biology and management of phalaris minor in rice–wheat system

Crop Protection 23 (2004) 1157–1168

ARTICLE IN PRESS

Contents

1. Int

1.1

1.2

2. Bio

2.1

2.2

2.3

2.4

2.5

3. Ma

3.1

3.2

3.3

3.4

3.5

3.6

Referen

*Correspondi

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doi:10.1016/j.cro

Review

Biology and management of Phalaris minor in rice–wheat system

Hari Om*, S. Kumar, S.D. Dhiman

CCS Haryana Agricultural University, Rice Research Station, Kaul, Kaithal 136 021, India

Received 5 June 2003; received in revised form 27 January 2004; accepted 16 March 2004

Abstract

Phalaris minor Retz. (Littleseed canarygrass) is a pernicious weed, which infests several crops during the winter season,

particularly the wheat crops in rice–wheat sequence. Considering the limitations of cultural and chemical methods of weed control,

the understanding of its biology with respect to different environmental, edaphic and management factors may offer a useful key to

strengthen weed management strategies. This review considers various aspects on dormancy, viability and agro-ecology with

emphasis on management practices in host and succeeding crops. Due attention has been given to the approaches required to

manage the resistant biotypes under present conditions and hence to avoid further escalation of the epidemic. The various studies

indicate that P. minor utilizes beneficially the prevailing environmental and management conditions of both the wheat and

succeeding rice crop in rice–wheat system for its survival and growth. Its seed is highly sensitive to variable moisture and

temperature regimes for germination and exhibits tolerance to anoxia during anaerobic respiration in rice. Tillage options, residue

management, spatial–temporal considerations and other factors influence the seed dynamics, pattern and depth of emergence and

growth of P. minor. A comprehensive and conceptual understanding of these aspects may provide useful guidelines in formulating

cautious and opportunistic weed management strategies.

r 2004 Published by Elsevier Ltd.

Keywords: Weeds; Resistance; Dormancy; Viability; Emergence; Seed distribution; Tillage; Puddling; Straw burning; Allelopathy

roduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158

. Development of resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158

. Conditions favouring P. minor infestation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158

logy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158

. Seed distribution pattern in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159

. Dormancy and viability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159

. Germination and growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161

. Tiller production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161

. Allelopathic effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1162

nagement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1162

. Varietal competitiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1162

. Seed rate, time and method of sowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1163

. Crop diversification/rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1163

. Physical methods of control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1164

. Herbicides and their rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1164

. Integrated weed management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166

ces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166

ng author. Tel.: +91-1746-254263.

ss: [email protected] (H. Om).

front matter r 2004 Published by Elsevier Ltd.

pro.2004.03.009

IN PRESSH. Om et al. / Crop Protection 23 (2004) 1157–11681158

1. Introduction

ARTICLE

Phalaris minor Retz. (Littleseed canarygrass) is anannual grass weed. It is the most troublesome weed inthe wheat crop mainly in a rice–wheat system, which is apredominant system in Indo-gangetic plains of India.Nearly 40 per cent of the total wheat area of the countryis covered under this system. Most of these areas areheavily infested by P. minor which emerges with thegerminating wheat crop, competes for water andnutrient requirement and reduces the grain yield (Bhanand Kumar, 1997). Surveys of wheat in the states ofPunjab (Bir and Sidhu, 1979; Zahir and Gupta, 1979)and Haryana (Malik et al., 1981, 1985; Singh et al.,1995b) have established the prevalence of P. minor.Globally, P. minor has been reported in more than 60countries of the world, widely covering all the continentsexcept polar regions (Anderson, 1961; Baldini, 1995;Singh et al., 1999).

1.1. Development of resistance

The continuous use of urea-based herbicides, parti-cularly isoproturon, for more than a decade in wheatunder a rice–wheat system has resulted in the evolutionof herbicide resistant biotypes of P. minor (Malik andSingh, 1993; Malik et al., 1995; Yaduraju and Ahuja,1995; Walia et al., 1997). The resistant biotypes havebeen reported to tolerate between 2 and 8 times higherdose of application than susceptible ones in Haryana(Malik and Singh, 1995) and more than 2 times inPunjab (Walia et al., 1997).Three new herbicides (sulfosulfuron, clodinafop and

fenoxaprop) have been recommended and adopted forthe control of isoproturon-resistant biotypes of P. minor

in rice–wheat system, but the recommendation of theseherbicides are full of uncertainties as the resistantmechanism is metabolic in nature (Kirkwood et al.,1997; Singh et al., 1998). Many cases of cross-resistancehave been documented against Alopecurus myosuroides

Huds. (Read et al., 1997; Menendez et al., 1997) as wellas indications of cross-resistance against P. minor

(Yadav et al., 2002).

1.2. Conditions favouring P. minor infestation

Indigenous tall wheat varieties are sown early in theseason and grow before the emergence of P. minor andoffer sufficient competition to this weed. High yieldingsemi-dwarf varieties grow faster, but the higher tem-peratures at early sowings adversely affect tillering andgrowth of these cultivars. It is thus natural thatinfestation of P. minor is less severe in tall wheat thanin the later sown group of semi-dwarf varieties. Theoptimum time of sowing of these semi-dwarf varietieswhich have replaced tall indigenous wheat varieties on a

large scale after mid-60s is ideal for the emergence of P.minor. Moreover, the cultivation of these varietiesrequires early and frequent irrigations with highfertilizer doses which have modified the field environ-ment and seems to have changed the ecologicalconditions that are conducive and favourable for thegrowth and development of P. minor.Besides this, rice crop management system supports

the survival of P. minor seed in the rice–wheat system(Parasher and Singh, 1984, 1985; Bhan and Kumar,1997; Singh et al., 1999). The conditions favouring thesurvival mechanism of P. minor seed in the soil underthis system are:

i.
The seed is susceptible to solarization. The presenceof water in rice fields lowers the temperature of soilwhich helps in its survival.
ii.
Puddling before rice planting helps in deep place-ment of seed and thus exposure to lower tempera-tures.
iii.
The increased and prolonged alcohol dehydrogen-ase activity in P. minor seed plays a detoxifying roleunder anaerobic respiration.
iv.
The tolerance to anoxia might be due to inherentability of seed in using NO3 as an alternate electronacceptor in electron transport system (ETS).
v.
The seed has hard seed coat, which is impermeableto the entry of water.
Keeping the above considerations in view, i.e.,conditions of the host and succeeding crop in a croppingsystem favouring survival mechanism of the weed,development of resistance and possibilities of crossresistance, it is necessary to evaluate the sensitivities ofP. minor seed with respect to environment, soil andmanagement factors and herbicides as well. Reviewingand documentation of existing knowledge in a singleattempt would help strengthening the understanding ofthe behaviour of P. minor seed under a particular orvariable set of environments and provide flow charts forrestructuring integrated weed management strategiesand formulating and projecting the future line of actionin research.

2. Biology

The studies on biology help in formulating short- orlong-term strategies for the control of weeds anddeciding the suitable time and method of weed control.Only a few studies on P. minor germination andemergence pattern, seed dormancy, viability and long-evity under field conditions in rice–wheat system andhow these are affected by management factors such astillage options, water regimes, stubble management andenvironmental factors have been reported. While

ARTICLE IN PRESSH. Om et al. / Crop Protection 23 (2004) 1157–1168 1159

reviewing the scenario engendered by increasing menaceof P. minor due to the development of herbicideresistance, the Editorial of Indian Society of WeedScience Newsletter mentioned in its summary remarks,‘‘It also focuses over hollowness in our research for weknow very little about the biology and ecology of P.minor—the weed which has been with us for over threedecades’’ (Anon., 2000). In the light of this, theknowledge of various aspects such as seed bankdynamics and its distribution pattern in soil, seeddormancy, viability and loss of vigour, its germinationand response to various environmental and managementfactors is needed for effective management of this weedparticularly when the cases of cross resistance have beennoted.

2.1. Seed distribution pattern in soil

Without knowing the place and site of maximumbioactivity of the seed, the exercise of weed control mayprove futile. Dhiman et al. (2002) recorded that thehighest density (3.62 seeds cm�3) of P. minor seed inpuddled soil (required for transplanting rice in rice–wheat system) was found in the uppermost 0.2 cm layerfollowed by the adjacent lower layer (0.2–1.25 cm). Nineper cent viable seeds remained concentrated in theuppermost 0.2 cm layer. More than one-third of the seedbank (37.6 per cent) accumulated in the upper 2.5 cmlayer, whereas, 54.8 per cent seed rested in upper 5.0 cmlayer of soil. The density also varied with the level ofwater in the field at the time of puddling. There was adecrease in the concentration of P. minor seed from 21.2to 12.2 per cent in the upper 1.0 cm layer with theincrease in puddling frequency from 2 to 4 at low waterlevel (2–3 cm), whereas, the trend was reversed when thepuddling frequency was increased at high water level (6–7 cm) recording 20.7 (2 puddlings) and 69.5 per cent (4puddlings) seed in this layer (Om et al., 2002a, b). Inanother pot study, the maximum number of seeds(83.4 per cent) of P. minor emerged when the seeds weresown at 1.0 cm depth of soil followed by the uppermost0.2 cm depth (82.7 per cent). Below 1.0 cm, there wasdrastic reduction in germination with increasing depthof placement of seed up to 10.0 cm. However, Singh andGhosh (1982) found the highest number of seeds on thesoil surface and this decreased with depth up to 15 cm.No significant difference in total number of seeds in

the soil or the relative vertical distribution of seedsthrough the soil profile between two tillage systems (zeroand conventional tillage) was found by Franke et al.(2002). They recorded that the seeds were relativelyequally distributed over the upper 10 cm of the soil forboth tillage systems and few seeds were found below10 cm depth. This is likely to be the result of theextensive tillage operations associated with the preced-ing rice cultivation, which equally distributed the seeds

over the upper 10 cm of the soil, while below 10 cm, thesoil was left undisturbed.Yadav (2002) reported that more seeds of P. minor

(39.9 per cent) were observed in 0–5 cm soil depth undertransplanted and wet seeded rice, but in case of dryseeded rice, 33.7 per cent seeds were extracted from 5–10 cm soil profile. He further concluded that theconcentration of P. minor seeds which accumulated inupper soil profile during puddling process might haveresulted into more weed population in wheat aftertransplanted rice system. These studies reveal that theseeds of P. minor remain concentrated in the uppershallow layers of soil even after the tillage operations inthe succeeding rice crop. However, the level of water atthe time of puddling before rice transplanting andpuddling frequency can influence seed distributionpattern in the soil profile.

2.2. Dormancy and viability

The knowledge on dormancy helps to understand thephases of bioactivity in the seed with the passage oftime. Studies on viability register the information withrespect to the persistence and survival of weed seedunder a particular set of conditions or cropping system.This knowledge facilitates deciding the most suitabletime, method and duration of weed control measuresrequired. These two aspects in P. minor are profoundlyaffected by environment, edaphic and managementfactors (Om et al., 2002a, b).The term after-ripening is used to describe the

transition of dormant seeds to a more readily germina-tive state. It is not an abrupt change from a dormant toa fully germinative state, rather seeds in a populationbecome more responsive to a range of conditions atwhich they are able to germinate and less responsive to arange of conditions that restrict germination (Baskinand Baskin, 1998; Bewley and Black, 1994). When seedsgerminate over only a narrow temperature rangepossible for the species, they are in conditionaldormancy (Vegis, 1964; Baskin and Baskin, 1985).Studies conducted at CCS Haryana Agricultural Uni-versity Rice Research station Kaul (Kaithal) Indiarevealed that soil as a medium of seed placement,temperature and moisture hastened the process of after-ripening in P. minor seed individually as well ascumulatively. The seed of P. minor collected from thesoil on April 17 just after wheat harvesting exhibitedhigher germination than that threshed directly from thenaturally mature earheads. The seeds retrieved from soilsamples on April 17, 30, May 15 and 30 showed steadyincreases in germination in both years studied (2001 and2002). Moisture content advanced the process of after-ripening, but germination was strongly inhibited whenthe seed was kept in soil at more than field capacity or inwater. The seed incubated in soil at field capacity (FC)

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at 30�C increased germination while a temperature of40�C favoured after-ripening of seed when mixed in drysoil or kept dry without any medium. Release fromsecondary/conditional dormancy was quicker and high-er in the seed retrieved from the soil which wasincubated at 20�C for 70 days. Germination of the seedimproved with the rise in temperature from 30�C to40�C when the seed was retrieved from the soilincubated at FC for 70 days. The seed kept immersedin water was least responsive to the rise in temperature.Seed recovered from dry soil or kept without anymedium responded quickly at both the temperatures.The 40�C temperature decayed the seed when kept inwet soil (at FC or saturation) or in water. Lightenhanced the germination of P. minor seed. The seedsretrieved from field after rice harvesting lost vigour interms of test weight, germination, length of radicle andplumule, respectively, compared with the seed stored atambient laboratory conditions. There was 13, 67, 74, 79and 86 per cent loss of germination in the seed whichwas buried in puddled rice field in rhizosphere andexhumed at 10, 17, 24, 31 and 38 days after its burial,respectively, from a field in which continuous submer-gence was maintained over that in which irrigation(572 cm) was given at 1 day after disappearance ofponded water (Anon., 2003; Dhiman et al., 2002).Freshly harvested mature seeds of P. minor remained

dormant for a period of 3 months and germinationincreased in the range of 88–96 per cent after 12 monthscompared to 5 months (Singh, 1998). He also observedthat thousand-seed weight varied from 1.5 to 2.1 gdepending upon soil type and growth conditions.Similarly, Jimenez-Hidalgo et al. (1993) found consider-able variation in germination of Phalaris species fromdifferent locations and years, but no differences werefound between seeds aged 6 and 18 months. On theother hand, Hari et al. (2003) observed that dormancy inP. minor was less than 60 days under natural fieldconditions in rice–wheat system as the seeds retrievedfrom the soil samples from farmers’ fields after harvest-ing of wheat crop in the last week of May exhibited 80–96 per cent germination.The presence of chemical inhibitors in the seeds might

be responsible for true dormancy (Rost, 1975). The seedof P. minor tolerated anaerobic conditions by enteringinto secondary dormancy and avoiding anaerobicdecomposition (Parasher and Singh, 1985). The chemi-cal status of seeds under anaerobic conditions suggestedthat P. minor seed exhibited resistance to oxygen stressprobably due to the formation of chemical metabolitesand changes in the membrane permeability (Parasherand Singh, 1985). This could be the reason for the highinfestation of P. minor under the areas of rice–wheatsequence (Singh et al., 1995b).Burning of crop residues affects germination and

viability of seed considerably. Hari et al. (2003) reported

that burning of wheat straw caused 15–25 per cent lossof the P. minor seeds present at the surface of the soiland 98 per cent of the remaining seeds lost theirviability. The effect of transmittance of heat to deeperlayers was more pronounced in puddled soils than non-puddled soils. Yadav (2002) working at Pantnagarobserved that the shattering of weed seeds on the soilsurface was more (2554m�2) after combine harvestingof wheat than after manually harvested crop (2154m�2).Straw burning after combine harvesting resulted into67.7 per cent loss of seeds of P. minor and the remainingseeds completely lost their viability. Seeds of P. minor

collected from manually harvested wheat fields hadslightly higher germination (91.2 per cent) than seedscollected from combine-harvested fields (86.1 per cent).He concluded that the burning helped in reducing theweed density by destroying their viability and suggestedthat if wheat residues after combine harvesting were notburnt, there would be large increases in the weed seedbank. On the other hand, burning of crop residueswould add to the environmental pollution and the use ofstraw as cattle feed would be affected. These issues areof paramount importance and require consideration.Om et al. (2002a) reported that the emergence of P.minor in farmers’ field of wheat was less by 42–80 percent in the fields where wheat straw was burnt (64.2plantsm�2) after combine harvesting than its removal(183.2 plantsm�2).Half-life of seeds buried at Hisar was 10 months,

while this was more than 15 months for seeds buried atKarnal (Franke et al., 2002). They explained that thedifference in seed longevity/viability between Hisar andKarnal locations might be due to the differences in soilconditions, as Karnal soil was heavier with more clayand more compact than sandy loam soils of Hisar. Therice–wheat rotation at Karnal required high amounts ofadditional irrigation water, while the fallow field atHisar did not receive any irrigation water. Heavycompacted soils with high soil moisture content, similarto Karnal soils, are usually poorly aerated, which mayinfluence P. minor seed decomposition. Depth of burialis another important factor which may determineseed viability. Seed half-life in a study conducted atKarnal buried at 20 cm depth was 11.3 months ascompared to more than 15 months for seed buried at30 cm depth (Franke et al., 2002). This study suggestedthat depth of burial of P. minor seed, soil type and itsphysical condition had a significant effect on seedviability by affecting seed decomposition and germina-tion rate.Germination of seeds of resistant biotypes of P. minor

was more than those of susceptible biotypes in the threestudies at most sampling times. This result indicates apossibility of difference in physiology between R(resistant) and S (susceptible) biotypes just after burial,which may be related to the increased activity of the


cytochrome P-450 enzyme which resistant seeds exhibit(Singh et al., 1999).Om et al. (2002a) reported that there was complete

loss of viability in 10 monthsold P. minor seeds retrievedfrom the soils underwent rice conditions. The seedsretrieved from the soil after rice harvesting kept undercontinuous submergence for 60 days exhibited 26 and 57per cent loss of germination over semi-submergence andsemi-wet conditions, respectively.The information available on the aspects indicate that

the periodic variability in dormancy and viability in P.

minor seed is affected by various factors includingmoisture, temperature, light, water management in thesucceeding rice crop, residue management, method ofcrop harvesting, soil texture and its physical condition,depth and period of seed burial in soil, etc.

2.3. Germination and growth

Water is a basic requirement for seed germination.Dry seed has very low moisture content and metabolitesremain inactive. The moisture content at field capacity isoptimum for germination. The physiological effects ofwater stress on plants are most pronounced in rapidlygrowing tissue, a fact that is particularly exemplified bydevelopmental phases of germination, emergence andinitial seedling growth.Yadav (2002) reported that maximum germination

(96.7 per cent) of P. minor seeds was observed at ‘0’ barwater potential and the level of germination reduceddrastically when water potential level changed from 0 to�6 bar beyond which no germination took place. Thehighest germination of P. minor seeds (92.7 per cent) wasat pH 6.0 and a significant reduction in the germinationabove and below this level of soil reaction was recorded.No germination was observed at pH’s 3.0, 9.0 and 10.0.Hari et al. (2003) observed that the germination of P.

minor increased with up to 24 h imbibition of seed indistilled water. This was constant from 24 to 72 h at10�C and 22�C. The germination decreased when theseed was imbibed for more than 6 h at 34�C and reduceddrastically after 30min imbibition at 46�C. They alsoreported that there was nearly 10�C higher temperaturein upper 2 cm soil profile than ambient air temperature(42�C) in a field, which was ploughed and kept dry afterwheat harvesting in the month of May. The soiltemperature decreased down the soil profile. This rangeof temperature (38–44�C) seems to be sufficient toreduce the soil seed bank of P. minor under suitable fieldmanagement.In general, the earliest germinating seeds from soil

depths of 3–4 cm produced more tillers and seeds withhighest weight (Kumar and Kataria, 1977). The greatestproportion of seedlings (60 per cent), however, emergedfrom the upper 2 cm layer after irrigation. Bhan andChaudhary (1976) reported that plants emerging in late

December had more tillers than plants emerging inNovember. Singh and Ghosh (1982) reported a tem-perature of 17–20�C to be ideal for germination.Similarly, Mehra and Gill (1988) observed the optimumtemperature for germination of P. minor seeds inlaboratory was 15–22�C and more dark brown seedsgerminated quickly compared to light yellow and greenones. While Bhan and Chaudhary (1976) found thatPhalaris seeds germinated between 10 to 20�C and nogermination could take place above 30�C and below5�C.Yadav (2002) working on rice–wheat sequence

observed that in bed-planted wheat, after transplantedrice, 16.8 plants of P. minor emerged (m�2 day�1) during15–20 days after sowing (DAS) and only 1.4 plantsm�2 day�1 during 20–40 DAS. Similarly, 90 per cent ofP. minor emerged from the bottom layer of 0–3 cm depthand only 22 per cent from top of the bed. He furtherreported that periodicity of emergence (plantsm�2 day�1) of P. minor before first irrigation was lower(6.2) in case of conventionally tilled wheat as comparedto zero-tilled (10.0), but the trend reversed after firstirrigation where 20 plants m�2 day�1 emerged in theformer and 7.0 plants m�2 day�1 in the later method ofwheat sowing. After the completion of one cycle of rice–wheat system, the population of total weeds was less inzero-tilled wheat than in conventional, whereas thepopulation was significantly higher in zero-tilled wheatafter the completion of second cycle of rice–wheatsystem.Plant density of P. minor was lower in zero tillage

than conventional wheat sowing (Gupta et al., 2000;Hari et al., 2003). Cross ploughing in the upper 2–5 cmsoil (shallow tillage) and drill-sowing of wheat 1 weekafter shallow tillage reduced germination of P. minor by44 and 37 per cent and increased grain yield by 21 and47 per cent over zero-till and conventional methods,respectively (Hari et al., 2003).

2.4. Tiller production

P. minor produces more tillers than normal wheatvarieties under non-competitive conditions and itsbranching habit contributes to higher seed production(300–400 seeds panicle-1). Under non-competitive con-ditions, up to 42 tillers per plant have been recorded inP. minor. Field studies show that delay in wheat sowingfrom early to late November or mid-December had asignificant effect on the emergence and growth patternsof wheat and its associated weeds. Heading (flowerinitiation) was observed in P. minor at 66 and 98 DASwhen sown on 20th December and 10th November,respectively, and increased to 50 per cent within a week.Shoot dry weight of P. minor was 89 per cent lower thanwheat from 10th December sowing when recorded at 90DAS in a pure stand, but from 20th December, it was 22


per cent higher than wheat. It indicates that P. minor ismore competitive in late sown crop of wheat (Singhet al., 1999).The rate of biomass accumulation of P. minor was

slow during the critical growth stages, dry weightincrease was higher between 60 and 90 DAS comparedto the first 60 days (Malik and Singh, 1993). Plant heightvaried depending upon the growing conditions, gener-ally attaining 50–70 cm height at 90 DAS (Malik andSingh, 1995). They mentioned that at maturity, theinflorescence may be taller than the dwarf varieties ofwheat. Various studies (Singh et al., 1999) suggest thatenvironment and crop management factors influence thegermination and growth of P. minor. However, colourof seed, mobilization of total phenols from the seed, soilreaction, depth of seed placement and some otherrelated factors have their impact on seed germinationand plant growth.

2.5. Allelopathic effects

Tamak et al. (1994) found that 10 per cent concentra-tion of rice straw + stubble extract resulted in higherinhibition of seed germination and growth of P. minor.There was greater inhibition of seed germination whenextract was added at 2 DAS, compared to 6 and 10DAS. However, Gill and Sandhu (1994) reported thatthe plant residues of P. minor improved seed germina-tion, root and shoot growth and dry matter productionof wheat in pot culture study. On the other hand, Porwaland Gupta (1989) reported that root exudates of P.minor decreased shoot and ear length and dry matterproduction by wheat.Om et al. (2002b) reported that extract of sunflower

(Helianthus annuus) and dhaincha (Sesbania aculeata)reduced plant population of P. minor by 100 per centusing in vivo tests, whereas, used as green manure underfield conditions reduced its population by 42 and 15 percent, respectively, thus, suggesting an inhibitory role ofallelo-chemicals. Grain yield of wheat was significantlyhigher when the green manuring of sunflower and S.aculeata was done before rice transplanting oversummer fallow. Allelopathic potential of broadleavedweeds of wheat in inhibiting germination of P. minor

was in the order of Chenopodium album=Medicago

denticulata=Melilotus indica=Convolvulus arvensis (100per cent inhibition)>Vicia hirsuta (86 per cent)>Cir-

sium arvense (48 per cent)>Lathyrus aphaca (38 percent)>Rumex acetosella (9 per cent). Exudates of ricevarieties HKR 126, IR 64, Jaya and Haryana Basmati-1registered >50 per cent inhibition of germination of P.minor. As the extract was diluted, effect of non-scentedcultivars was considerably reduced in comparison toscented group. Out of 11 wheat varieties, WH533provided the highest inhibition of germination (30 percent) followed by WH 542 (21 per cent).

The findings of this study, although limited, show thatplant residues of the companion as well as thesucceeding crops or the weeds of these crops exert aninfluence in inhibiting germination of P. minor.

3. Management

Effective control of P. minor in wheat crop may beinfluenced by a number of factors such as (i) selection ofvarieties with early vigour and extent of canopy cover-age (ii) increased seed rate with cross sowing at rightangle or narrow spacing and (iii) an optimum fertilizerrate synchronized with irrigation at proper time. Thesemeasures not only favour crop competition against grassweeds, but also increase herbicide efficacy. The iso-proturon resistant biotypes appear to be equallycompetitive with their susceptible biotypes. The methodand time of herbicide application along with theperfection of pump and nozzle and accurate quantityof water and herbicide also influence the efficiency of itscontrol. For example, the activity of isoproturon ishigher at 2–3 leaf stage of P. minor compared to othergrowth stages. However, it is the integration ofagronomic practices with herbicide application, whichhelps in effective management of P. minor particularlyits resistant biotypes.As described in the preceding section, off-season

management provides an opportunity to handle the seedbank for reducing its vigour/viability. Disposal of cropresidues particularly after combine harvesting of wheat,water management in rice crop, level of water at the timeof puddling, puddling frequency, allelopathic potentialof green manure crop and rice cultivars in inhibiting thegermination of P. minor are some of the managementoptions which influence P. minor emergence and itsintensity. Besides this, shallow depth of emergence,sensitivity of P. minor seed to higher temperatures andlow soil moisture and tillage options/seeding techniquescan help in minimizing P. minor intensity in the cropseason.

3.1. Varietal competitiveness

The competitive ability of a crop variety is reflectedeither by its ability to reduce weed growth and its seedproduction or by its ability to tolerate weed interferenceand maintain higher level of grain yield. However, cropspecies and cultivars are known to differ in theircompetitiveness with weeds (Lemerle et al., 1995).Therefore, development of weed competitive cultivarsmay be the first and foremost requirement for investi-gating integrated weed management strategies. Thedifferent characteristics of a good competitive cultivarare that it must have rapid germination and initial quickgrowing habit with higher tillering capacity and leaf

ARTICLE IN PRESS

Table 1

Competitive ability of wheat cultivars against P. minor

Wheat

genotypes

Dry wt. of P.

minor at 90 DAS

(gm�2)

Wheat plant

height at 60 DAS

(cm)

Dry matter accumulation

by wheat at harvest

(gm�2)

Effective tillers

(m�1 row)

Grain yield

(q ha�1)

WH 157 52.5 43.0 1750 67.9 55.1

PBW 343 44.7 44.0 1863 83.4 58.9

WH 542 51.0 39.6 1767 71.4 56.0

HD 2687 39.3 44.5 2172 86.1 67.1

CD at 5% 5.0 3.4 94 4.8 2.9

H. Om et al. / Crop Protection 23 (2004) 1157–1168 1163

area. These characteristics appear to confer competi-tiveness with respect to crop height and tilleringcapability.Chauhan et al. (2001) reported that HD 2687 was

more competitive variety followed by PBW 343 ascompared to WH 542 and WH 157. Significantly less drymatter accumulation of P. minor was recorded with HD2687 and PBW 343 wheat varieties, which might be dueto significantly more effective tillers of these varieties(Table 1). Consequently with better smothering poten-tial, these varieties i.e. HD 2687 and PBW 343 producedhigher grain yields than WH 157 and WH 542.Walia (2002) observed that PBW 154, WH 435 and

PBW 343 were highly competitive with P. minor as theper cent reduction in yield in weedy plot under thesevarieties was found to be less than PBW 233, a durumvariety. More competition offered by the formervarieties may be due to more leaf area index ascompared to durum variety PBW 233 and triticalevariety TL 1210.

3.2. Seed rate, time and method of sowing

The competitive nature of wheat was found to beimproved with the increase in seed rate from 100 to150 kg ha�1 even in light soils under optimum fertilizerand irrigation conditions. High seed rates of 150 kg ha�1

with herbicide use could provide as good yield as weedfree conditions. There was a reduction in density of P.minor, Rumax acetosella and Melilotus indica with theincreasing seed rate of wheat from 125 to 150 kg ha�1

(Anon., 2001). The use of stale seedbed or higher seedrate (100 vs. 150 kg ha�1) alone under normal sowingwere not found to be effective in reducing P. minor

(Dhiman et al., 1985). It was observed that the cropsplanted closely (15 cm line to line spacing) resulted insignificant increase in yield of wheat as compared tonormal space of 22.5 cm. This might be due to densecanopy cover, which could have smothered the weedgrowth reflected by a reduction in dry matter by 15.2 percent (Brar, 2002).Kurchonia et al. (1993) found that wheat variety WH

147 sown at the normal (20, 30 November), mid-late (5,15 December) and late (20, 30 December) sowing dates

produced mean grain yields of 45.5, 42.6 and31.6 q ha�1, respectively. Delayed sowing decreasedweed infestation (weed dry weight in normal-78.9 gm�2, mid late—58.1 gm�2 and late—48.9 gm�2)of P. minor, Medicago hispida, Melilotus spp. andTrifolium fragiferum. They also reported that total weeddry weight including that of P. minor was lowered bycross sowing with an increase in grain yield. Sowing ofwheat crop at 20 cm spacing produced a grain yield of38.8 q ha�1 compared to 41.0 q ha�1 in cross sowing(20� 20 cm2). Johri et al. (1992) observed that closerrow spacing (15 cm), higher seed rate (150 kg ha�1) andcross sowing significantly decreased the NPK removalby grass weeds (Avena ludoviciana, Cynodon dactylon

and P. minor) compared with wider row spacing, normalseed rate and uni-direction sowing and simultaneouslyincreased NPK uptake by wheat crop. In similarpattern, Prakash et al. (1986) observed that cross sowing(22.5� 22.5 cm2) produced more wheat spikes (m�1 rowlength) and a lower dry matter of weeds whenisoproturon was applied at 2 weeks after sowing. Inthe absence of herbicide, cross sowing resulted in a lowercrop yield than closer row sowing.Kolar and Mehra (1992) observed that November

sown wheat produced higher grain yield (38.3 q ha�1)compared to October (20.5 q ha�1) or December(30.5 q ha�1) sowing. The dry matter accumulation byP. minor was found to be higher in November sowing(96.0 gm�2) as compared to October and December(45.0 gm�2) sown crop.

3.3. Crop diversification/rotation

Crop rotation has been cited extensively (Bhan andkumar, 1997; Chhokar and Malik, 2002) as an effectivepractice for management of P. minor because selectionpressure is diversified by changing patterns of distur-bances. Diversification of the area under rice–wheatcropping system not only brings changes in weedspectrum, but also makes soil conditions unfavourablefor P. minor emergence and growth. Replacing wheatwith other crops (Table 2) like berseem (Trifolium

alexandrinum), potato (Solanum tuberosum), sunflower(Helianthus annuus), and gobhi sarson (Brassica

ARTICLE IN PRESS

Table 2

Effect of crop rotation on dry matter accumulation by P. minor

Treatment Dry matter of

P. minor (gm�2)

Rice-wheat (herbicide) 20.9

Rice-wheat (control) 455.3

Rice-potato-wheat 0.0

Rice-potato-sunflower 0.0

Rice-berseem 0.0

Rice-gobhi sarson (GSL-2) 1.3

CD at 5% 4.0

H. Om et al. / Crop Protection 23 (2004) 1157–11681164

compestris) for 2–3 years in rice–wheat cropping system,the population of P. minor was found to be reducedsignificantly (Brar, 2002). The results recorded infarmers’ fields in Kapurthala and Patiala districtsrevealed that there was no seed or plant of P. minor

where rice–sunflower or rice–berseem crops were grownin rotation.A survey of the affected area in Haryana during 1993

revealed that the occurrence of resistance was only 8–16per cent, where wheat was rotated with sugarcane,vegetables, pigeon pea, clover or sunflower compared to67 per cent under rice–wheat cropping system (Malikand Singh, 1995). Alternate crops like sugarcane andsunflower with their ultimate aggressive vegetativegrowth may have a suppressing effect on P. minor.Berseem (Trifolium alexadrinum) used as a green fodderfor cattle can be used successfully to control P. minor.Potato, winter maize (Zea maize), oilseeds and pulsesare potential crops, which can be successfully rotatedwith wheat. Similarly, Bhan and Kumar (1997) reportedthe effect of various cropping sequences on thepopulation of P. minor in wheat and opined that analternate crop will allow breaking the life cycle of P.minor and initiate utilization of different weed manage-ment practices. In addition, once the wheat areachanged, other weeds will bring basic changes in weedspectrum. Secondly, due to the small holdings of themajority of farmers, only a limited area under wheatcultivation can be rotated. Generally the farmers do notobtain good prices of the produce from alternate cropsand the marketing and other system constraints restrictthe farmers to cultivate these crops.

3.4. Physical methods of control

The morphological similarities of P. minor with wheatmake it very difficult to distinguish in early growthstage. Therefore, physical removal of P. minor can onlybe carried out by trained labour. Pinkish colour at thecollar region may be one of the characteristics toidentify P. minor from wheat during hand weeding.Effective removal of P. minor with the help of wheel hoeis cumbersome in heavy soils of the rice–wheat system.

Dhiman et al. (1985) reported that inter-row cultureby hand hoe (kasola), or wheel hoe increased wheatyields by 26–29 per cent over unweeded checks, butchemical control using isoproturon at 1.0 kg ha�1

increased yield by 41 per cent. Hand weeding twice at20 and 40 DAS has been found to be effective inreducing weeds and increasing the grain yield of wheatat several locations. Manual weeding was less effectiveunder heavy soils and grassy weed infestation.Conventional methods of manual weeding can be

more efficient in light soils. Sharma et al. (1985)reported that hand weeding and hand hoeing 4–5 weeksafter sowing reduced the dry matter of P. minor by 39and 69 per cent, respectively, compared with unweededcontrol. Under rice soils (altered physical conditions dueto puddling), hand weeding is less effective as P. minor isfrequently chopped from ground level or the cloddylumps of soil help its re-establishment because of thepresence of moisture as a result of irrigation or rainfall(Walia and Gill, 1985).

3.5. Herbicides and their rotation

Evaluation of existing or new herbicides for thecontrol of P. minor and associated weeds is often routinework of scientists under different agro-climatic condi-tions. It is always desirable to have several herbicides toallow choice for weed control. Herbicides are aneffective and efficient tool of weed management andtheir selective use will not only provide economic yields,but also help to avoid weed resistance.Isoproturon applied at the 2-leaf stage showed a

maximum reduction in dry matter of P. minor and A.

ludoviciana (Bhan et al., 1985). Balyan et al. (1989)evaluated the effect of time of application of isoprotur-on on P. minor, A. ludoviciana, C. album and L. aphaca

in wheat (Variety WH 283) on sandy loam soil. All theweeds were susceptible to isoproturon (1.0 kg ha�1),when applied at 20–30 DAS. Tolerance increased withdelayed application. Visible phytotoxicity to wheatwas also observed following its application at 20–25days after sowing, though yield was not significantlyaffected. In wheat, isoproturon treatment could reducephotosynthetic activity, but recovery occurred within2 weeks. Susceptibility of P. minor to isoproturonwas similar at 2- and 4-leaf stage, but at 6-leaf stage,P. minor showed a marked tolerance to isoproturon(Yaduraju, 1991).The poor control of P. minor by isoproturon has

become a serious problem, which is expected to becomemore serious in the future because of increasing reportsof herbicide resistant-biotypes from other parts of India.If the resistance problem is not tackled properly andtimely, it may lead to a serious threat to wheatproduction in the belt of Indo-gangetic plains; thegrain-bowl of the country.

ARTICLE IN PRESS

Table 3

Influence of different treatments on dry matter accumulation of P.

minor

Treatments Dry matter accumulation (g pot�1)

1999–2000 2000–2001

Weed control treatments

Control 2.287 6.314

Isoproturon 0.94 kg ha�1 1.698 5.271

Tralkoxydim 0.35 kg ha�1 0.528 3.384

Fenoxaprop 0.10 kg ha�1 1.123 2.308

Clodinafop 0.06 kg ha�1 0.054 1.492

Sulfosulfuron 0.025 kg ha�1 0.051 1.438

CD at 5% 0.214 0.316

Escapes of herbicide

Clodinafop (4) 0.752 3.573

Fenoxaprop (4) 1.050 3.406

Sulfosulfuron (13) 0.890 3.069

Tralkoxydim (2) 1.136 3.756

Diclofop (2) 0.954 3.429

Isoproturon (5) — 2.877

CD at 5% 0.195 0.316

Figures in parenthesis are number of escapes of P. minor.

H. Om et al. / Crop Protection 23 (2004) 1157–1168 1165

Singh (2002) observed that the use of paraquat andglyphosate resulted in the evolution of some resistantweed biotypes and suggested a need to modify herbicideuse pattern and techniques for their increased effective-ness. Herbicide rotation, mixtures, use of surfactantsand application technology could help in delaying theresistance problem and providing economic gain bytheir judicious use. The continuous use of propanil orbutachlor in rice may result in the evolution of resistantbiotypes of Echinochloa spp. In India the rotation ofanilofos with butachlor helped in avoiding the appear-ance of resistant biotypes and emergence of new weedsin rice and increased the effectiveness of both theherbicides. The emergence of wrinkle grass (Ischaemum

rugosum) became major problem in transplanted areasof Punjab (India) due to continuous use of butachlor(Pillai, 1994). There was poor control of Ischaemum

rugosum and Cyperus iria with the application ofbutachlor and anilofos, respectively. Anilofos gave goodcontrol of Ischaemum rugosum, whereas butachlor andpretilachlor controlled Cyperus iria effectively (Dhimanand Nandal, 1995).For effective control of P. minor in the areas where

resistance for isoproturon has not occurred, substitutedurea herbicides are being used as post-emergenceapplications after first irrigation. Field experience hasshown that farmers do not use proper dose and methodof application of herbicides, which results in poorcontrol of weeds. If herbicide application is delayed,weeds escape the action of herbicides leading toincreased infestation. Diclofop-methyl or metribuzincan be used to control P. minor in the fields where poorcontrol was observed in the previous year with theisoproturon (Bhan and Kumar, 1997).Dry matter accumulation of P. minor by the applica-

tion of clodinafop (0.6 gm�2) and sulfosulfuron(0.25 gm�2) was equal and both treatments reduceddry matter (Table 3) compared to fenoxaprop-p-ethyl(1.0 gm�2), tralkoxydim (3.5 gm�2) and isoproturon(9.4 gm�2). Moreover, fenoxaprop-p-ethyl and tralkox-ydim recorded significantly lower dry matter of P. minor

as compared to isoproturon. All the herbicides reducedthe dry matter accumulation significantly compared tocontrol during both years. The number of escapes of P.minor was highest in sulfosulfuron (Bhullar et al., 2002).From 2 years study in Punjab and Haryana, Majumdaret al. (2002) concluded that clodinafop (Topik 15WP) isa boon to the farmers in controlling isoproturonresistant biotypes of P. minor. Both the herbicides(sulfosulfuron and clodinafop) were observed to reducethe population of P. minor significantly in wheat crop,but broadleaved weeds were not suppressed by clodina-fop. Rumex acetosella was beyond the control of boththe herbicides, which is creating a major problem inHaryana particularly in zero-tillage systems (Anon.,2001). The application of sulfosulfuron had just

suppressed this weed but clodinafop was quite ineffec-tive in its control. A visual phytotoxicity in wheat hasbeen observed with the use of metsulfuron, although itgives good control of broadleaved weeds includingRumex spp. (Dhiman et al., 2002).Yadav et al. (2002) commented that the use of

isoproturon should be discouraged in resistanceaffected rice–wheat growing areas with utmost priorityand more emphasis should be given on herbiciderotation. Alternate herbicides should be used properlyand farmers should be trained for proper spraying.To control the weed flora complex in resistance affectedareas, 2,4-D sodium salt and metsulfuron should beapplied alongwith alternate herbicides for better weedcontrol.The rotation of herbicides with different modes of

action may be important in avoiding the problem ofresistance. Chlorotoluron, which was used initially forthe control of P. minor had been found to provide goodcontrol of resistant biotypes (Singh et al., 1997).Moreover, cholotoroluron was found to be more activeon the S-biotypes than isoproturon. A resistant biotypeof P. minor from Israel has been reported resistant tofenoxaprop-p, but not to isoproturon or methabenzthia-zuron (Tal et al., 1996). This biotype also exhibitedcross-resistance to tralkoxydim, sethoxydim and cyclox-ydim, whereas, the Indian biotypes of resistant P. minor

are sensitive to these herbicides (Singh et al., 1995a).Cross-resistance to clodinafop-propergyl and fenoxa-prop-p-ethyl has yet to be confirmed, though differentialresponses were observed in the resistant biotypes ofPiminor in a pot study (Mahajan and Brar, 2001;Bhullar et al., 2002).

ARTICLE IN PRESS

e

Smothering • Early sowing time • Higher seed rate • Closer row spacing • Cross sowing • Better water management in rice • Green manuring before rice planting

Cultural methods

Selection • Crop diversification • Quick growing variety• Quality seed

Tillage• Zero tillage technology • Stale bed system• Shallow tillage & drill • Puddling frequency and

water level in rice

INTEGRATED WEED

MANAGEMENT

Chemical methods • Perfection of pump • Crop stage • Quantity of water • Proper herbicide dose

Mechanical methods• Type of weed flora • Intensity

New herbicide

Herbicides rotation

Farmers’ training Hoeing Hand weeding

Fig. 1. Technology choices for management of P. minor in wheat crop.

H. Om et al. / Crop Protection 23 (2004) 1157–11681166

3.6. Integrated weed management

Both wheat and P. minor are similarly well adapted tothe winter cropping regimes and improper inputmanagement can shift the competitive advantage infavour of weeds. Accordingly, a range of controlmeasures require consideration including choice of (i)wheat varieties, which provide early and vigorousgrowth, (ii) sowing date which ensures that wheatestablishes early and rapidly, (iii) levels of moisture,soil fertility and sowing depth, which favour wheat croprather than P. minor and (iv) higher seed rate, narrowrow spacing or bi-directional drilling coupled withoptimum fertilizer dose which can provide wheat witha competitive advantage over P. minor. Maintaining thecompetitive advantage is unlikely to be achieved by thecrop in the absence of use of selective herbicidesespecially where late season sowings are undertaken.Consequently, herbicide choice becomes a key manage-ment decision.Herbicide choice must be matched to the weed flora,

its stage of growth and cropping sequence. Althoughweed management is of primary significance in limitingloss of crop yield and quality, other crop protectionmeasures should also be followed to maintain propercrop health. Similarly, the post-harvest management ofcrop residues requires proper consideration to minimizeany potential loss of soil applied herbicide efficacy whilemaintaining soil nutrient status. Malik et al. (1988)suggested a proper combination of all cultural practices,i.e. improved tillage, crop diversification, herbiciderotation, herbicide application technique and trainingof users as the key ingredients of a sustainable weedmanagement system for P. minor in rice–wheat system.This means a return into traditional methods of farming

coupled with the utilization of innovative technologies.It is very clear that herbicide alone will not be thesolution to problem of resistance. Therefore, perfectsowing and weed control techniques that allow integra-tion of mechanical methods with herbicides or culturalmethods require urgent attention.After considering the above management practices of

P. minor, a flow chart (Fig. 1) could be drawn torepresent the ingredients of integrated weed manage-ment. Combination of selected practices as required andusing them in the correct weed management schedulemay be chosen as an integrated weed managementstrategy for P. minor. The management during wheatcrop and the management factors in the rice croptogether with disposal of crop residues influence themanagement of P. minor for wheat crop in rice–wheatsystem.

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