concentrations of these salts will render waterunpalatable to livestock but much lower concentrations, particularly of Na 2, S04, will interfere withthe metabolism of some trace elements, notably Cu and Se, in animals thereby inducingdeficiencies of these elements.

The trace element content of herbage varies with soil type, the concentration of these elements inthe soil and the ability of the plant to extract the element from the soil. Single or multipledeficiencies of all the listed trace elements have been reported in grazing animals. The traceelements are required for many of the enzymes used in metabolic processes within the animal andin many of the energy transfer pathways. Some plants are very efficient in extracting some traceelements from the soil and may accumulate them in toxic quantities. For example Astragalus

bisulcata will accumulate selenium at levels up to ten times that which is toxic to sheep and cattleand is a frequent cause of death in these animals grazing range infested with this plant. Sweetclover ( Melilotus spp ) may accumulate molybdenum, if grown on soils of high Mo content, tolevels that may become toxic to livestock.

The mineral nutrition of livestock is very complex and goes beyond the scope of these lectures.However, it is important to understand that many of the minerals interact with each other. Forexample, the functions and requirements of an animal for P are very closely dependant on theamount of Ca in the diet. Ca requirement of animals is greatly increased when they are grazingplants which accumulate high levels of oxalates, such as Kochia ( Kochia scoparia ) or Greasewood


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(Sarcobatus vermiculatus ). Adequate levels of Cu in plants may become inadequate for animalhealth and production if the diet also contains high levels of Mo, in which case insoluble coppermolybdate is formed, or if the diet or water or both contain high levels of sulphates in which caseinsoluble copper sulphide is formed in the rumen. If both Mo and S0 4 are present, the Cu becomesbound as copper thiomolybdate and is almost totally unavailable to the animal. When this occurs,very high levels of Cu must be fed or Cu injected into the muscle of animals to prevent Cu

deficiency occurring. These high levels of Cu would be toxic to animals eating herbage and/orwater with low contents of Mo and/or S0 4.

When in a deficient state, all minerals will reduce feed intake, animal growth, reproductive functionand milk and wool production. In high concentrations all minerals can exert varying degrees of toxicity.

Vitamin A deficiency does not occur when grazing animals consume green herbage but may beimportant on arid ranges or where animals are required to graze senesced herbage for long periodsof time. Vitamin D rarely becomes deficient in grazing livestock because all animals, includingman, can convert ultra-violet rays from the sun into Vitamin D in the skin. In ruminant animals,the B vitamins, which are a large and complex group of vitamins, are rarely a problem with respectto deficiency because the rumen micro-organisms can synthesize these vitamins in adequateamounts to meet the ruminant animal's requirements. The exception to this is vitamin B12 whichcan only be synthesized if adequate Co is present in the diet.

The rest of the discussion in this section will be restricted to energy and protein because these arealmost always the first two limiting factors in the nutrition of grazing animals and rarely is amineral or vitamin deficiency uncomplicated by concurrent energy and/or protein deficiency.

Energy

Energy is measured as heat energy or as the heat of combustion of the feed. Energy in animalnutrition is partitioned into gross energy (GE), digestible energy (DE), metabolizable energy (ME)and net energy (NE). This is shown schematically in Figure 15.

It should be noted that GE for all plant materials varies very little from 4.37 kcals/g, with theexception of plants that have high levels of oil in the seeds or leaves. The latter may have GEvalues of 5 to 7 kcals/g. However, the digestibility of plant materials varies considerably betweenand within plant species depending on soil type, climate and plant phenology. Therefore, thedifferences in DE between and within plant species can be very large. Because of the difficulty of measuring energy loss in urine, methane and heat increment in grazing animals the energy nutritionof range animals is usually described in terms of DE.


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Figure 15. Partition of feed energy within ruminant animals.

The most important animal related factors limiting energy intake and utilization in rangelandanimals are:

a) digestibility of the herbage

Apparent Digestibility = Matter in Feed - Matter in Feces x 100 %Matter in Feed

True Digestibility = Matter in Feed-Matter in Feces-Endogenous Matter x 100%Matter in Feed

The greater the digestibility of the diet, the greater the amount of energy captured from the diet.

b) rate of passage of the digesta through the intestine and particularly the rumen

The faster the rate of passage, the faster the clearance of material from the rumen and hence thegreater the ability of the animal to consume more feed. Some animals have adapted to lowdigestibility diets by increasing the rate of passage. For example, the digestibility coefficient of aherbage is greater when fed to cattle than deer but the rate of passage through deer is faster. Thismeans that the rumen of deer is cleared more rapidly than cattle allowing more space for intake of additional feed. Because the coefficient of digestibility diminishes with increasing time of residencyin the rumen, this means that the energy intake is usually greater for deer than cattle whenconsuming the same feed. Also, plants differ in their rate of passage through an animal Forexample, legumes have a faster rate of passage than grasses of the same digestibility.


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c) metabolites of digestion

Nutrient energy in ruminants comes largely from the carbohydrates in the feed. Carbohydratesmake-up the biggest proportion of substances present in plant matter. The carbohydrates arefermented by the rumen microflora and microfauna and the end products of this fermentation arethe volatile fatty acids (VFA). The most important VFA's are acetic, butyric and propionic acids.

The higher the fermentation to propionic acid the more efficient is the transfer of energy fromplants (referred to as net primary product or NPP) to the animal (referred to as net secondaryproduct or NSP) as shown below:

Propionic acid TCA(2 mols of 3 C + 2H +)

109%Forage Cellulose

62%Glucose Pyruvic acid Acetic acid TCA

(1 mol of 6 C) (2 mols of 3 C) (2 mols of 2 C)Grain Starch

O2CO 2 + CH 4

78%

Butyric acid(1 mol of 4 C)

Fat

The efficiency of conversion of Pyruvic acid (CH 3 - C0 - C00H) to:Acetic acid (CH 3 - C00H) is 62% because CO 2 and CH 4 are lostPropionic acid (CH 3 - CH2 - C00H) is 109% because 2 atoms of H are addedButyric acid (CH 3 - CH 2 - CH 2 - C00H) is 48% because additional O 2 is lost.

The amounts and proportions of the volatile acids depend on the substrate (feed) and the varyingpopulations of rumen micro-flora and fauna fermenting the feed.

High cellulose diets, such as those consumed by range animals, lead to high proportions of aceticacid in the fermentation products. Thus the transfer of energy from plants to animals is inefficientat this point (Table 26).

Table 26. Typical fermentation patterns of hay, grain and range herbage diets. Acetic (%) Butyric (%) Propionic (%)

Good quality hay 60 9 25Grain 52 8 38RangeYoung green 65-70 10 20-25Mature dry 75-80 10 10-15


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Energy nutrition of ruminants is also greatly influenced by climate (see the section on winterfeeding) and other environmental conditions such as the terrain of the range or pasture. Forexample, topography will influence the energy cost associated with walking. Table 27 shows theadditional energy cost for an animal to walk in a vertical compared with a horizontal plane.

Table 27. The energy cost of an animal to walk 1m in a horizontal and vertical plane.

___________________________________________________Plane Energy cost (J/Wkg/m)___________________________________________________Horizontal 2Vertical 26___________________________________________________

The energy costs or walking, grazing and ruminating are considerably higher for cattle on range orpasture compared with cattle in pens (Table 28).

Table 28. % Increase in metabolic rate above resting for cattle in pens and on range whileperforming certain functions.

______________________________________________________Function Pens Range______________________________________________________Walking 6% 45%Grazing/eating 12% 30%Ruminating 4% 9%______________________________________________________

These increases are associated with the greater distance required for cattle to walk to obtain theirfeed and water on range compared with cattle in pens; the longer time required for prehending theirfeed; and the longer time spent ruminating because of the more fibrous nature of range foragecompared with hay, silage and grains fed to penned cattle. These, and other factors, result in cattleon open pasture and rangeland having maintenance energy requirements which are almost twicethose of cattle in pens (Table 29).

Table 29. The maintenance energy requirements of cattle at pasture and in pens.________________________________________________________

Maintenance Energy Requirements(KJ/W 0.75 /day)

________________________________________________________Pens 480Open pasture 740-830Rangeland 740-990________________________________________________________

Protein

Figure 16 provides a diagram of the pathways between dietary N intake and protein synthesis inthe ruminant animal. Note this is somewhat more complicated than Figure 15 because, unlikeenergy which passes through the system only once, nitrogen re-cycles within the ruminant just as itrecycles in soil-plant systems.


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Note that ruminants are unique in that they can utilize dietary non protein nitrogen (NPN) as asource of protein because of the ability of the rumen microbial population to convert NPN intomicrobial protein which is digested in the small intestine and becomes a very important source of amino acids for the ruminant.

Figure 16. The pathways between dietary protein intake and protein metabolism in the ruminant.

Also note that urea can be recycled back to the rumen via the saliva and ruminants can partially

offset a dietary protein deficiency by conserving urea excretion in the urine and diverting it back tothe rumen via the saliva. Urea has a very high concentration of nitrogen.

NH2 Feed grade (0 = C ) urea is 45% N or 281% CP equivalents. NH2

However, urea contains no precursors for the formation of a carbon skeleton for protein so highenergy feeds must be fed simultaneously to maximize microbial protein production if the animal isto receive maximum benefit from the urea. The energy must be readily available such as molasses or


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processed grain since, once in the rumen, the urea is rapidly hydrolyzed to NH 3 and absorbedthrough the rumen wall.

The conversion of urea to protein is outlined below.

Microbial urease

1. Urea C0 2 + NH 3

Microbial enzymes2. CH0 VFA + keto acids

Microbial enzymes3 . Keto acids + NH 3 Amino Acids

Microbial enzymes4. Amino acids microbial protein

Enzymes in abomasum& S.I.5. Microbial protein Free amino acids

6. Free amino acids are absorbed.

Note also that protein can be used as an energy source by the conversion of amino acids to glucose.

Distinguishing Between Protein and Energy Deficiencies

It is very difficult to distinguish between deficiencies of energy and protein under range conditions.This is because the two are so closely correlated. For example:

1) Low protein herbage also has a low digestibility.2) Protein deficiency in herbage leads to a protein deficit in the rumen which reduces

microbial activity and herbage digestibility is reduced. This in itself reduces the transfer of energyfrom the plant to the animal but it also reduces the rate of passage of the digesta and herbage intakeis reduced so energy intake is also reduced.

3) Apart from the influence of protein intake on rumen function, the amount of proteinreaching the small intestine also influences the amount of herbage consumed. This has been

demonstrated by the infusion of casein solution into the duodenum via a duodenal cannula. Infusedanimals consumed more herbage than animals infused with the same volume of liquid as water.This lead to an understanding of the role of 'by-pass' or non-degradable rumen protein in ruminantfeeds.

It is often claimed that energy is the first limiting factor to animal productivity on range. This hasbeen based on studies which have indicated that the amount of nitrogen present as NH 3 in therumens of animals on range was always in excess of the amount of N required by the rumen micro-organisms to sustain the level of fermentation (i.e. VFA production) taking place. Thus it has beenassumed that herbage intake, and subsequent productivity has been limited by the digestibility of the


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herbage consumed or the ability of the microorganisms to digest the herbage and liberate its energyto the ruminant animal in the form of VFA. This is sometimes supported by the general lack of response to urea supplements given to sheep and cattle on range compared with the positiveresponse from urea supplements which occurs when sheep and cattle are fed high energy diets, forexample in a North American feedlot.

While this is true at the rumen level it is not the whole story. This was first demonstrated in a seriesof experiments with cattle on winter range in Australia. Cattle were given supplements of (1)sorghum grain (high energy/low protein) and (2) by-pass protein (protein which was not degradedin the rumen but which was digested and absorbed in the small intestine). The supplements weregiven in factorial combinations and the range herbage had an organic matter digestibility of 40% andcrude protein content of 3.6%. The protein supplement was a mixture of 70% cottonseed meal,20% meatmeal and 10% fishmeal.The results of those experiments are summarized in Figure 17.

Figure 17. The effect of feeding an energy (sorghum) or protein (cottonseed, fish and meat-meals)on herbage intake of grazing beef cattle.

For every unit of ME provided in the sorghum supplement there was a reduction in ME intake fromthe herbage (i.e. the sorghum energy became a substitute for herbage energy). The correspondingincrease in liveweight gain was small, indicating that the energy limitations of the range herbagewere only small. On the other hand, there was a substantial increase in ME intake from the herbagewhen the protein supplement was eaten and a correspondingly large increase in liveweight gainindicating a major influence of protein deficiency on productivity of cattle on winter range. Thesame limitations no doubt apply to wildlife. The relationship between herbage organic matter intake(HOMI) and intakes of sorghum (S) and the protein supplement (P) is shown in the followingequation.


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HOMI = 2.96 - 0.76 S + 2.33 P = 1.45 P 2 (R = 0.79; P < 0.05).

Production data are shown in Tables 29, 30 AND 31.

Table 29. The effect of protein and energy supplements on weight gains of steers grazing winterrange from 6 to 15 months of age._________________________________________________________

ADG (kg/day)

No supplement -0.04560 g sorghum/day +0.10600 g by-pass protein/day +0.35

Table 30. The effects of a protein supplement on weight gain and pregnancy rate of heifersgrazing winter range from 0 to 15 months of age.

______________________________________________________ADG Pregnancy

(kg/day) %

No supplement -0.08 0800 g by-pass protein/day +0.5 92

Table 31. The effects of protein supplements on herbage intake and liveweight change of cows45-90 days post partum and on milk yield measured on day 71. (Calves were creepfed during the experiment so that production data are not applicable directly to cowsupplementation and are not included).

________________________________________________________________

Amount of Protein Supplement (g/kg W 0.75 0 5.3 10.6 15.9 21.2

Herbage Intake(kg DOM/day) 4.41 5.41 6.63 7.97 7.44

Liveweight Change(kg/day) -2.40 -2.37 -0.99 -0.06 +0.08

Milk Yield Day 71 (kg)

2.3 2.8 3.6 4.4 4.5

The calves were creep fed so their weight changes relative to cow milk yield were not reported.

Similar data have more recently been reported from USA and Canada (Tables 32 and 33)


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Table 32. The effect of cottonseed cake supplement on intake and weight change of steersgrazing blue grama grass in New Mexico from January to May.

Control Cottonseed Cake

Intake (g/kg BW) 10.8 12.9

Weight Change (kg/d) -0.03 0.24

Table 33. The effect of canola meal supplements on cow weight changes when grazing roughfescue pasture in Alberta from December 1993 to January 1994.

Canola Supplement(kg/d)

Weight Change(kg)

Back Fat(mm)

0 -35.1 -1.050.4 -46.1 -0.94

0.8 -25.1 -0.561.2 -19.5 -0.44

Relationship Between the Quantity of Intake and the Q uality of the Herbage Consumed .

Because of the difficulty of measuring how much a grazing animal eats and the quality of theherbage which it consumes while on range there are few relationships reported in the literatureshowing the effect that herbage quality has on the quantity of herbage consumed. One report forcattle is cited as an example:

OMI/W 0.75 = 0.107* DIG + 0.030 * N (R 2 = 0.84; RSD = 0.022; P


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PASTURE MANAGEMENT TO ENHANCE THE NUTRITION OF CATTLE

"The range resource is by far the most variable commodity that is encountered in livestock nutrition. It can vary in nutrient quality from a low comparable to sawdust to a high comparable to

top quality alfalfa. It is capable of producing gains approaching 1.4 kg/d in growing classes of livestock at certain times of the year or of allowing starvation in the same cattle at other times."This quote is taken from Raleigh and Lesperance (1972) in Church et al. Practical Nutrition.

It is also very important to remember that the beef cow has her greatest nutrient requirements whenshe is at pasture because this is when she is lactating and re-breeding (Figure 2). Also, the calf hasincreasing nutrient requirements that must be met from the pasture in order to maintain goodgrowth rates. So, arguably, the grazing season is the most important time in a beef operation.However, unfortunately it is often the most neglected part of a beef operation.

Factors affecting nutrient intake and productivity of grazing animals

The factors influencing nutrient intake and productivity of cattle on range or pasture aresummarized in Figure 17.

Chemical composition

Nutritional DigestibilityValue

Type and proportions of end-products of digestion

Animal Nutrient Forage yield and physicalProductivity Intake availability

Physiological state of the animal

Environmental factors such asForage temperature,humidity,Intake topography

Plant acceptability

Digestibility and rate of

passagePasture structure

Figure 17. Factors affecting the intake and productivity of cattle on range or pasture.

As the grazing season advances plants mature with a subsequent decline in their nutritive value andthe productivity (liveweight gain) of herbivores grazing them Figure 18).


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Figure 18. Effect of plant phenology on forage quality

Figure 19 shows schematically how the digestible protein and energy contents of range and pasturedeclines with advancing maturity and the consequent decline in liveweight gains of yearling cattleand calves.

Getting the most out of the grazing season

The phenological cycle of growth is not the same for all species of plants. Some plants start growthearly in the spring (cool season plants) and some do not commence growth until late in the springor even early summer (warm season plants). The commencement of growth is temperaturecontrolled, but it can be ameliorated by soil moisture and fertility. Similarly, not all plants enter thereproductive phase at the same time or senesce (death of the above ground parts leaves andstems) at the same time. Some plants have a short growing season while some plants have a longgrowing season. For example, crested wheatgrass starts growth early but has a short growing

season and therefore matures early and loses quality rapidly so that its nutritional value is poor aftermid July. In areas where late summer rainfall is reliable, crested wheatgrass will provide valuableforage from re-growth. However, in areas where summer is not reliable the reliability of re-growthis reduced and cannot be counted on providing useful grazing beyond mid-July. In contrast,Russian wild ryegrass, commences growth almost as early as crested wheatgrass but more slowly.However, it has a much longer growing season and holds its nutritional quality much longer thancrested wheatgrass. Therefore, it is a good grass for late season grazing. This means that these twograsses complement each other and can be used in a complementary grazing system in which afield of crested wheatgrass can be used for early season grazing and a field of Russian wild


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Figure 19. The amount of digestible nitrogen and energy require for maintenance and growth of yearling steers (A,B,C) and cows & calves (D,E,F) on range.


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ryegrass can be used for late season grazing. Complementary grazing and/or various forms of rotation grazing such as those described in the Managing Saskatchewan Rangelands book ( http://www.agr.gov.sk.ca/saf/default.htm ) can be used to both lengthen the grazing season and toincrease the quality of the forage available to the grazing animal, thus increasing animal production.

Proper rotation grazing can achieve two important grazing management objectives. It can increase

the period of vegetative growth by forcing the plant to produce more vegetative tillers and this inturn increases the quality of the herbage since new vegetative growth is always of greaternutritional quality than mature growth. The extent to which this can be used depends, however, onthe availability of moisture. Without moisture plants do not grow. Therefore, the practice of rotation grazing usually is more beneficial in the more mesic (wetter) regions compared with morearid (dry) regions.

Examples of the different growth curves of different species are given in Figure 20

Figure 20. Relative yield and period of growth of native grass and seeded pastures.

Examples of how the information in Figure 20 can be used to design various complementarygrazing systems for the different soil zones of Saskatchewan are given in Figure 21.


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Figure 21. Grazing systems for Saskatchewan soil zones.


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PRINCIPLES OF RANGE MANAGEMENT

Basic terms in Range Science

1) Climax communities. These are plant communities in equilibrium with their environment.

2) Succession. There are four types of succession in range ecosystems:a) Primary succession. This refers to the establishment of plants on land not previously

vegetated. For example, on the edge of a gradually filling slough or a sand bar in a river.

b) Secondary succession. This refers to the invasion of land that has been previouslyvegetated. For example after fire, logging, cultivation.

c) Progressive succession. This leads to communities with greater and greatercomplexity and biomass and to habitats that are progressively less severe and more stable.

d) Retrogressive succession. This leads backwards toward communities with lowerdiversity and less moderate habitats that are less stable. Caused by drought, fire or grazing.Avoidance of this type of succession is a primary objective of range management.

"By far the most important factor contributing to retrogression on range is improper grazing"(Stoddart, Smith & Box, 1980) and this is the major problem facing the world's rangelands today.

Stages in Grazing Retrogression

There are two major stages in retrogressive succession:

1) Physiological disturbances of the climax plants. For example, the most preferredclimax plants are grazed heavily, lose vigor and produce less.

2) Composition changes in the climax cover. For example, reduced photosynthesis in theclimax species as a result of overgrazing encourages competition from other less palatableplants that may eventually eliminate the preferred palatable plants.

Those plants most susceptible to grazing pressure are called DECREASERS. Some examples of range plants that are decreasers are winter fat, rough fescue, northern wheatgrass, greenneedlegrass.

The less preferred or more resistant plants which are at a competitive advantage are calledINCREASERS. Some examples of range plants that are increasers are the grasses - Blue grama,June grass, needle and thread; forbs such as pasture sage, club moss, yarrow; and shrubby plants

such as cinquefoil, silver sagebrush, western snowberry and rose.Continued grazing pressure may cause reductions in the increasers and lead to the invasion of weedy species called INVADERS. Some examples of common invaders are dandelion, Canadathistle, kochia, lambs quarters and leafy spurge.

These changes are illustrated in Figures 22 and 23.


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SUCCESSIONAL % CLIMAX SPECIES CONDITIONSTAGE CLASS

Figure 22. Approximate relationship between range condition and degree of retrogression froma climax ecosystem.

Figure 23. The effect of grazing intensity on range condition and the relative proportions of decreasers, increasers and invaders.

CLIMAXCOMMUNITY EXCELLENT

GOOD

FAIR

POOR

WEED STAGE

75-100%

51-75%

26-50%

0-25%


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Several things can be done to reduce the extent of retrogressive succession on rangelands.

1. Practice Good Range and Pasture Management

3) Don't graze too early.

Defoliation during the early growth period of plants reduces leaf area index that reduces theconversion of solar energy to plant energy by photosynthetic tissue in the leaves. This reducesplant vigor and the ability of the preferred (grazed) species to compete with the non-preferred(ungrazed) species. It also reduces energy storage in the roots. Therefore, the grazed plants do notcompete well with non-preferred species for light, moisture and nutrients in subsequent years witha resulting long-term change in range botanical composition and condition. Cattle in Saskatchewanshould not graze native range until mid-June. Although many rangelands in Saskatchewan aregrazed in mid-May, research at Swift Current has shown that their yield (and hence carryingcapacity) can be increased by 50% or more if grazing is delayed until early to mid June (Table 34).

Table 34. The production of a mixed grass prairie at Swift Current based on clipped plot yields.________________________________________________________________________________Date of First Summer Yield % Increase in YieldClipping Kg/DM/ha Due to Delayed Clipping________________________________________________________________________________

May 16 664 -June 5 1067 60.7June 20 1137 71.2July 5 1218 83.4______________________________________________________________________________

Delaying grazing also reduces the risk of ingestion of toxic plants since many toxic plants growearly in spring and at that time may make up a high proportion of the available forage.

4) Don't graze too heavily.

Approximately 40-45% of the total herbage yield should be left to carry over through winter for anumber of reasons:

1 allows storage of energy reserves in roots2) allows trapping of snow and moisture penetration in spring is increased3) ensures adequate seed production4) reduces water evaporation from the soil

5) reduces wind erosion6) reduces soil compaction and erosion from rain7) protects the crown of the plant from winter kill8) increases soil temperature in the winter thus increasing plant survival9) decreases summer soil temperature which increases plant growth

3) Use Appropriate Grazing Systems

(i) Rotational Grazing(ii) Short Duration Grazing - High Intensity/Low Frequency


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5) Deferred Grazing6) Complementary Grazing

These grazing systems

1) allow recovery of plant species which are susceptible to grazing

2) allow established plants to gain in vigor which increases survival3) allow cattle to obtain more palatable herbage4) allow more uniform utilization of the sward5) range improvement techniques are easier to apply because of smaller areas6) animal distribution is easier to control because of smaller areas7) breeding efficiency increases due to smaller areas8) possible internal parasite control.

However,

9) must have sufficient water for each field10) fencing costs are high - can reduce costs by using solar generated electric fences11) more frequent handling of livestock 12) fire hazard of accumulated forage ??13) increased requirement for breeding bulls ??

3. Range Improvement

Seeded range and the use of several species of grass to complement native range can extend thegrazing season and the nutritive quality of the herbage during the grazing season. Fire can be usedto control undesirable and woody species. Although the use of fertilizer on native range is usuallynot economical, it will increase herbage yield and quality that result in increased carrying capacityand animal production. Fertilizer is more likely to be an economic proposition if applied to seededpasture than to rangeland because seeded species have a greater potential to respond to increasedfertility. Control of toxic plants and weedy invader species.

4. Livestock Distribution

Proper distribution of stock-watering sites, salt, and shelter ensures better use of pastures.Grasslands more that 1km (3/4 mile) from watering sites will be under-grazed. Salt blocks,mineral feeders and oilers should be located away from water and occasionally relocated to attractlivestock to under-grazed areas. Range riders can be used to ensure good animal distribution.Cross fencing, though expensive, will frequently provide economic returns affording bettergrazing control and carrying capacities.

5. Provision of Supplements

Energy supplements such as hay, grain, range pellets, range cubes, etc. provide a substitute source

of energy for grazing animals and as a result they will reduce their grazing activity and intake of range herbage. Thus if range or pasture is being overgrazed, cattle should be removed or if this isnot possible, provision of an energy supplement will relieve the grazing pressure on the range.

Protein supplements, particularly those with a high rumen by-pass value, on the other hand,stimulate herbage intake so should only be provided in situations where the range herbage hasmatured or senesced and is in abundant supply.

Mineral supplements should be available at all times. These typically should contain calcium,phosphorus and trace minerals (Cu, Se, Zn, Mn) depending upon local soil types and deficiencies.


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Cobalt iodized (blue) salt should be available at all times and consumption calculated on the basisof 1.5 kg/cow/month in spring reducing to about 0.125 kg/cow/month in July-August.

Creep Feeding

Energy supplementation of calves on range and seeded pasture to alleviate their increasing inabilityto meet the calf's energy requirement for satisfactory growth (i.e. about 1kg/day). The creepconsists usually of ground good quality hay and grain or straight grain, preferably oats, as oatsprovide fewer digestive upsets. Consumption will increase from about 0.25 kg/day at 1-2 monthsof age to 1.5 kg/day at 7 months of age. More creep will be required where the forage supply isseverely limited.

STOCKING RATE, GRAZING PRESSURE AND CARRYING CAPACITY

The following definitions are taken from Terminology for Grazing Lands and Grazing Animalsprepared by the Forage and Grazing Terminology Committee and officially adopted by the RangeManagement Society. A thorough understanding of the terminology is essential to the study of range management. Although officially adopted in 1991, there is still considerable confusionamong range management personnel and who have adopted several variations of these definitionswith detrimental consequences to range management.

Stocking rate is the number of animals per unit area of land for a stated time.

Stocking density is the number of animals on a specified unit of land at a particular point in time.

Grazing Pressure or Grazing Intensity refers to the number of animal units or forage intake unitsper unit of available forage dry matter at any one point in time.

An animal unit (AU) is one non-lactating bovine weighing 500 kg and fed at maintenance level. Itis expressed as weight 0.75 in other kinds of animals.

A forage intake unit is an animal unit with a rate of forage consumption equal to 8 kg dry matter / day. This is based on the assumption that the maintenance requirement of a mature non-lactatingbovine weighing 500 kg is 8 kg of forage dry matter / day whether the forage has a digestibility of 80% or 40% which is a false assumption.

An animal unit month (AUM) is the amount of forage consumed by one animal unit for one month.

Forage allowance is the amount of forage dry matter available per animal unit or per forage intakeunit at one point in time.

The Carrying capacity of a pasture is the maximum stocking rate that will achieve a target level of

performance, in a specified grazing system, that can be applied over a defined time period withoutdeterioration of the ecosystem. The carrying capacity of a pasture is not static from season toseason or year to year. Therefore, the average carrying capacity refers to the long-term carryingcapacity averaged over a number of years.

The term Animal Unit Month is commonly used in Saskatchewan but its use is not withoutproblems. Firstly it is not always clear what definition of animal unit is used and whether all partiesare using the same definition. The old definition of an animal unit was a 1000lb (450kg) cow withcalf at foot and that of a forage intake unit was 30 lb/day (13.5 kg). If this definition is still beingused it is not appropriate since most cows in Saskatchewan now weigh at least 550kg with many


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weighing 600-700kg. The forage intake unit for a lactating cow weighing 550kg using the currentdefinition would be about 13.5kg DM when allowance is made lactation for the additional weightof a mature cow. This is close to the old value of 13.7kg. However, it is not clear if allowance ismade for pasture intake of the calf, which could average at least 3kg/day over the grazing season.Nor is it clear if allowance is made for the removal of only 55% of the available herbage that isrecommended in order to sustain range condition and productivity. Current publications in

Saskatchewan still define a yearling steer as 0.7 AU which suggests that the old definitions are stillbeing used, at least by some agencies, since, in Saskatchewan, a backgrounded yearling steergoing on to pasture in spring would probably weigh 300-350kg (700-750lb). That steer could beexpected to gain 70-120kg (150-250lb) while on pasture. This means it would be equivalent to0.7-0.95 AU.

Effects of Stocking Rate on Pas ture

Pasture production is increased by defoliation. Defoliation stimulates the production of new growth(tillers). The rangelands of the Northern Great Plains and Prairies evolved under grazing. Themacro-grazer during this evolution was the Bison. The macro-grazer of today is the beef cow. Thedeterioration of rangelands during the last 100 years is not the fault of the cow per se but the wayin which we have allowed the cows to graze. Therefore, the lobby to remove cattle form westernrangelands in the US is mis-guided since this will not result in the return of the original climaxcommunity but a different climax community. The encroachment of Aspen trees into the parklandregions of the prairies is a living example of progression towards a different climax community asa result of changing land use following settlement. Before settlement, the First Nations peopleburnt these areas to maintain them as grasslands in order to attract the Bison. Since settlement,these areas have not been burnt which has resulted in the encroachment of aspens on the un-ploughed areas of the parkland regions.

However, pasture growth rate is reduced at high stocking rates. The reduction in leaf areafollowing intense grazing pressure results in reduced interception of solar radiation by the leaf andhence lower photosynthetic activity in the plant. The carbohydrate reserves in the roots andcrowns of heavily grazed plants are therefore reduced. Pasture growth at high stocking rates mayalso be reduced by exhaustion of soil moisture because of greater evaporative loss directly from thesoil, if grazing increases the amount of bare ground, or through the plants, if the stimulation of new growth increases the amount of evapo-transpiration through the plant. However, pasturegrowth may also be reduced at low stocking rates because of increased senescence and a greaterproportion of matured leaves that are photosynthetically less efficient. It, therefore, is veryimportant to define these extremes if the most efficient use of the pasture is to be made.

High stocking rates frequently increase the proportion of less palatable species (increasers). Also,reduction in the proportion of palatable legumes (decreasers) in a seeded pasture will not onlyreduce the overall palatability of the pasture but it will also reduce nitrogen fixation and henceoverall pasture productivity. Excessive grazing creates an unstable pasture community and adepleted ecosystem.

Stocking Rate and Soil Properties

High stocking rates increase the bulk density of the soil and decrease pore space (Tables 35 and36). This reduces the rate of water infiltration and increases the amount of run-off water therebyincreasing the risk of erosion. This compaction occurs because of three things:

i) the direct effect of treading or trampling by the animals which increases at high stocking rates.ii) a decline in root mass in the soil following the weakening of plants with high stocking rates andiii) reduced protection of the soil from the erosive action of wind and rain.


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Table 35. The effect of static loads exerted by vehicles and animals when stationary on range.

LOAD kg/cm 2

Crawler tractor 0.32-0.63

Sheep 0.65Wheel tractor 1.4-2.1Horse and cow 1.7Truck 3.5-7.0

Table 36. The effect of degree of range use (stocking rate) on soil pore space and moistureinfiltration rate.

Degree of RangeUse

Soil PoreSpace (%)

MoistureInfiltration Rate

(cm/hour)Moderately grazed 68.1 10.5Over grazed 59.1 5.5Depleted 46.5 2.1

Stocking Rate and Animal Nutrition

The direct effect of high stocking rates is to reduce the amount of herbage available to each animalon the pasture and so to restrict individual animal intake. This will result in a decrease in animalproductivity. This is especially important at those times of the year when herbage is in short supplyand/or when the physiological demands of the animals are high, such as in late pregnancy and earlylactation. Increasing stocking rate to increase production per unit area of land may therefore requiresome adjustment of calving dates to match nutritive demands with feed supply.

Stocking rate, if it affects botanical composition, which it invariably will, may also affect dietarydigestibility and protein content and so have a further indirect effect on feed intake and utilization.

Stocking Rate and Animal Production

The effects of stocking rate on animal production are greatest at those times of the year when thefeed supply is inadequate. In Canada this corresponds to late fall through early spring. Highstocking rates result in reduced animal liveweight gains, particularly at these times of greatest feed

shortage. This is particularly important at breeding since estrus and ovulation rates are closelyassociated with liveweight gain. As a result the number of offspring per 100 females is reduced athigh stocking rates. Increases in stocking rate also cause reduced milk production with a resultingdecrease in the growth rate of the offspring. Wool growth is also decreased with increasingstocking rate in sheep. In addition, at very high stocking rates the wool fibre may be weakened("tender" wool) and more likely to break thus reducing the spinning quality of the wool.

Finally, animal health may be adversely affected at high stocking rates. Increased mortality islikely at high stocking rates due to under-nutrition. Increased wear on the incisor teeth is commonthus reducing the animals' foraging ability and shortening their life span. The increased wear on


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the teeth is caused by the shortness of the leaf length of overgrazed grass causing the animals tocontinually eat very close to the soil. There is also evidence that parasite burdens increase at highstocking rates and bacterial and viral diseases are more readily spread at high concentrations of animals.

Quantifying Animal Production in Relation to Stocking Rate

There is considerable debate among scientists over the relationship of stocking rate to animalproduction. It is unlikely that the shape of the curve will be the same for all pasture types and allanimal production traits. There are three most likely shapes (Figure 24 a, b and c).

ADG ADG ADG(a) (b) (c)

Stocking Rate Stocking Rate Stocking Rate

Figure 24. The relationship between stocking rate and average daily gain (ADG) of beef cattle.

In figure 24(a) the plant response to increasing stocking rate is very small at low levels of stockingrate. Thus the effects of stocking rate on herbage yield, availability, botanical composition andnutritional value are very small and barely detectable in terms of animal product. As the stockingrate increases, however, the effects on pasture botanical composition, yield, availability andnutritional value become increasingly important and more readily detectable in declining animalproduct. The rate of decline in animal production, therefore, increases at a more rapid rate for eachincremental increase in stocking rate.

In Figure 24(b) very low stocking rates have a detrimental effect on animal production. This maybe caused by increased senescence and hence reduced nutritional value of the pasture at these lowstocking rates. In theses cases a moderate increase in stocking rate will increase the rate of defoliation which will stimulate vegetative re-growth and tiller production. However, further moresevere increases in stocking rate will cause more severe defoliation with resulting changes inbotanical composition and productivity of the pasture which will ultimately reduce animalproduction. Another example is the development of wolf plants in bunch grass species such ascrested wheatgrass, Russian wild ryegrass etc. at low stocking rates. At low stocking rates, cattlewill not consume all plants in a pasture at the same rate. Thus some will be left ungrazed and will

become coarse, senescent and unpatatable. Cattle will avoid these plants and prefer to return topreviously grazed plants with new regrowth and tillers that are much more palatable and nutritious.These plants become overgrazed while the ungrazed plants become more coarse and less and lesspalatable. The leaves and stems of these unpalatable plants die but do not decay during winter.New growth the following spring commences from the crown and is protected by all the remainingdead, coarse unpalatable material. As a result, the new season growth remains untouched while thenew season growth on those plants grazed the previous season becomes further overgrazed.Eventually, these ungrazed plants become large and rank and are termed wolf plants while therest of the pasture becomes increasingly overgrazed.


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However, most experimental results indicate that the stocking rate curve is linear (24c). Forexample, Jones and Sandland (J. Agric. Sci. Camb. (1974) 83:335) examined data from a largenumber of stocking rate experiments and concluded that in all cases the relationship did not departsignificantly from the linear model

Y = a - bxwhere Y = animal production and x = stocking rate and a,b are the regression coefficients of

intercept and slope. They therefore concluded that the relationship between animal production andstocking rate could be determined from just two stocking rates spaced anywhere along the line. Itseems unlikely that this is true at the extremes of stocking rates and experimental linearity is mostlikely due to the choice of stocking rates by the researchers within the linear part of figures 22(a)and 22 (b).

This choice has probably been a conscious one by researchers because:

i) this part of the curve is of more interest and value in terms of predicting productiveefficiency

ii) very low stocking rates are expensive and impractical because they require a large area of land

iii) very high stocking rates are also expensive because animal productivity is very low andmay even result in animal mortality (death).

Knowledge of the value of the regression coefficients a and b can provide the pasture manager orresearcher with a considerable amount of information. For example, the intercept coefficient "a"provides a dimensionless estimate of the quality of the grassland herbage. This is because the lineintercepts the Y axis (animal production) when x = 0 or at zero stocking rate. At zero, or morecorrectly in mathematical terms, infinitely low stocking rate, there is an infinite amount of herbageavailable per animal so animal production is not restricted by herbage availability. If herbageavailability is not restricted then if animal production is less than the maximum genetic potential forthe animal, production in a normal healthy animal must be restricted by the quality of herbage onoffer.

Similarly, the regression coefficient for slope, "b", provides a dimensionless estimate of the yieldor growth response of the pasture to increasing stocking rate or the effect of stocking rate on thequantity of herbage available to the animal. For example, a high value for the "b" coefficientindicates a rapid and severe degeneration of the range or grassland with increasing stocking rate. Alow value for "b" indicates only a small effect of stocking rate on the grassland ecology.

Consider, for example, Figure 25 (a) and (b) showing the response in average daily gain (kg) of cattle grazing two grasslands A and B. In Figure 25(a), the slope of the regression of ADG (Y) onstocking rate (x) is the same for both pastures that is b A = b B). Thus both grasslands are showingthe same response to increasing stocking rate. However, the intercept a A is much greater than theintercept a B indicating that at all levels of stocking indicating that the nutritional value of grasslandA is greater than that of grassland B.

In Figure 25(b) however, it can be seen that at an infinitely low stocking, or in the climax state,grassland C has a greater nutritional value than grassland D but it is more sensitive to increases instocking rate than is grassland D.


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ADG (Y) ADG (Y)

(a) (b)aC

aAbA aD bD

aB A DbB bC

B C

Stocking Rate (x) Stocking Rate (x)

Figure 25. Examples of possible responses of different pastures to increasing stocking rate.

Furthermore, if the effect of stocking rate (x) an individual animal production (Y) is linearY = a - bx

then multiplying both sides of the equation by x gives

Yx = ax = bx 2where Yx now = animal production per unit area of land.

This relationship is now shown in Figure 26.

Figure 26. The relationship between stocking rate and average daily gain / animal and / ha.


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It can be shown mathematically that:3) Maximum animal production per unit area of land (Y max) occurs at a stocking rate equal to

a/2b.

This is defined as the optimum stocking rate since higher or lower stocking rates will result indecreased animal product per unit area of land.

ii) Optimum stocking rate occurs when animal production per head is half that which ispossible at zero, or infinitely low, stocking rate (a/2)

iii) At twice the optimum stocking rate a/b, animal production per unit area of land becomeszero.

This method of calculating optimum stocking rate is known as the half intercept method. However,it should be noted that it is only appropriate if the grasslands reach equilibrium at each of thestocking rates applied. That this may take several years. Further, the production per animal at Y max(a/2) must be compatible with production goals. For example, stocking 100 steers on 100 ha mayallow for the sustained expression of Y max or maximum beef production per unit area of land butthis may be achieved at individual animal gains which are less than the goals of the pasturemanager. Therefore to meet these goals, the stocking rate x must be reduced.

In addition, there are confounding factors when a multi-specific pasture is being consideredbecause the optimum time to commence grazing will vary for each species of plant. Therefore, thepasture should be managed for the key species, which may or may not be the most susceptibledecreaser species. Thus grazing multi-specific pastures will always involve an element of compromise between maintaining good range condition and maximum animal production.

PALATIBILITY, PREFERENCE AND SELECTION

Palatability and preference are plant/animal interrelated factors. For example, preferred plants areobviously palatable. In most cases preference refers to animal reactions to a plant and palatabilityrefers to the plant characteristics that promote that reaction.

Selection expresses the degree to which animals harvest plants or plant parts differently fromrandom removal. The selectivity ratio expresses the dietary composition in proportion to rangecomposition. The higher the selectivity ratio the greater the selection or preference (Figure 27).

The answers to the following questions are important to the understanding of grazing animalbehavior, nutrition, production and the effects of the grazing animal on range and pasturecondition: Why does a single species of animal have a preference for particular plants or particularparts of plants? Why do different species of animals have different preferences? (Table 37) Theanswers are complex and by no means complete. While there is a great deal of information on whatherbivores eat there is very little information on why they eat it and because of the complexity of

the subject only a brief summary can be presented here. Some very broad generalizations arepossible:

1) Green plant material is preferred to dead plant material2) Grass and legume leaves are preferred to stems3) Different species of animals select different diets (Table 37)4) Within any one species of animal the diet selected varies with the season (Table 38).


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Selection

SHEEP CATTLE

Legumes 2.22.0

1.8 Legumes

Forb Leaves 1.6

Leaves 1.4

1.2

1.0 Grasses

Grasses 0.8 Leaves

0.6

0.4 Forb Leaves

Figure 27. Selectivity ratios exhibited by sheep and cattle in California. Plants or plant parts at thetop of the scale were highly preferred while those at the bottom of the scale were poorlypreferred.

Table 37. The selectivity ratio for six different plant species grazed by sheep and cattle sharing thesame rangeland in California, USA, during summer._____________________________________________________

Plant Species Selectivity RatioSheep Cattle

_____________________________________________________

Phalaris aquatica 10.0 2.6

Stipa pulchra 3.1 2.2Trifolium spp 1.8 1.9

Bromus spp . (annual) 0.9 1.0

Avena barbata 0.4 0.6

Aira caryophyllea 0.3 0.5_____________________________________________________


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It will be noticed from Table 37 that sheep are more highly selective than cattle. The gradient fromthe most preferred species ( Phalaris aquatica ) to the least preferred species ( Aira caryophyllea ) ismuch wider for sheep than for cattle but the order of preference is the same. It is also apparent thatsheep exert a much higher degree of grazing pressure on Phalaris aquatica than do cattle. This

plant is well adapted to grazing but, if it were not, it is obvious that sheep would deplete thepasture of this species at a much faster rate than cattle.

Table 38. The selectivity ratios for grasses, forbs and browse of goats grazing rangeland in Texasduring the four seasons.

___________________________________________________________

Season Selectivity RatioGrasses Forbs Browse

___________________________________________________________

Winter 0.7 --- 1.6

Spring 0.7 8.3 1.0

Summer 1.1 8.0 0.7

Fall 1.0 4.0 0.8___________________________________________________________

It can be seen from Table 38 that goats selected grasses in close equilibrium (balance) with theirpresence on the range in all seasons. The selection of forbs was very high during spring, summer,and fall and non existent in winter, probably because after such high selection pressure very fewforbs remained through winter. The preference of goats for forbs has lead to the practice of usinggoats to "clean up" weedy pastures. During winter the goats turned more selection pressure ontobrowse.

There are three types of ungulate herbivores:

1. Bulk/roughage feeders such as cattle and bison. These animals graze comparativelyindiscriminately, use the tongue to gather the food into the mouth followed by a short jerkingmotion of the head during the biting process. They take comparatively large quantities of herbage with each prehensile motion. They have a large rumen volume in proportion to theirbody weight and a long retention time of ingesta in the rumen and as a result intake is low inrelation to their body weight. A relatively large proportion of the rumen biota are protozoa.

2. Concentrate feedrers such as deer, moose and horses. These animals characteristically havesoft pliable lips and in the case of horses, upper and lower incisors. They are therefore able tobe highly selective with a comparatively small bite size. They are able to eat very discretelyselecting single leaves or even parts of leaves. In contrast to bulk/roughage feeders, concentratefeeders have a comparatively small rumen volume in relation to body weight, rumen retentiontime is low and as a result intake is high in relation to their body weight. Only a smallproportion of the rumen biota is protozoa.


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3. Intermediate feeders such as goats, elk and carribou. The diets of these animals arecharacterized by variety and frequent compositional changes but they are also able to exert ahigh degree of selectivity. Sheep have many characteristics of both intermediate andbulk/roughage feeders. Other characteristic are intermediate to bulk/roughage and concentratefeeders.

Palatability

a. Physical Factors

(i) Presence of awns, spines, hairs, stickiness and coarseness of texture all influencepalatability.

(ii) Growth stage. As herbaceous plants mature they generally decrease in palatability andnutritive value. The whole plant becomes higher in fibre content and the leaf : stem : fruit ratioschange towards a higher proportion of stem. Succulence decreases and coarseness increases dueto the position and extent of lignification. (Deer will eat twigs in spring and summer when they aresucculent but avoid them in winter when they are mature).

In rare instances, the palatability of some species, for example Medicago hispida (Burclover)increases as maturity progresses. However, this may be associated with a reduction in alkaloidcontent as the plant matures (see section b.(v).

(iii) Environment - Climate, topography and soil all affect palatability of plants and animalpreferences for plants. For example, a simgle plant species on different sites will vary in chemicalcomposition, succulence, proportion of leaf (or leaf:stem ratio) and coarseness of the foliage.Different animals prefer different sites and site affects their selection of food (see later section onanimal behavior). Moisture from rain or dew may increase the palatability of plants.

(b) Chemical Factors

(i) Sugars - the literature is confusing on the influence of sugars on palatability. For everyreport of a positive correlation between total sugar content (or soluble carbohydrate content) andpreference there is a report that they were not correlated. One of the problems in sorting thisrelationship out is that the methods of extraction between various studies has differed and there isusually some doubt as to what carbohydrates were actually being determined because of thecomplexity of plant carbohydrate chemistry. However there are several reports that whenunacceptable herbage was sprayed with sucrose or molasses it was eaten and this was the basis of the early development of molasses/urea feeding or spraying dry standing pasture and crop stubblewith urea/molasses to increase its value as a feed.

(ii) Organic acids - Citric malic, malonic, succinic, quinic and shikimic acids have shown a closepositive correlation to preference in sheep.

(iii) Tannins - these substances have been negatively correlated with preference.

(iv) Coumarins - To humans these have a sweet smell and bitter taste. The smell of coumarin isobjectionable to sheep and some plants (eg. Melilotus spp such as sweet clover ) contain highconcentrations of coumarins and these may be the reason for poor acceptability of these plants bysheep. However, sweet clover is very palatable to cattle.

v) Alkaloids - A large number of plants contain alkaloids in varying amounts (as much as 2.5%of the dry matter of some plants) and these substances reduce the palatability of plants. In most


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(iii) The structure of the plant canopy. For example, an experiment in Australia studied thereasons for a lower milk production when dairy cows grazed Setaria sphacelata compared with

Digitaria decumbens . The two pastures were not different in the amount or nutritive value of thedry matter available to the cows. However, the cows grazing the Setaria sphacelata pasture took significantly more bites per kg of dry matter consumed and consumed significantly less dry matterin a day than did cows grazing the Digitaria decumbens . The cows grazing the Setaria sphacelata

produced less milk because they consumed less forage and the researchers concluded that they ateless forage because the Setaria sphacelata was more difficult to prehend and the cows sufferedfrom grazing fatigue. This phenomenon was attributed to the differences in canopy structure.Setaria sphacelata is a tall bunch type grass while Digitaria decumbens is aggressivelystoloniferous, short and more dense, thus making it easier for the cows to prehend. Anotherexample is that of a wolfy bunchgrass such as crested wheatgrass or Russian wild ryegrasspasture. For example it has been reported that as much as 60% of the total forage in a wolfy crestedwheatgrass pasture on highway 19 south of Saskatoon was present as wolfy plants that wereunacceptable to cattle.

(c) The Role of Special Senses

The senses of sight, touch to the lips and/or tongue, taste and smell are all involved in dietpreference and selection.

Sight appears to be used primarily to orient animals to other animals and their environment. Whileanimals do use sight to recognize conspicuous food plants this sense appears to be relativelyunimportant in selecting a preferred diet. For example, one experiment compared the diets selectedby blindfolded (sight impaired) and non-blindfolded sheep and it was found that there were nosignificant differences between the two groups of sheep with respect to the types or parts plantsselected or the selectivity ratios (Table 39).

Table 39. Effect of Sight on Herbage Selection by Sheep

_________________________________________________________

Pasture Species % of Herbage Eaten+Sight -Sight

_________________________________________________________

Phalaris tuberosa 65 55 Medicago sativa 95 85Trifolium pratense 50 60Festuca arundinacea 90 85

Eragrostis curvula 0 10 Dactylis glomerata 65 75Stipa hyalina 55 55

Lolium perenne 45 45_________________________________________________________

The same researchers conducted a series of experiments in which sheep were treated surgically toproduce single or multiple impairment of the senses of touch, taste and smell. They found thatwhile all these three senses were used to determine preference, taste was the most important. Forexample, only in one of five cases did smell impaired sheep select a diet different from normalsheep while taste impaired sheep consistently selected a different diet from normal sheep and touchimpaired sheep always selected a similar diet to normal sheep. In these experiments, 140 different


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plant species were available to the sheep so a large opportunity existed for the sheep to displaychanges of preference when sense impaired.

Since taste appears to be the most important sense used in diet selection, it seems likely that aconsiderable amount of herbage that is prehended must be rejected and not consumed, since the

material must first be taken into the mouth before being tasted. While this rejected material isreturned to the pasture as plant litter and ultimately decomposed and re-cycled it may neverthelessrepresent a considerable amount of unnecessary defoliation of plants that are not consumed.

(d) The Role of Physiological Status on Preference.

The animal's response to sense stimuli is moderated by its current nutritional status. For example,hungry animals usually have lower thresholds of rejection for taste. These thresholds (criticallevels) are determined both by the number and kind of molecules transmitted from the plant to theanimal and by the number and type of receptors in the animal. Thus, palatability is probablydetermined largely by the number and kind of molecules transmitted from the plant to the animaland preference by the number and type of receptors in the animal that can receive and disseminateinformation from these molecules from the plant. The number and kinds of sensory receptors differbetween species of animals and this may partly account the differences between species of animalswith respect to diet preference. There appear to be only small differences between lambs and oldersheep or calves and cows with respect to preference. In addition, there is no evidence thatpregnancy or lactation influence preference.

(e) The Role of Previous Grazing Experience on Preference

Previous grazing experience, especially that learned when an animal is young, influences dietpreference throughout the lifetime of the animal. However this can be modified by subsequentexperience. However there are examples where preference is cannot be modified by subsequentexperience in some herbivores such as with panda bears, koala bears and mountain gorillas.

(f) The Role of Nutritional Wisdom on Preference

Does an animal prefer a particular plant because it is nutritionally good for it and avoid anotherplant because it is toxic? There is much debate over this question. This kind of nutritional wisdomis unlikely since death following ingestion of toxic plants is not uncommon among herbivores butsome herbivores are undoubtedly wiser than others. However, most herbivores do not eat discretemeals of a single food but a meal can last for many hours, include a large number of foods that areregurgitated and chewed again. It therefore seems unlikely that animals can relate illness or benefitto a particular food component. It is more likely that "apparent" nutritional wisdom is related topalatability through the sense of taste. For example, alkaloids are bitter and unpalatable and for thatreason are likely to be rejected unless the animal is hungry. Also, it has been suggested thatphosphorus deficient sheep select a diet which is higher in phosphorus than that measured in cutherbage samples. However, it has also been shown that phosphorus adequate sheep select a diet

which is higher in phosphorus than the cut herbage. It is likely that the sheep are not selecting forhigh phosphorus but for lower content of free phenols since phosphorus content and free phenolcontent is negatively related in plants and phenols such as tannins are negatively related topalatability. Nevertheless, it has been widely demonstrated that phosphorus deficient cattle willseek out and chew old bones (osteophagia); sodium deficient sheep and cattle will lick soil(geophagia); and coprophagia (eating of feces), which is commonly practiced in lagomorphs(rabbits), is known to be a means of compensating for diets deficient in protein &/or B vitamins.These examples of deviant feeding behavior are called pica.

Factors affecting palatability are summarized in Table 40.


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Table 40. Factors that commonly improve or reduce palatability of plants to grazing herbivores.

FACTORS WHICH

IMPROVE PALATABILITY REDUCE PALATABILITY

Plant physical factors of growthHigh succulence Low succulence, high dry matterHigh leaf:stem ratio Low leaf:stem ratioSeedstalks scarce Seedstalks abundantNew growth/regrowth abundant; long growing season Old growth abundant; plant dormantLeaves fine, tender Leaves coarse, tough, rank Twigs small, spaced Twigs large, compacted, sharp

Plant physical factors of morphology

No thorns, awns spines etc. Thorns, awns, spines presentLeaves glabrous (smooth) Leaves pubescent (hairy)

Plant chemical factors

Sugar, soluble carbohydrate, organic Sugar, soluble carbohydrate, organicacid contents high acid contents lowTannin, alkaloid contents low Tannin, alkaloid contents high

Environmental factors

Weather promotes growth Weather imposes dormancyPrevious growth normal Previous growth rapid & coarsePlant surface moist from rain, dew Plant surface dryHigh sunlight except where maturation Low sunlight except whereadvanced maturation retardedPlant surface clean Plant surface covered with dust, mud,

feces, urinePlants not affected by disease, insects etc. Plants affected by disease, insects etc.

Factors that influence diet selection in grazing herbivores are summarized in Figure 28.


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ANIMAL

Motor outflow

Reflexes of attention, approach, examination and consumption or rejection

Senses of sight, smell, touch and taste

PLANTSPlant species present and their chemical and physical

characteristics and relative availability

Modified by

PLANT ENVIRONMENTSoil textureSoil fertility

Plant community

Modified by

PHYSICAL ENVIRONMENTTopography (slope, aspect, site e.g. slough, rocky knoll etc.)

Distance plant is from waterDistance plant is from shade

Modified by

ANIMAL FACTORSAnimal species

Animal individualityPhysiological status (food demand, disease etc.)

Grazing behaviorSocial behavior

Modified by

PREVIOUS EXPERIENCE

DIET COMPOSITION

Figure 28. Schematic diagram of diet selection in the grazing herbivore.


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EFFECTS OF GRAZING ANIMALS ON RANGE AND SEEDED PASTURE LANDS

Grazing animals have both physical and chemical effects on grassland. The interaction of thegrazing animal on plant and soil ecology is summarized in Figure 29.

Figure 29. The soil-plant-animal ecosystem of range and pasture lands

Physical effects

DefoliationEffects of defoliation on plant morphology

Defoliation of plants is not detrimental to plant survival but severe defoliation is detrimental. Theseverity of defoliation is defined by the frequency and timing of the defoliation and also by theamount of plant material removed. Correct defoliation is almost always beneficial and it isfallacious to assume that the removal of livestock from public lands will be the answer to theproblems of land degradation. For example, grazing of most grasses stimulates new growth andincreases the amount of tilling and grazing of shrubs (browsing) causes hedging (new growth of succulent stems and leaves from the old wood of these plants.

In general, plant species that are resistant to defoliation have four main characteristics:

(i) They maintain vegetative buds in or close to the soil surface(ii) They do not elevate the apical meristem more than 2 or 3cm until rapid elongation and

flowering take place.(iii) They produce numerous fruiting stems.(iv) They have the capacity to initiate abundant new culm development (tillers from basal buds).

The degree of development of the above characteristics in a plant species will determine its abilityto withstand grazing and survive.


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Effects of Defoliation on Plant Growth

The disappearance or persistence of grazed plants is largely regulated by the amount of energyreserves stored in the plant. Over-grazing generally reduces the non-structural carbohydrates inroots and perenniating stem bases and can have a detrimental effect on plant growth and survival.

However, structural carbohydrates are not considered when discussing the effects of defoliation ongrowth because they rarely become energy sources for plant growth. The non-structuralcarbohydrates are the major sources of stored energy reserves and these are referred to as TotalAvailable Carbohydrates (TAC).

TAC compounds originate in the leaves and other sites of photosynthesis and are stored in theleaves, roots, rhizomes, stolons and lower stems (i.e. the points of origin of new growth).Variations in TAC between plants are related to:

(i) The species of plant(ii) The plant growth cycle(iii) The part of the plant being examined(iv) The site where the plant is growing.

Because of these variations, TAC is not always a good measure of plant response to grazing but itcan be used as an indicator. Management of grazing should not necessarily aim for maximumlevels of TAC but should not exceed critical minimum levels. A lot of research is directed atdetermining these critical minimum TAC levels.Defoliation reduces the leaf area index (LAI) or the ratio of living leaf area to the ground surface, atleast temporarily, and this reduces the conversion of solar energy to plant energy so themorphological response of a plant to defoliation has a marked effect on its growth.

Effects of Intensity of Grazing on Plant Survival

The intensity of grazing relates to the amount of material remaining after grazing not to the amountof material removed during grazing. The intensity of defoliation or grazing will influence plantsurvival depending upon the morphological response of plants to grazing. For example, alfalfadoes not withstand high intensity grazing because it damages the crown whereas white clover(Trifolium repens ) can withstand high intensity grazing because it producing rhizomes and undermoderately heavy grazing it will actually be an increaser.

Effects of Frequency of Defoliation on Plant Survival

The frequency of defoliation refers to the interval of time between defoliations and the number of defoliations. Frequency is closely related to intensity of defoliation but is not the same, sincerepeated defoliation at the same height would hold grazing intensity constant but vary the grazingfrequency. For example, consider two plant species that are grazed constantly to the same height.One species responds to defoliation by producing new growth twice as rapidly as the second

species. The first species can then be grazed twice as frequently as the second species, but sincethe same amount of plant material is left in both species, the intensity of grazing is the same forboth species.

The ability of a plant to produce new growth is dependent on many things such as plantmorphological response, soil fertility, soil moisture, temperature, day length, site aspect (north orsouth facing slopes of hills) and so on. However, plants within one ecosystem may responddifferently to defoliation depending upon the season in which the defoliation occurs.


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Effect of Season of Defoliation on Plant Survival

The definition of season of defoliation should be made in relation to the growth curve of the plant.For example, plants that commence growth very early in spring are more sensitive to defoliation inearly spring than are plants which commence growth in late spring and are still dormant in earlyspring. In late spring, however, the situation is reversed.

Some grasses and forbs, those that have elevated growing points, are highly susceptible todefoliation and lose vigor when growing points are removed at any time. Others, that do not haveelevated growing points, show little effect as measured by dry weight and seed production. Thesensitivity of some plants to defoliation increases rapidly when the flower stalks begin to developand decreases rapidly as the plants approach maturity.

Plant growth curves differ from species to species depending on such seasonal factors as daylength and temperature and moisture, so the season during which defoliation occurs will influenceplant response differently between species. However, all plants are most susceptible to defoliationduring the early part of their growth. Nevertheless, very recent and unpublished research fromAgriculture Canada Research Station at Swift Current has indicated that the productivity of uplandand lowland mixed prairie native pasture and crested wheatgrass is unaffected by season of grazingprovided grazing occurred only once/year and was restricted to 6-10 days.

Prehensile Technique

Animals frequently pull whole plants from the soil. This is a drastic form of defoliation but duringmilder defoliation, animals tear developing grass stems from this sheaths and remove pieces of plants which they do not eat but discard. Plants differ in their resistance to pulling from the soildepending upon root structure, soil type and soil moisture but an overgrazed plant with a depletedroot system surrounded by bare and eroded soil is much more likely to be completely removedfrom the soil. Animals also differ in their eating action.

Bark wounding (removal of bark from trees), which if severe will cause death of the tree, is aspecial form of plant damage. Most bark wounding is caused by wildlife such as elk and deerwhen removing velvet from their antlers. The most spectacular destroyer of woody plants is theAfrican elephant that has eliminated whole forests by breaking limbs from trees, pushing downtrees and tearing whole shrubs and small trees from the ground. Over a long period of time this hasconverted much woodland in Kenya into grassland.

Treading

The treading action of animals breaks plants. This can be both beneficial and detrimental. If treading breaks dead standing herbage it brings it into contact with the soil surface and in so doingit speeds the process of decay and re-cycling of organic matter and nutrients. Treading can alsohelp to bring seeds into contact with the soil and thereby increase the rate of seedling germinationand survival. However, if treading breaks green plant material or newly emerging tillers it not only

renders this herbage unavailable to the grazing animal but also damages the plant.Treading compacts the soil which:

Reduces seed emergenceReduces root growth which in turn reduces nutrient uptake by the plant and as a consequenceplant growth is reducedReduces moisture infiltration, increases moisture run-off which increases the risk of erosionand reduces plant growth and survival (see Table 35, page 68). The degree to which treadingwill compact soil depends on the stocking rate, texture, structure, porosity and moisturecontent of the soil.


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Fouling

Fouling, or the covering of vegetation with feces, by livestock has both beneficial and detrimentaleffects. Feces and urine are important components in the re-cycling of nutrients on range andseeded pasture. This will be discussed in a later lecture. However, fouling can also damage thevegetation. For example one report from New Zealand indicated that dairy cows deposited feces at

an average rate of 13.9 time/day and that each deposit covered 0.07m2

. This amounts to355m 2 /cow/year (in New Zealand grazing continues for 365 days/year) or 0.035ha/cow/year or3.5ha for a herd of 100 cows. The experiment also found that 75% of the vegetation under themanure was killed and did not re-grow the following year. Beef cattle defecate less frequently thandairy cattle but in southern Saskatchewan where the rainfall is considerable less than in NewZealand, the cow pies take much longer to decay and the damage to the vegetation is probablymuch longer lasting, though there are no data available to confirm or deny this suggestion. Theextent of this damage depends on the species of animal (sheep, goats, deer etc. cause less damagethan cattle because their feces are deposited in small pellets and are much drier, allowing thevegetation to grow through.

Re-distribution of plants by grazing animals

Animals are important distributors of plants over native range and seeded pasture lands. They candistribute plants by the physical adherence of seed to the hair, wool, horns, hooves, etc. Thepresence of hooks, barbs, awns, and sticky exudates on seed will influence the physicaldistribution of plants in this way. Whether or not animals are cloven hoofed (divided feet such ascattle or sheep) or not (such as horses or mules) will affect plant distribution by this method. Sheepand cattle carry more seed between the claws of their feet than horses.

Seeds are also redistributed in the manure. This has been recognized an important way of re-vegetating seeded pastures in Australia for 20 years. For example, white clover ( Trifolium repens )has been re-introduced into pasture from small nursery areas. The nursery was located in a pasturewhere the clover seed was allowed to ripen and cows were then allowed access to both the nurseryand the pasture. In 3 years the pasture had been re-vegetated. Manure provides a favorableenvironment for nutrients and moisture for the developing seedlings but not all seedlings survive.However, in that experiment, 30% of the white clover seed passed through the digestive tract of the cows and 85-90% of them germinated. Although this may be a slow way to introduce thelegume into the pasture compared with re-seeding it was effective, economical and environmentallyfriendly.

Birds are also important in redistributing seeds over rangelands. Of course not all seedsredistributed by animals and birds in this manner are desirable and in North America we have beenperhaps pre-occupied with the negative effects of plant re-distribution. For example, a study 30years ago in California suggested that in a single grazing season one cow could redistribute 36species totaling more than 900,000 viable seeds of weed species. However a report from Utah in1996 indicated that crested wheatgrass could be introduced into a pasture by feeding 1 / 3lb of seed/acre.

Cows are more successful at re-distributing seeds on range and pasture than sheep of horsesbecause their feces are more moist and they do not masticate their food to the same extent as sheepand horses. For these reasons, wildlife are also probably less likely to redistribute plants thancattle. Finally, the rejection of fouled areas by animals for sources of feed also provides animportant source of parent material for seed setting and helps to ensure the survival of plantspecies.


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Chemical Effects

Nutrient Cycling

Figure 30 provides a generalized diagram of nutrient cycling in range and pasture systems

Figure 30. Generalized representation of nutrient cycling on grazed range and pasture

Grazing animals return a large proportion of the consumed plant nutrients to the soil. A mature cowmay produce as much as 25kg feces and 9kg urine daily. Average chemical composition on apercentage net weight basis of the excreted materials is shown in Table 40.

Table 40. Average nutrient contents of feces and urine of beef cattle._________________________________________________

Nitrogen P 205 K20% % %

In feces 0.38 0.18 0.22

In urine 1.1 0.01 1.15_________________________________________________

Most of the voided phosphorus occurs in the feces whereas the urine is rich in nitrogen andpotassium.


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Table 41. Nitrogen, phosphorus and potassium excretion in the feces and urine of cattle grazingnative range.

N P 2O5 K2O

In feces (%) 0.38 0.18 0.22In urine (%) 1.10 0.01 1.15

Total (g/d) 194 46 159

Fertilizer equivalent (kg/ha) 15.2 1.6 7.7

Table 42 compares the amount of nutrients removed from the soil in a crop of spring wheat and acrop of calves. These data indicate clearly why it is not possible to maintain satisfactory yields of wheat without the application of fertilizer. In contrast, the grazing of cattle on range and seededpasture is the most sustainable form of agricultural production because nutrient removal inminimal.

Table 42. Amount of various elements removed from the soil by a crop of grain or calves (R kg)expressed as a proportion of those elements taken up by the plants (V t).

N P K Ca Mg

Wheat R/V (kg/t) 735 852 86 204 500

Calves R/V (kg/t) 39 36 4 71 1

Without herbivores, nitrogen and minerals cycle from soil to plants, to litter and back to soil.Animals add many more pathways because the nitrogen and minerals pass through animals, maybe changed in chemical structure, may be exogenous or endogenous and are returned to the soil infeces or urine. They are removed from the rangeland the form of meat, milk, wool, etc. and, whilethe yield of these animal products is very much lower than that of grain and oilseed products, thereduced efficiency of production translates into increased sustainability of production. Forexample, in the U.K. it has been reported that pasture yield increased from 10,000 kg/ha for cutand removed pasture to 12,000 kg/ha for grazed pasture with the same rates of fertilization. Theincreased production from the grazed pastures is caused by the re-cycling of nutrients as opposedto their removal from the cut and carry operation.

The nutrients in urine are readily available to plants, though there is some loss, particularly of nitrogen, through volatilization. Rates of manure decomposition depend on the climate (rainfall andtemperature) and coprophagous insects and other biota and micro-biota are also important. Theactivities of these organisms incorporate manure into the soil, reduce infective stages of parasiticworms

principles of grazing animal nutrition.pdf

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