Biology and control of Phalaris minor Retz. (littleseed canarygrass) in wheat

Crop Protection 18 (1999) 1 — 16

Review Article

Biology and control of Phalaris minor Retz. (littleseed canarygrass) in wheat Samunder Singh , R.C. Kirkwood , G. Marshall
* Department of Bioscience and Biotechnology, University of Strathclyde, Glasgow G4 0NR, UK
Division of Plant Science, Scottish Agricultural College, Auchincruive, Ayr KA6 5HW, UK Received 9 June 1998; received in revised form 15 July 1998; accepted 29 October 1998

Abstract Phalaris minor Retz. (littleseed canarygrass) is an important winter season weed of several crops across many continents. P. minor is a prolific and competitive weed especially in wheat crops. This review considers its distribution, biology and agro-ecology. Special importance is attached to considering the value and limitations of cultural and chemical control methods together with a crop management blueprint. While the use of selective herbicides is critical to maintain economic returns of wheat production, there are issues associated with their continuous use. Undoubtedly the development of herbicide-resistant biotypes of P. minor is an epidemic in India of economic, cultural and scientific importance. Consideration is given to the nature of the resistance problem and management approaches designated to minimise the impact of resistance and perhaps avoid further spread of the epidemic. Future research strategies are discussed to address the nature of this important grass weed problem. These include the importance of agronomic and physiological research to understand the basis of weed behaviour.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Distribution; Herbicide resistance; Economics; Integrated control

1. Introduction Phalaris minor an annual grass (Poideae) infests winter season crops and may greatly affect their yield and quality. P. minor occurs in several winter crops, but it has become more pernicious in wheat due to its similar morphology and growing requirements. There are several components which govern the crop-weed competition, viz. cultivation practices, crops and varieties, climatic and edaphic factors and control practices. The initial period of 4—6 weeks following crop sowing is most critical for crop-weed competition; infestation by P. minor 45 days after sowing (DAS) has no significant effect on yield. P. minor has become the most dominant weed of wheat crops in Haryana and Punjab States of India and other wheat growing areas in north India as well as in many other countries. Recently, P. minor has

* Corresponding author. Tel.: 01292 525305; Fax: 01292 52537; E-mail: [email protected]

become resistant to the phenylurea herbicide isoproturon applied to wheat crops in India and therefore problematic in the successful cultivation of winter season crops. The present paper aims to review the biology and management strategies for the effective control of P. minor in sustainable crop production systems.

2. Origin and distribution At present 22 species of Phalaris are recognised in the world of which 11 are native to the Mediterranean, including Phalaris minor Retz. and four in southwestern USA. P. minor was reported to be a major weed in Latin America and probably reached India through the import of Mexican wheat; it was becoming a problem by the 1970s (Bhan and Chaudhary, 1976). Earlier publications (Anderson, 1961), however, indicate P. minor infestation in some parts of India. Introduction of the Mediterranean species with wheat may have dominated the native species. Adoption of high yielding dwarf wheat varieties

0261-2194/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 9 8 ) 0 0 0 9 0 - 8

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which were less competitive with grass weeds under increased fertiliser and irrigation practices favoured the dominance of P. minor in India. Surveys of wheat crops in the states of Punjab (Bir and Sidhu, 1979; Zahir and Gupta, 1979) and Haryana (Malik et al., 1981, 1985; Singh et al., 1995a) established P. minor as the most dominant weed of wheat in northwest India. Of the 10 important weeds in Pakistan, P. minor was reported to be the most dominant (Ghafoor et al. 1987). P. minor is widely distributed in all the continents of the world from Macronesia to Mediterranean, Irano-Turanic and Saharo-Sindic regions, eastern and South Africa, north and south America, Australia and the Far East (Fig. 1). It is, however, not mentioned in the list of the World’s worst weeds by Holms et al., (1997). P. minor was reported in Afghanistan, Agerbaizan, Algeria, Argentina, Armenia, Austria, Australia including Tasmania, Azores islands, Bahrain, Belgium, Belorussia, Bolivia, Brazil, Canada, Canary Islands, Colombia, Corsica, Costa Rica, Croatia, Cyperus, Egypt, Eritrea, Ethiopia, France, Georgia, Germany, Great Britain, Greece, Holland, Iberian Peninsula, India, Iran, Iraq, Israel, Italy, Jordan, Kenya, Kuwait, Libya, Luxembourg, Madeira Islands, Malta, Mexico, Morocco, Nepal, New Zealand, Oman, Pakistan, Peru, Portugal, Palestine, Qatar, Spain including Balearic Islands, S. Africa, S. Arabia, St. Helena, Sweden, Syria, Tunisia, Turkey, USA including Hawaiian islands, Uruguay and Zimbabwe (Anderson, 1961; Baldini, 1995 and personal verification of specimens at Kew Gardens Herbarium, UK). This shows that Phalaris is present in

every part of the world except Antarctica and the north Pole.

3. Taxonomy and morphology P. minor is self pollinated (2n"28, rarely 29) with a C3 photosynthetic pathway, similar to wheat. Taxonomically, the species is quite uniform except for a form with short sterile florets observed in 1828 in India and Afghanistan which was named as P. nepalensis (Anderson, 1961). This form differs from P. minor only in the length of sterile floret; no differences exist in other morphological features and it is now considered as P. minor. Morphologically, P. minor is similar to wheat until the flowering stage. There are, however, significant differences in the leaf characteristics and growth habits between wheat and P. minor. Both tillering and branching occurs in P. minor, whereas wheat has no branching habit. The length of ligule in P. minor (3.0—8.5 cm) is 3 times that of wheat but the auricles are absent. P. minor has a purplish pigmentation at the base of the stem and internode; leaves are narrow, drooping and purplish-orange sap oozes out when the leaf is broken. The length of the P. minor earhead is normally half (6 vs. 12 cm) that of wheat, maturity of P. minor is unsynchronous and starts earlier than wheat. Inflorescence maturity of P. minor starts at the top and progresses towards the base with continuous shattering of seeds before harvesting of the crop. The test weight of P. minor seeds is '20-fold lower than that of wheat.

Fig. 1. Geographical distribution of Phalaris minor Retz.

S. Singh et al. / Crop Protection 18 (1999) 1—16

4. Biology 4.1. Germination periodicity A large proportion of P. minor seeds germinate between mid-November and mid-December (Ray et al., 1982). Germination progresses rapidly between 10—20°C (Bhan and Chaudhary, 1976; Mehra and Gill, 1988), dark brown coated seeds germinating better than light yellow and green seeds (Mehra and Gill, 1988). Jimenez-Hidalgo et al. (1993) found that initially, germination of P. minor was more rapid at 20°C than at 10°C but after 3 weeks germination percentages were similar at both temperatures. Germination in Phalaris spp. was much lower in the dark (on average 13%) than in the light (on average 76%). Germination of P. minor sown at various soil depths was found to spread over a 9 week period (Kumar and Kataria, 1977) and could last up to 10—12 weeks (Walia and Gill, 1985a). The first three flushes being of much significance for crop-weed competition; in the later stages the crop closes in the rows. The emergence pattern of P. minor was affected by irrigation and hoeing (Tiwari and Bisen, 1982). Kumar and Kataria (1977) suggested that germination was initiated by the availability of soil moisture at various depths, whereas Okereke et al. (1981) observed the germination of P. minor to be unaffected by soil moisture. In general, the earliest germinating seeds from soil depths of 3—4 cm produced more tillers and seeds with higher test weights (Kumar and Kataria, 1977); the greatest proportion of seedlings (60%), however, emerged from the upper 2 cm layer after irrigation. Bhan and Chaudhary (1976) however, found that plants emerging in late December had more tillers than plants emerging in November. Singh and Ghosh (1982) found the highest number of seeds on the soil surface and this decreased with an increase in depth to 15 cm. These authors found a temperature of 17—21°C to be ideal for germination, any increase or decrease from the optimum temperature affected germination. Depths of sowing of 1.2, 2.0 or 3.3 cm had no effect on the final germination of P. minor at 18 DAS, although the time of emergence increased with increasing depths (Okereke et al., 1981). Freshly harvested mature seeds remained dormant for a period of 6 months (Singh 1998), and germination increased to 88—96% after 12 months compared to 4—24% after 5 months. The test weight (1000 seed weight) varied from 1.5 to 2.1 g depending upon soil type and growth conditions. In Spain, variation in germination capacity of different seed samples of Phalaris species collected from clay and calcareous soils appeared to be associated with soil type, probably depending upon water-holding capacity (Garcia-Baudin et al., 1980). No dormancy was observed 13 months after harvesting of P. minor. Similarly, Jimenez-Hidalgo et al. (1993)

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found considerable variations in germination of Phalaris species seeds from different locations and years but no differences were found between seeds aged 6 and 18 months. P. minor seeds tolerated anaerobic conditions by entering into secondary dormancy and by avoiding anaerobic decomposition (Parashar and Singh, 1985). The chemical status of seeds under anaerobic conditions suggested that P. minor resisted oxygen stress probably due to the formation of chemical metabolites and changes in membrane permeability (Parashar and Singh 1985). This could be the reason for the high infestation of P. minor under rice—wheat rotation areas (Singh et al., 1995a). The seed coat of P. minor is hard and may not be affected by anaerobic conditions under rice cultivation prior to wheat sowing.

4.2. Growth and development P. minor has more tillers/plant than wheat under noncompetitive conditions and its branching habit contributes to greater seed production (300—450 seeds per panicle); under non-competitive conditions up to 42 tillers/plant have been recorded in P. minor. Field studies carried out by Singh and Malik (unpublished) at Hisar (North India) with major grass and broad leaf weeds of wheat found that delay in wheat sowing from early to late November or mid-December had a significant effect on the emergence and growth patterns of wheat and its associated weeds. Flowering initiation (heading) was observed in P. minor at 66—98 days after sowing (DAS) when sown on 20 December and 10 November, respectively and increased to 50% within a week. On average, P. minor matured 20 days earlier than wheat under late (December) and normal (November) sowings, respectively. Shoot dry weight (DW) of P. minor was 89% lower than wheat in the 10 November sowing when recorded at 90 DAS in a pure stand; in the 20 December sowing, however, it was 22% higher than wheat, emphasising the greater competitiveness of P. minor in late sowing of wheat. Similarly, uptake of nitrogen (N), phosphate (P) and potash (K) by P. minor was 6, 7 and 54% higher than wheat, respectively at the 20 December sowings (Anon., 1990). The rate of biomass accumulation of P. minor is slow during the initial growth stages, dry weight increase being greater from 60 to 90 DAS compared to the first 60 days (Malik and Singh, 1993); plant height varied depending upon the growing conditions, generally growing 50—70 cm at 90 DAS (Malik and Singh, 1995). At maturity the P. minor inflorescence may be taller than the dwarf wheat varieties. The minimum and maximum heights of P. minor vary from (30 to '100 cm.

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5. Agro-ecology 5.1. Crop—weed interaction P. minor thrives under high fertility and moisture conditions (Singh and Malik, 1992a; Singh et al., 1995a) and competes vigorously with wheat reducing its yield by up to 80% depending upon weed intensity, cultivation practices and soil and environmental factors. A population of 50—500 plants/m of P. minor has been found to reduce wheat yield by 8—50% (Mehra and Gill, 1988; Singh and Malik, 1994a; Khera et al., 1995). Balyan and Malik (1989) reported that a population of 20 plants/m of P. minor had no significant effect on wheat yield; a higher population of 2000 plants/m of P. minor recorded in farmers fields in Haryana State resulted in complete crop failure (Malik and Singh, 1993, 1995). On average, a population of 200—400 plants/m of P. minor is normally observed under field conditions in Haryana and Punjab states in India. Differential effects of weed populations on wheat yields were also observed by Cudney and Hill (1979) in USA, Godinho and Costa (1981) in Portugal; Montazeri (1993) in Iran, and Afentouli and Eleftherohorinos (1996) in Greece. Abundance of P. minor in moist (irrigated) soils in wheat fields of heavy soils indicates considerable adaptation to exploit water and nutrient availability. Depletion of soil moisture was greater from the soil profile in the presence of P. minor at higher levels of N (Khera et al., 1995). Under controlled environmental conditions, water requirement of wheat and P. minor was comparable when 20—25 day old plants were placed in nutrient solution for two weeks (Singh et al., 1998a). P. minor removed 54 kg N/ha from unweeded wheat field plots compared to 10 and 7 kg/ha, respectively, from plots treated with 0.94 and 1.41 kg/ha isoproturon (Walia et al., 1997a). At 85 DAS uptake of N and K by P. minor in wheat was considerably higher compared to broad-leaved weeds, but was reduced by application of isoproturon (1.0 kg/ha) (Yadav et al., 1986). When sown in separate plots on 10, 30 November and 20 December, uptake of N, P and K by P. minor was also significantly higher at 90 DAS at the 30 November sowing (Anon., 1990). On average, uncontrolled weeds at 90 DAS removed 69, 15 and 52 kg/ha of N, P and K following 48, 41 and 54% crop loss in N, P and K uptake in the weedy plots (Johri et al., 1992); again the uptake of nutrients was greater by grass compared to broadleaved weeds. Under Punjab conditions, soil incorporation of residues from the previous crop (rice) resulted in increased N uptake by P. minor (Walia et al., 1997a). 5.2. Effect of fertiliser application on crop competition Surveys conducted in Haryana State revealed that under high fertility conditions, P. minor occurred in 92% fields compared to 67 and 32% under medium and low

fertility soils, respectively (Singh and Malik, 1992a), indicating that P. minor is a vigorous competitor of wheat under improved nutrient and moisture regimes. Weed competition with wheat was severe up to 80 kg N/ha and decreased at 120 and 160 kg N/ha due to vigorous crop growth, reduced weed population and dry matter production (Singh et al., 1984). Walia and Gill (1985a, b) also reported that higher levels of nitrogen (120 and 160 kg/ha) suppressed the population of P. minor; however, effects on wheat grain yield were nonsignificant between 120 and 160 kg N/ha. Isoproturon (0.94 kg/ha) with 40 kg N/ha provided similar yields to that of 160 kg N/ha with two hand weedings (hoeing). Increasing the nitrogen rates from 30 to 120 kg/ha resulted in a significant increase in the DW of wheat (9%) and P. minor (42%), particularly the latter (Balyan and Malik, 1989). The corresponding yield reductions under unweeded control plots at 30 and 120 kg N/ha by 40 plants/m of P. minor was 14 and 15% respectively, compared to weed-free plots. Increasing the population of P. minor under similar conditions to 160 plants/m resulted in 14 and 28% yield reductions, respectively. Prasad et al. (1993) reported that nitrogen application up to 150 kg/ha increased the DW of grassy weeds (P. minor) and resulted in a significant decrease in DW of broadleaf weeds; similar results were reported by Malik and Singh (1993). Clearly, under conditions of dense weed infestations and high levels of nitrogen application, grass weeds are highly competitive with wheat. Selection of an appropriate crop cultivar may have a profound effect on crop—weed competition (e.g. Satpaul and Gill, 1979, Gill and Mehra, 1981). They evaluated the competitive ability of different wheat varieties with P. minor, C. album and Melilotus alba. Tall (115 cm) wheat varieties (C-273 and C-306) strongly suppressed weed density, plant height and dry weight of weeds, while dwarf wheat (WL-334) allowed vigorous weed growth and development. Since the tall varieties are low yielding it is more desirable to grow medium height varieties in order to combine the reasonable yield potential and adequate smothering effect on weeds. Medium height wheat variety WH 147 and HD 2285 were found to be more competitive than HD 2009 under weedy conditions (Singh et al., 1990). A reduction in grain yield of 27—28% was observed in cv. HD 2285 and WH 147 compared to 59% in HD 2009, respectively. Improved wheat genotypes with aggressive initial growth and canopy cover are being evaluated in India for their smothering effect on weeds and increased herbicides efficiency. 6. Cultural and mechanical weed control 6.1. Sowing method Bi-directional (cross-rows, perpendicular to each other) sowing of wheat reduced the dry weight of C.

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album but had no effect on grass weeds (Malik et al., 1988a). Cross-row sowing combined with isoproturon treatment had a complementary effect and was found more desirable. Sharma et al. (1985) found that closer row spacing (15 cm) and cross-rows (22.5 cm) resulted in a significant reduction in P. minor density compared to unweeded check plots. Narrow or cross-row spacing coupled with 0.5 kg/ha of isoproturon at 2 weeks after sowing (WAS) significantly reduced P. minor DW and increased tiller numbers and yield of wheat. Similarly, Prakash et al. (1986) observed that cross-row sowings (22.5;22.5 cm) produced large number of wheat spikes/m of row length and a lower DW of weeds when isoproturon was applied at 2 WAS. In the absence of herbicide, cross-sowing resulted in a lower crop yield than closer-row sowing. Under late sowing conditions, cross-sowing of wheat (22.5 cm spaced rows) resulted in 26, 10 and 18% higher grain yield than broadcast, close (15 cm) and normal sowing (22.5 cm), respectively (Singh and Singh, 1996). The DW of P. minor under cross-row sowing was reduced by 59, 23, and 38% compared to broadcast, close and normal rows, respectively (Singh and Singh, 1996). Increased seed rate (150 kg/ha) and bi-directional sowing significantly reduced the N, P and K uptake by weeds compared with wider rows and normal seed rate (100 kg/ha), resulting in higher nutrient uptake by wheat (Johri et al., 1992). The efficacy of isoproturon increased under increased seed rates and cross-row sowing as evidenced by increased yields of wheat (Panwar et al., 1989a, 1995). On average increased seed rates and cross-row sowing results in 10—15% higher yields of wheat. 6.2. Sowing time Ray et al. (1982) observed that pre-sowing irrigation helped to germinate the first major flush of P. minor which was effectively killed by soil scarification or by application of paraquat (0.5 kg/ha). Weed free conditions for 2—3 weeks provided a good start for wheat and the later flushes of P. minor were effectively controlled by post-emergence metoxuron (1.25 kg/ha). Singh et al. (1985) observed that delayed wheat sowing decreased the density and growth of P. minor. Delayed sowing beyond November, however, resulted in reduced growth of some weeds like ». sativa, ¸. aphaca (Singh and Malik, unpublished) and A. ludoviciana (Singh et al., 1995b), but the reverse was true in the case of P. minor which showed greater emergence when sowing was delayed. Kolar and Mehra (1992) observed that November sown wheat produced a higher grain yield (3830 kg/ha) compared to October (2050 kg/ha) or December (3050 kg/ha) sown. Under Punjab conditions the DW accumulation by P. minor was found to be greater in November sowing (960 kg/ha) compared to October or December (450 kg/ha). In a study carried out at Jabalpur, wheat

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(cv. WH-147) under normal (20, 30 November), mid-late (5, 15 December) or late (20, 30 December), sowings produced 4.55, 4.26 or 3.16 t/ha, respectively (Kurchania et al., 1993). Delayed sowing also decreased weed infestations and their DW from 789 to 581 and 489 kg/ha for normal, mid late and late sown wheat, respectively. Narrow or cross-row sowings of wheat from midNovember to early December can provide a competitive edge to the crop under P. minor infestation; delayed sowings by mid to late December are optimum only where A. ludoviciana is a major weed but not with P. minor. Delayed sowing will not only reduce wheat yields due to the short growing duration but also due to increased competition where P. minor is a dominant weed. Early sowings of October are not feasible in rice—wheat rotation areas where P. minor is a major weed due to late harvesting of rice and less time being available for field preparation for wheat sowing. 6.3. Seed rate The competitive nature of wheat was found to improve with increase in seed rate from 100 to 150 kg/ha even in light soils under optimum fertiliser and irrigation conditions. High seed rates of 150 kg/ha with herbicide use can provide as good yields as weed free conditions. Increasing the seed rates from 100 to 175 kg/ha in the wheat variety WH-423 decreased the DW of weeds from 135 to 96 g/m under unweeded plots (Panwar et al., 1995) and increased average wheat yield from 4.57 to 5.44 t/ha under higher seed rates. Similar results were obtained earlier where high seed rate, cross-row sowing and isoproturon (1.0 kg/ha at 35 DAS) produced significantly higher grain yields (Panwar et al., 1989a). The use of a stale seedbed and a higher seed rate (100 vs. 150 kg/ha) alone under normal sowings (22.5 cm) were not found to be effective in reducing P. minor (Dhiman et al., 1985). 6.4. Manual/mechanical control Dhiman et al. (1985) reported that inter row-culture by hand hoe (kasola), or wheel hoe increased wheat yields by 26—29% over unweeded checks; but chemical control using isoproturon at 1.0 kg/ha increased yield by 41%. Hand weeding twice at 20 and 40 DAS has been found to be effective in reducing weeds and increasing the grain yield of wheat at several locations, though manual weeding is less effective under heavy soils and grass weed infestation. Under rice soils (altered physical conditions due to puddling) hand hoeing is less efficient as frequently P. minor is chopped at ground level or the cloddy lumps of soil help re-establish it due to the presence of moisture as a result of irrigation or rainfall (Walia and Gill, 1985a). The conventional methods of manual weeding are more efficient under light soils. Hand weeding and hand hoeing at 4—5 WAS reduced the DW of P. minor by 38 and 69%,

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respectively compared with unweeded control plots (Sharma et al., 1985). Similarly, Singh and Singh (1996) found two hand weedings to increase the yield of wheat by 46% over the weedy check; the gross margin, however, was reduced by 30% compared with isoproturon #2,4D treatments. Kataria and Kumar (1981) also found herbicides more effective than two hand weedings in a wheat field infested with P. minor and other weeds. Hand weeding is ineffective particularly with grass weeds. Increased labour cost and the non-availability of labour during peak periods of weeding make this practice impracticable. By contrast, mechanical weeding has replaced manual weeding in some locations. In Bahawalpur, Pakistan, cultural and chemical methods of weed control were evaluated in a wheat field infested with P. minor and other broad-leaved weeds. Bar harrowing after the first irrigation provided 11% higher yield than unweeded plots, whereas the highest yield was achieved with chlorotoluron (55% increase over control) applied 22 DAS (Cheema et al., 1988). Experiments are being conducted to ascertain the efficacy of bar (tooth) harrows in the flat bed and cultivators in raised bed under resistance infested fields in Haryana and Punjab states and the initial results were found to be encouraging (Malik, 1997, pers. comm.). Mechanical cleaning of seed at harvest is another useful method of checking the movement of weed seeds. Zimmer et al. (1985) tested three machines for removing weed seeds and other unwanted material. Passing seed through an air screen machine followed by gravity table removed most impurities. The purity level of crop seed was further enhanced by installing a cylinder separator which removed the seed of P. minor along with other weed species. A similar design modification of harvesters is undergoing evaluation in Australia to collect the seeds of resistant ¸olium rigidum as an integrated control practice.

7. Chemical weed control Several herbicides were evaluated and used for the control of P. minor and associated weeds in wheat under different agro-climatic conditions (Table 1). It is always desirable to have several candidate herbicides to allow choice of chemical weed control under different cropping systems. Herbicides are effective and efficient tools of weed management and their selective use will not only provide economic yields but also help to avoid/delay weed resistance. It is clear that chemical weed control out

Dr. R.K. Malik, Professor of Weed Science, CCS Haryana Agricultural University, Hisar, India.

performs mechanical and manual methods, except with resistant weed biotypes. 7.1. Weed stage at herbicide application Isoproturon applied at the 2 leaf stage showed a maximum reduction in DW of P. minor and A. fatua, whereas application at the 6 leaf stage had no effect on A. fatua (Bhan et al., 1985a). Balyan et al. (1989) evaluated the effect of time of application of isoproturon on P. minor, A. ludoviciana, C. album and ¸. aphaca infesting wheat (cv. WH-283) on a sandy loam soil. All the weeds were most susceptible to isoproturon (1.0 kg/ha) when applied at 20—30 DAS; tolerance was observed with delayed application. Visible phytotoxicity to wheat was observed following application at 20—25 DAS, though yield was not significantly affected. In wheat, isoproturon treatment can reduce photosynthetic activity but recovery occurred within two weeks (Singh et al., 1997). Application of isoproturon at 25 DAS not only resulted in a saving of 25% in herbicide rates but also increased the efficacy of weed control (Singh and Malik, 1994a). At 25 DAS, 0.75 kg/ha of isoproturon provided 93 and 85% control of P. minor and associated weeds compared to 83 and 78% control with 1.0 kg isoproturon applied 35 DAS, respectively, for two consecutive years. The corresponding yields of wheat were 5345 and 4084 kg/ha following application of isoproturon (0.75 kg/ha) at 25 DAS compared to 5010 and 4020 kg/ha with 1.0 kg/ha isoproturon applied 35 DAS (Singh and Malik, 1994a). Sharma et al. (1985) also found that application of isoproturon (0.50 kg/ha) at 2 WAS had a similar effect on P. minor to that of 1.0 kg/ha, applied 5 WAS. Under late sowing conditions lower application rates of isoproturon were required when applied at the first irrigation (20—25 DAS) as the weeds at the 2—3 leaf stage were more susceptible to herbicides than their advanced vegetative growth stages (Singh and Malik, 1993). Interaction of sowing time and application of isoproturon significantly influenced the populations of P. minor and A. ludoviciana. Susceptibility of P. minor to isoproturon was similar at the 2 or 4 leaf stage but at the 6 leaf stage P. minor showed a marked tolerance to isoproturon (Yaduraju, 1991). 7.2. Crop residue management and herbicide efficacy In situ burning of rice straw prior to wheat sowing was formerly a common practice in rice—wheat rotation areas of North India however, poor efficacy of isoproturon in wheat was often linked to straw burning (Malik and Singh, 1993). Previous work by Moss (1979) in the UK showed that the straw disposal method can have a large influence on the performance of the herbicides chlorotoluron and isoproturon against grass weed Alopecurus

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Table 1 Herbicides used for the control of P. minor in wheat under different agroclimatic conditions Herbicides

Application rate (kg/ha)

Comments

References

Chlorotoluron

1.0

Gill and Brar (1977); Cheema et al. (1988).

Diclofop-methyl

1.0—1.5

Isoproturon

1.0

Methabenzthiazuron

0.70—1.3

Metoxuron

1.2—1.75

Nitrofen

1.20—2.5

Pendimethalin Terbutryne

1.5 1.0

Tralkoxydim

0.2-0.40

Excellent control of P. minor and other broadleaf weeds Poor control of P. minor at different locations Controlled a range of broad-leaf weeds along with P. minor Good control of P. minor from 25 to 35 days after wheat sowing Broad spectrum weed control, poor heavy infestation than isoproturon or Metoxuron; poor control of C. album was also observed at at some locations Effective against P. minor both pre- and post-emergence Phytotoxic effect on crop and poor wheat yields recorded Excellent control of P. minor and 43% higher yield than control, Specific to P. minor only, however, activity recorded against A. ludoviciana and other broad-leaved weeds, but phytotoxic to crop Provided wheat yield as good as weed free plots Crop phytotoxicity at higher application rates; provided good control of P. minor and other grass and broad leaf weeds, crop toxicity largely associated with low organic content soils No effect on broadleaf weeds, P. minor control varied

myosuroides. Mehra and Sandhu (1989) conducted experiments on sandy loam soil in Punjab on the effects of burning or incorporating 5 t/ha of rice straw and the efficacy of metoxuron on P. minor. These authors found that burning or incorporation of straw had no effect on crop or weed growth. Similarly, Brar et al. (1995) reported higher yields of wheat when rice straw was burned. The effect of straw burning and efficacy of herbicides has been investigated by many workers. Ash content due to straw burning has been found to increase the adsorptive capacity of soil and to reduce the activity of isoproturon, chlorotoluron and other herbicides. The effect of organic matter on adsorption of herbicides is well documented. Burnt straw comprises only a small part of the soil organic carbon but it increases the adsorptive values to a great extent especially when present near the soil surface. The adsorptive capacity of burnt straw has been found to decline over time when mixed with soil (Hurle, 1978); its longer term effect, however, was observed by Moss (1985). These effects of ash mixing with soil due to field preparation and any decline in the adsorptive values have not been characterised under Indian conditions. Burning of straw may destroy some weed seeds lying on the soil surface; it could also stimulate the germination of others (Moss, 1979, 1980).

Yadav et al. (1984) Singh and Dhaliwal (1984); Mirkamali (1987) Shaktawat (1987) Boyall et al. (1979); Gill et al. (1979); Yadav et al. (1984); Walia and Gill (1985a); Khan and Makhdum (1987); Shaktawat (1987) Rathi and Tiwari (1981); Tomar et al. (1983); Gill and Brar (1975) Joshi and Singh (1981); Gill and Walia, 1985a; Shaktawat (1987) Shaktawat (1987) Yadav et al. (1984); Shaktawat (1987); at some locations Gill et al. (1979); Joshi and Singh, (1981); Gill and Mehra, 1987 Gill and Brar (1972, 1975, 1977) Hooda et al. (1974): Gill et al. (1979); Kataria and Kumar, 1980

Gill et al. (1979); Yadav et al. (1984) Singh et al. (1974); Gill and Brar, (1977) Tomar et al. (1983); Shaktawat (1987)

Panwar et al. (1989b) Yaduraju et al. (1992)

Burning of rice straw has been found to significantly lower the efficacy of isoproturon against P. minor in Haryana (Singh, 1996). Under Punjab conditions, significantly higher yields were obtained with 1.41 kg/ha isoproturon compared to a 0.91 kg/ha application rates with residue removal treatment. By contrast, no significant differences between isoproturon application rates with burning of residues or their incorporation were reported by Brar et al. (1995). These authors recommended the incorporation of straw residues for their long-term benefits compared with the adverse effects of burning on soil nutrient loss and environmental pollution.

8. Herbicide mixtures/surfactants Mixing of herbicides of different modes of action has been advocated as a strategy to increase weed control efficacy and to avoid resistance evolution. Several combinations of herbicides have been found effective in controlling the broad spectrum weed flora infesting wheat crops. The combination of isoproturon# dicamba, isoproturon#2,4-D and isoproturon#tralkoxydim were found to provide good control of grass

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and broad-leaved weeds (Bhan et al., 1985b; Panwar et al., 1988; Singh and Malik, 1992b, 1994b; Singh et al., 1993a; Singh and Singh, 1996). Isoproturon, methabenzthiazuron, metoxuron and diclofop-methyl were evaluated alone and with surfactant against a number of weeds and the best results were obtained when the surfactant was tank mixed with metoxuron followed by isoproturon and methabenzthiazuron for P. minor (Malik et al., 1988b). The addition of 0.1% wetter (Hyoxid X-100) increased the efficacy of methabenzthiazuron against P. minor enabling the application rate to be lowered from 0.8 to 0.52 kg/ha without affecting the level of control (Gill and Mehra 1987). Similarly, increased control of P. minor and other grass weeds was achieved by the addition of mineral oil (Welsh, 1994) or a safener, CGA 185072 to clodinafop-propargyl (Hugo and Biljon, 1993). Tank mixing of fenoxaprop-P-ethyl with flumioxazin, a protoporphyrin IX synthase was found to be effective against fenoxaprop-P-ethyl resistant biotypes of P. minor in Israel (Yaacoby et al., 1996). Fenoxaprop-P-ethyl resistance in a P. minor biotype from Israel had target site modification (Tal et al., 1996), whereas the P. minor biotypes from India seems to have acquired enhanced cytochrome P450 monooxygenase activities to detoxify isoproturon (Singh et al., 1996; 1998a, b). Addition of surfactant (Silwet L77, 0.05%) to isoproturon under glasshouse studies had no advantage against the resistant biotype (Singh et al., 1995c) however, a higher concentration of surfactant was reported to increase isoproturon control in pot studies (Yaduraju and Ahuja, 1995). The activity of sulfosulfuron (Mon 37500) was greatly increased against the resistant biotypes with the addition of surfactant under pot and field studies (Malik and Yadav, 1997; Singh, 1998). 8.1. Weed management summary It is evident from the literature that careful management is required if we are to minimise the competitive impact of P. minor in wheat cropping. Both wheat and P. minor are similarly well adapted to the winter cropping regimes that an individual management input is unlikely to move the competitive advantage in favour of wheat. Accordingly, a range of measures require consideration including choice of (1) wheat cultivars which provide aggressive canopy growth, (2) a sowing date which ensures that wheat establishes rapidly, (3) levels of soil moisture, soil fertility and sowing date which favours wheat rather than P. minor, and (4) high seed rates, narrow row spacing or bi-directional sowings coupled with high nitrogen fertiliser rates can provide wheat with a competitive advantage over P. minor. Maintaining the competitive advantage is unlikely to be achieved in the absence of the use of selective herbicides,

especially where late season sowings are dictated. Consequently, herbicide choice becomes a key management decision. Herbicide choice must be matched to the weed flora, stage of development and crop rotations. While weed control is of primary importance in limiting loss of crop yield and quality, other crop protection measures may also be required to maintain crop growth e.g. seed treatments, fungicide application. Similarly the post-harvest management of crop residues requires consideration to minimise any potential loss of soil applied herbicide efficacy but also to maintain soil nutrient status. The combination of these prudent agronomic practices will help to ensure an economic return to farmers, a sustainable cropping ecosystem which pays due attention to minimising selection pressure on weeds to evolve herbicide resistance.

9. Understanding the basis of herbicide resistance In northwest India, P. minor evolved resistance to isoproturon in wheat due to continuous use of isoproturon for more than a decade under an unbroken rice—wheat cropping pattern (Malik and Singh, 1993, 1995; Singh et al., 1993b, Walia et al., 1997b). The resistant biotypes from Haryana required a 2—8 times higher application rates of isoproturon to effect control equivalent to that in the susceptible biotypes (Malik and Singh, 1995). Similarly, the resistant biotypes of P. minor from Punjab were not controlled by even double the recommended application rates of isoproturon (1.88 kg/ha), which was phytotoxic to wheat (Walia et al., 1997b). P. minor infested most of the northwestern states of India by 1987 where wheat was a major crop and isoproturon was used in its control. Isoproturon-resistant biotypes were detected in 1991—92 in farmers’ fields and by 1997 the resistance to isoproturon had increased to 0.8 m ha wheat area in the states of Haryana and Punjab (Fig. 2). In India, P. minor resistance to isoproturon evolved with small undetected incremental build up of resistant alleles in the population. Due to cost constraints, Indian farmers used not only lower than recommended application rates, but poor application methods (sand or fertiliser mix broadcast) and inaccurate timing (35 DAS) further accelerated the evolutionary process. The use of lower than recommended application rates provided 60—70% control initially but later the level of control declined significantly due to cultivation practices (straw burning) and adulterated herbicide formulations; the latter had lower than prescribed levels of active ingredients. Increasing the application rates to full or even higher than the recommended level worked only for a transient period of 2—3 years. Marginal selectivity to isoproturon in wheat resulted in phytotoxic effects due to increased

S. Singh et al. / Crop Protection 18 (1999) 1—16

Fig. 2. Infestation of Phalaris minor in the Indian states 䊏 in 1987 and evolution of isoproturon resistance where P. minor is a serious weed but not necessary in the whole region).

application rates thus affecting the yield adversely. The absence of any other grass herbicide recommended for use in wheat by Indian farmers resulted in complete crop failures in many areas resulting in ploughing up of fields or harvesting of wheat as a green fodder for animals (Malik and Singh, 1993, 1995). Resistance to isoproturon in P. minor was not found to be due to any target site modification as isoproturon was equally effective under in vitro photosynthesis on both resistant and susceptible biotypes; in in vivo conditions, however, the resistant biotype had a lower reduction in photosynthesis and recovery was rapid and greater (Singh et al., 1997). Similarly, no differences in uptake

9

in 1997. (Grey scale depicts the states

and translocation of [C] isoproturon were observed between the resistant and susceptible biotypes; degradation, however, was more rapid in the resistant biotype (Singh et al., 1996). Increased detoxification of isoproturon in the resistant biotypes seemed to be governed by increased activity of cytochrome P450 monooxygenase enzymes as addition of cytochrome P450 inhibitors, 1-aminobenzotraizole (ABT) and piperonyl butoxide (PBO) to isoproturon, resulted in loss of resistance and inhibited its degradation in resistant biotypes (Singh et al., 1998a, b). The isoproturon-resistant biotypes have also shown cross-resistance to diclofop-methyl (Singh et al., 1995d;

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S. Singh et al. / Crop Protection 18 (1999) 1 — 16

Kirkwood et al., 1997) and to clodinafop-propargyl under pot studies (Singh et al., 1998c). In previous studies (Malik and Singh, 1995), requirement of 5 times higher application rates of diclofop-methyl by an isoproturon resistant biotype to achieve a 50% reduction in growth (GR ) was not considered evidence of cross-resistance as  diclofop-methyl provided good control under field conditions. There was no previous history of field application of diclofop-methyl in India. The second year of its field application, however, revealed reduced efficacy of diclofop-methyl and an increase in application rates by 100% failed to provide an acceptable control of isoproturon resistant P. minor biotypes (Malik, 1997, pers. comm.). Significant differences in the activity of sulfosulfuron (Malik and Yadav, 1997) and clodinafop-propargyl (Malik, 1997 pers. comm.) against the resistant and susceptible biotypes were observed under field conditions at different locations in India. Clodinafop-propargyl rapidly evolved cross-resistance to other grass weeds like A. myosuroides in the UK (Read et al., 1997; Ryan and Mills, 1997) and to Eleusine indica in Malaysia (Tiw et al., 1997). Differential response of P. minor biotypes to clodinafop were also reported from Mexico (Malik, 1996). Resistance was not found, however, to chlorotoluron in the isoproturon resistant biotypes of P. minor from India (Singh et al., 1998c). This contrasts with the situation in the UK where resistance to chlorotoluron in A. myosuroides is more than isoproturon. A fenoxaprop-P-resistant (target site) biotype of P. minor from Israel also had a low level of cross-resistance to other acetyl-coenzyme A carboxylase (ACCase) inhibitors, diclofop, clodinafop, sethoxydim and tralkoxydim, but was equally susceptible to propanil, isoproturon and methabenzthiazuron which do not inhibit this enzyme (Tal et al., 1996). The isoproturon resistant biotype of P. minor from India has not shown any cross-resistance to fenoxaprop, tralkoxydim or sethoxydim (Singh et al., 1995d; Kirkwood et al., 1997; Malik and Yadav, 1997). The 20-fold resistance to fenoxaprop-P in the Israeli biotype was considered to be due to an insensitive ACCase, whereas the Indian biotypes seemed to have metabolic resistance as cytochrome P450 monooxygenase inhibitors increased the activity of isoproturon, and to a lower extent of diclofop-methyl, against the resistant biotypes (Singh 1998). Metabolic resistance in the P. minor biotype quickly renders many herbicides ineffective if used continuously and an innovative approach is required to manage avoidance of resistance by integrating various control measures. Protoporphyrinogen oxidase inhibitors are a new group of herbicides to which resistance in weeds has yet to be observed. Flumioxazin and isopropazol have been found to provide good control of fenoxaprop-P-ethyl resistant biotypes of P. minor (Yaacoby et al., 1996) and

chlorotoluron resistant A. myosuroides (Moss and Rooke, 1997).

10. Strategies for avoidance of herbicide resistance 10.1. Herbicide rotation Control of isoproturon-resistant P. minor with fenoxaprop-P-ethyl under pot studies (Singh et al., 1995d; Kirkwood et al., 1997) was confirmed under field conditions (Malik and Yadav, 1997). Some variations in the activity of fenoxaprop reported by Malik and Yadav (1997) were due to the application stage of weeds at some locations. Montazeri (1993) and Mirkamali (1993) also found fenoxaprop to provide good control of P. minor and wild oat along with tralkoxydim and diclofop-methyl. Clodinafop applied at 48 g/ha was less effective and a higher application rate was required for satisfactory control of P. minor and other grass weeds (Mirkamali, 1993), whereas a mixture of clodinafop (18 g/ha) with mineral oil (1 l/ha) provided satisfactory control of P. minor when applied at the 2 leaf to second tiller stage (Welsh, 1994). While both fenoxaprop and clodinafop are vulnerable to the evolution of resistance, their rotation with herbicides of different modes of action might be expected to prolong the life span considerably and check the dominance of a particular weed flora. Contrary to expectation, chlorotoluron (with the same mode of action as isoproturon) was found to provide excellent control of the resistant and susceptible biotypes of P. minor under controlled environmental conditions (Singh et al., 1998c). Chlorotoluron was evaluated for its performance against P. minor prior to evolution of isoproturon resistance, and was found to be equally effective to that of isoproturon (Walia and Brar, 1996). Singh and Malik (1994a) found good control of broad-spectrum weeds and higher yields when chlorotoluron was applied by spraying than by broadcast mixed with sand or urea fertiliser. Tralkoxydim was also found to provide good control of P. minor under field conditions (Singh et al., 1993a; Walia and Brar, 1996); the absence of cross-resistance to tralkoxydim makes it more effective as it provides excellent control of another dominant grass weed, Avena ludoviciana. Improved control of A. ludoviciana, P. minor and some broadleaf weeds was achieved by tank mixing of tralkoxydim # isoproturon (Singh and Malik, 1992b; Singh et al., 1993a). Chlorotoluron can be substituted for isoproturon where resistance to the latter has been observed. Other promising herbicides which provided effective control of the resistant P. minor biotypes include terbutryne, metazachlor, propachlor, trifluralin, atrazine and pendimethalin (Singh et al., 1995d; Kirkwood et al., 1997), but some of them require field evaluation before

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recommendations can be made. Terbutryne was evaluated during the early stage of herbicide adoption under various locations of agro-climatic zones of India and was found to be effective in managing a broad spectrum of weeds; it offers a good alternative to control the resistant P. minor. Its application time needs to be perfected to avoid crop phytotoxicity, this being more prevalent in soils of low organic content. Similarly, field experiments conducted at Hisar in 1994 with trifluralin against various P. minor resistant biotypes confirmed the efficacy of trifluralin in controlling resistant biotypes (Singh and Malik, unpublished). Trifluralin has an adverse effect on wheat and needs an increased seed rate to compensate for a poor stand; depth of incorporation of trifluralin also has to be taken into account. Cross-resistance to pendimethalin was not observed under pot studies (Singh et al., 1993b, 1995d; Yaduraju and Ahuja, 1995; Kirkwood et al., 1997), the differential response under field conditions, however, may be due to variations in field conditions and availability of moisture (Malik and Singh, 1995). 10.2. Crop rotation Rotation of crops will integrate new agronomic practices and more competitive crops to help suppress the weed flora. The use of alternative herbicides which are not selective to wheat will also help to control P. minor. Resistance to isoproturon in P. minor was observed in 67% of fields under rice—wheat rotations compared to 8, 9 and 16% when rice—berseem—sunflower—wheat, sugarcane—vegetables—wheat and cotton—pigeonpea—wheat, respectively, were rotated (Malik and Singh, 1995). Sugarcane is one of the important crops to break the dominance of P. minor, not only because of its smothering effect during the later stage of growth but also due to rotation of herbicides, atrazine and simazine to which resistant P. minor is sensitive (Singh et al., 1995d; Yaduraju and Ahuja, 1995; Kirkwood et al., 1997). Similarly, adoption of winter maize and the use of triazine herbicides for weed control will be effective in controlling P. minor. In areas highly infested with resistant P. minor, delay of sunflower sowing until the end of December or early January encourages the germination of P. minor and its eradication in field preparation followed by pre-emergence application of dinitroaniline herbicides. Cultivation of oilseed crops with large biomass 45 DAS will suppress the growth and seed production of P. minor. Pulse crops are not good competitors with P. minor, but the use of an alternative herbicide will help in checking P. minor growth and infestation. Adoption of short duration vegetable crops and potato will not only provide a cash flow to farmers but will help in reducing P. minor infestation. Clethodim is not selective in wheat but it provides complete control of resistant biotypes of P. minor (Singh et al.,

11

1995), whereas rotating wheat with cabbage and the application of aziprotryne or clethodim can provide control of grass (P. minor) and broadleaf weeds (Dastgheib, 1995). Rotating wheat with green fodder crops like berseem (¹rifolium alexandrium) and lucerne (Medicago sativa) can considerably check the seed production of resistant P. minor. Barley, which has fast initial growth and biomass compared to wheat, smothers weeds, but is less remunerative under the rice—wheat rotation areas in India. Afentouli and Eleftherohorinos (1996) found that a population of 304 plants/m of P. minor had no effect on the grain yield of barley, whereas wheat yield was reduced by 36—39%. This probably reflects the fact that barley is generally considered to be more competitive compared to wheat in the presence of weeds (Dew, 1972; Fogelfors, 1977). The inhibition of seed germination, growth and seed production of various weeds due to the competitive ability and release of phytotoxic allelopathic substances by barley (Overland, 1966; Putnam and DeFrank, 1979, 1983), could be investigated with resistant P. minor. Tamak et al. (1994) reported that a 10% extract of straw and rice stubble completely inhibited the germination of P. minor. This, however, does not seem to occur under field conditions where farmers incorporate rice stubbles in soil prior to wheat sowing. 10.3. Agronomic practices Cultivation practices for field preparation, time and methods of sowing, seed rate, selection of variety and amount, method and time of fertiliser application and intercultural operations all play an important role in crop establishment and its competition with weeds. Studies on field preparation at night for wheat sowing need to be carried out to assess the effect of dark on P. minor germination; lower germination was recorded in the dark than light (Jimenez-Hidalgo et al., 1993). P. minor seed germinates largely from shallow depths, deep ploughing after wheat harvest can bury the seeds and have a profound effect on its germination and vigour in the following season. Shallow cultivation or use of the stale seedbed technique for wheat sowing with minimum soil disturbance may result in lower emergence of P. minor, thus decreasing crop competition. Zero tillage also offers advantages over conventional tillage as not only will it reduce the cost of field preparations, but may result in lower emergence of P. minor due to minimum soil disturbance. Use of the non-selective herbicides, paraquat and glyphosate for the pre-planting control of weeds can minimise the competition in normal sown wheat. Experiments conducted with a mixture of trifluralin#paraquat/glyphosate were found effective in the resistance affected areas in Haryana State (Malik, 1997, pers. comm.).

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11. Economic impact of losses Weeds not only reduce the yield and quality of crops but also utilise scarce and essential nutrients and moisture. In India, annual losses inflicted by pests including weeds were estimated to be US $ 175 million out of which weeds alone contributed a loss of 40% (Singh and Malik, 1992a). Grain yield of wheat, averaged from 11 locations in India ranged between 3119 and 4367 kg/ha under unweeded and herbicide treated plots, respectively. It has been estimated that yield reductions caused by weeds deprives farmers of US $158/ha taking the 1998 price of wheat in India at US $127.5/t; the cost of weed control using herbicide is lower than a tenth of this amount. Non-availability of manual labour for weeding at critical time for weed removal and poor efficiency with grass weeds are other constraints in achieving optimum yield targets. The increased yield obtained under herbicide treated fields was a function of 59, 50 and 46% higher uptake of N, P, and K/ha by wheat compared with unweeded conditions. On average, weeds utilised 40, 7 and 35 kg/ha of N, P, and K, respectively, costing $8/ha; application of herbicides increased nitrogen efficiency of wheat from 50 to 90% (Singh and Malik, 1992a). Application of 120 kg N/ha and isoproturon provided an improved gross margin of US $79 and $105 compared to US $72 and $106 at 160 kg N/ha, respectively, for two consecutive years (Walia and Gill, 1985a). The corresponding gross margin with unweeded control plots with 120 kg N/ha, was $32 and a loss of $3.4. Two hand weedings increased the margin to US$53 and $30/ha for the two subsequent years, respectively. Thus, the application of recommended rates of fertiliser and herbicide provided an 85% higher gross margin compared to unweeded conditions. Singh and Singh (1996) found that the sowing method in wheat influenced the financial returns. Among weed control methods, isoproturon#2,4-D treatment provided the highest cost : benefit ratio compared with two hand weedings and weedy check plots. Isoproturon #2,4-D gave the highest gross margin followed by isoproturon and pendimethalin. Manual weeding (twice) provided a lower margin than herbicidal control measures due to higher weeding costs. Herbicides are thus the most effective tools in managing the weeds and harvesting a good crop. Investing 1$ on herbicides generated an additional 9$ in increased wheat yields in India. The cost of weed control with newer herbicides to manage the resistant biotypes of P. minor could be more than the traditional phenylurea herbicides.

12. Future research requirements Diminishing economic returns from wheat production and the evolution of resistance to isoproturon in P. minor

has affected to a large extent the production of wheat in India. It is estimated that there will be a shortfall of 4.5 million tonnes in wheat production during 1998 compared to the 1997 harvest of 68.5 mt which necessitated importation of wheat (Anon., 1998). 12.1. Importance of agronomic research An integrated management strategy for the control of resistant biotypes of P. minor is required to prevent further development of cross-resistance to other herbicides. In areas where resistance has not yet evolved, weed management should be based on cultural and chemical methods. It is fundamental to prevent the movement of P. minor seed into the soil. This could be achieved by preventing the immigration of seed into the fields from external sources and by reducing or eliminating seed production by P. minor already in the field. Beneficial approaches include the use of certified seed, following quarantine procedures in checking emigration of resistant seed, adoption of clean cultivation practices to reduce the spread of seed by farm machinery and farmyard manure. Herbicide rotations require to be adopted; applying at appropriate times, at optimum rates and with accurate application methods. More research and development is required on herbicide mixing for increased efficacy, lowering application costs and the avoidance of resistance. There seems to be little merit in finding an ideal mixture as indicated by the previous information for the control of resistant P. minor which includes isoproturon as a mixture partner. It is desirable that the mixture partners should have different modes of action, crop safety and provide efficient weed control. It is essential, therefore, that herbicides be considered as just one component of an overall integrated system together with cultural control and other management strategies, and that agronomic principles be considered when developing this system. It is clear that in future weed management strategies will require to have an increased reliance upon integrating cultural and chemical methods. This is especially important in managing the continued spread of herbicide resistant biotypes. The practical requirements of such research programmes are extensive and require priorities to be given in order that progress can be made. Perhaps one of the key areas where progress is required is in relation to the seed bank dynamics of P. minor. The influence of cultural and chemical control on weed seed populations could provide an insight on the short- and long-term value of weed control methods. In addition novel treatments will be essential, e.g. soil heating via polyethylene film (solarisation) can kill weed seeds (Yaduraju and Ahuja, 1996). Cultivation of the soil at night may reduce the proportion of weed seeds which subsequently germinate under daylight. An improved understanding of the germination ecophysiology of

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P. minor could target research at examining seed dormancy, longevity and response to moisture. Such studies could provide a valuable link with more classical field trials examining the effect of tillage and weed control practices on the seedbank population of P. minor. The raised bed planting method which can help in improved mechanical control between crop rows and provide differential moisture regimes to favour wheat (P. minor largely germinates from soil surface under high moisture conditions) offers great scope to Indian farmers and needs extensive field evaluation as part of an integrated management approach. Practical, on-farm participatory research could be adopted to focus upon the characterisation of P. minor in crops as previously reported for other ecosystems (Derksen et al., 1994; Savary et al., 1994; McRoberts et al., 1995). In addition to improving our understanding of the biology and physiology of P. minor, little is known about the nature of genetic variation. Such studies might examine levels of intra and inter population genetic variations using different molecular-based techniques e.g. random amplified polymorphic DNA (RAPDs) (Moodie et al., 1997) and amplified fragment length polymorphism (AFLPs) (Andrews et al., 1998). Of special interest would be a comparison of biotypes which differ in their herbicide sensitivity or other polymorphisms, e.g. germination periodicity. It follows that the determination of the nature of the inheritance of herbicide resistance in P. minor would also be worthy of attention in the context of resistance management. It is estimated that there are now 234 weeds resistant to herbicides of different modes of action (Singh, 1998). If the present trend of resistance to herbicides remains unabated there will be only few effective herbicides for future use. Wheat is the most important winter season crop of north India and there is limited scope to rotate wheat areas to other crops. Herbicide rotation can be effective, but not as a sole method of success for the long term sustainability. Adoption of herbicide-tolerant crops is another potential opportunity for farmers to keep weeds under control (Marshall, 1998). Use of these genetically modified wheat crops, however, depends largely on the cost of seed and currently is beyond the reach of most farmers in the developing countries. As an alternative to genetically modified crop, CIMMYTs approach of identifying potential wheat genotypes for their aggressive growth and weed competitiveness for different agroclimatic regions and their integration with improved cultivation practices is being undertaken in India.

References Afentouli, C.G. Eleftherohorinos, I.G., 1996. Littleseed canarygrass (Phalaris minor) and short spiked canarygrass (Phalaris brachystachys) interference in wheat and barley. Weed Sci. 44, 560—565.

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Anderson, D.E., 1961. Taxonomy and distribution of the genus Phalaris. Iowa State J. Sci. 36, 1—96. Andrews, T.S., Morrison, I.N., Penner, G.A., 1998. Monitoring the spread of ACCase inhibitor resistance among wild oat (Avena fatua) patches using AFLP analysis. Weed Sci. 46, 196—199. Anonymous, 1990. Final Technical Report, All India Co-ordinated Research Programme on Weed Control. Haryana Agricultural University, Hisar, 180 pp. Anonymous, 1998. Pressure on Govt to raise wheat price. The Hindustan Times, New Delhi (India) 10 April 1998. Baldini, R.M., 1995. Revision of the genus Phalaris L. Gramineae. ¼ebbia 49, 265—329. Balyan, R.S., Malik, R.K., 1989. Influence of nitrogen on competition of wild canary grass (Phalaris minor Ritz.) in wheat (¹riticum aestivum L.). Pestology 13, 5—6. Balyan, R.S., Malik, R.K., Bhan, V.M., 1989. Effect of time of application of isoproturon on the control of weeds in wheat (¹riticum aestivum). Indian J. Weed Sci. 20, 10—14. Bhan, V.M., Chaudhary, D.B.B., 1976. Germination, growth and reproductive behaviour of Phalaris minor Retz. as affected by date of planting. Indian J. Weed Sci. 8, 126—130. Bhan, V.M., Gupta, V.K., Malik, R.K., 1985a. Effect of isoproturon at different stages of wheat and associated weeds. Indian Society of Weed Science Annual Conf., Anand, Gujrat, Abstract, 42 pp. Bhan, V.M., Yadav, S.K., Panwar, R.S., Singh, S.P., 1985b. The influence of substituted urea herbicides alone and in combination with 2,4-D on the control of weeds in wheat. Beitr. Trop. Landwirtsch. Veterinaermed. 23, 177—181. Bir, S.S., Sidhu, M., 1979. Observation in the weed flora of cultivable lands in Punjab-wheat fields in Patiala district. New Botanist 6, 79—89. Boyall, L.A., Ingram, G.H., Williams, D.J., 1980. Isoproturon, a selective herbicide for post-emergence grass weed control in Australian and Indian cereal crops. Proc. 7th Asian-Pacific Weed Science Society Conf., Sydney, 1979. Supplementary vol., pp. 55—58. Brar, S.S., Walia, S.S., Singh, J., 1995. Efficacy of isoproturon for control of Phalaris minor in wheat as influenced by residue management system under rice—wheat sequence. Indian J. Ecol. 22, 11—16. Catizone, P., Viggiani, P., 1980. Un quadriennio di ricerche sulle falaridi infestanti il grano. Atti Gioruate Fitopatalogiche, (Suppl.), 3, 213—228. Cheema, M.S., Afzal, M., Ahmed, M.S., 1988. Economics of weed control in wheat. Pak. J. Agric. Res. 9, 32—36. Cudney, D.W., Hill, J.E., 1979. The response of wheat grown with three population levels of canarygrass to various herbicide treatments. Proc. Western Weed Sci. Soc. (USA) 32, 55—56. Dastgheib, F., 1995. Weed control in cabbage with aziprotryne, clethodim and their combination. Proc. 48th New Zealand Plant Protection Conf., Hastings, New Zealand, 8—10 August, 1995, pp. 331—332. Derksen, D.A,, Thomas, A.G., Loeppky, H.A., Swanton, C.J., 1994. Impact of post-emergence herbicides on weed communities: fallow within tillage system. Weed Sci. 42, 184—194. Dew, D.A., 1972. Effect of wild oat density on yields of wheat, barley, rape and flax. Proc. North Central Weed Control Conf. USA, 27, pp. 38—39. Dhiman, S.D., Mohan, D.S.R., Sharma, H.C., 1985. Studies on cultural methods of weed control in wheat. Indian J. Agron. 30, 10—14. Fogelfors, H., 1977. The competition between barley and five weed species as influenced by MCPA treatments. Swedish J. Agric. Res. 7, 147—151. Garcia-Baudin, J.M., Salto, T., Aguirre, R., 1980. Preliminary note on the germination of Phalaris brachystachys Link. and P. minor Retz. Proc. 10th Int. Colloquim on Weed Ecology Biology and Systematics, Montpellier, France vol. 1, pp. 123—131. Ghafoor, A., Shad, R.A., Sher, M.A., 1987. Ten most important weeds in Pakistan. Prog. Farming 7, 17—20.

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Singh, D., Dhaliwal, H.S., 1984. Control of Phalaris minor Retz. and broad-leaved weeds in wheat with selective herbicides. Pesticides 18, 45—47. Singh, G., Singh, O.P., 1996. Response of late sown wheat (¹riticum aestivum) to seeding methods and weed control measures in flood prone areas. Indian J. Agron. 41, 237—242. Singh, G., Singh, D., Singh, N.K. 1985. Effect of dates of sowing of wheat on Phalaris minor and wheat yield. Indian Society of Weed Science Annual Conf., Anand, Gujrat, Abstract, 42 pp. Singh, H.G., Shaktawat, M.S., Rathore, A.S. 1974. Comparative efficacy of terbutryne and 2,4-D based herbicides for weed control in wheat. Pest Articles and News Summary 20, 300—303. Singh, R.D., Ghosh, A.K., 1982. Soil profile distribution and effect of temperature and soil depth on germination of Phalaris minor Retz. Indian Society of Weed Science Annual Conf., Hisar, Abstract, pp. 41—42. Singh, S., 1998. Studies on the mechanism of isoproturon resistance in Phalaris minor (littleseed canarygrass). Ph.D Thesis. University of Strathclyde, Glasgow, UK. Singh, S., 1996. Effect of rice straw burning, nitrogen and herbicides on wheat and Phalaris minor. Ph.D. Thesis, CCS Haryana Agricultural University, Hisar, India. Singh, S., Malik, R.K., 1992a. Weed management and fertiliser utilization. Fertiliser News 37, 59—63. Singh, S., Malik, R.K., 1992b. Evaluation of tralkoxydim against weeds in wheat. Test of Agrochemicals and Cultivars. Ann. Appl. Biol., 13, 60—61 (120) (Suppl.) Singh, S., Malik, R.K., 1993. Effect of time of application of isoproturon on the control of weeds in late sown wheat. Indian J. Weed Sci. 25, 66—69. Singh, S., Malik, R.K., 1994a. Effect of application methods of chlorotoluron on the control of Phalaris minor in wheat (¹riticum aestivum). Indian J. Agron. 39, 23—26. Singh, S., Malik, R.K. 1994b. Effect of 2,4—D and tribenuron on the control of broad-leaf weeds in wheat (¹riticum aestivum). Indian J. Agron. 39, 410—414. Singh, S., Kirkwood, R.C., Marshall, G., 1995c. The effect of Silwet L-77 or fenoxaprop-P-ethyl on the efficacy of isoproturon applied to isoproturon resistant Phalaris minor. Proc. Brighton Crop Protection Confer.-Weeds 1, 231—236. Singh, S., Kirkwood, R.C., Marshall, G., 1995d. Evaluation of isoproturon resistant littleseed canarygrass (Phalaris minor) to a range of graminicides. Annual Meeting Weed Science Society of America, 30 January—2 February, Seattle, USA, Abstract 162, p. 54. Singh, S., Kirkwood, R.C., Marshall, G., 1996. Uptake, translocation and metabolism of isoproturon in wheat, susceptible and resistant biotypes of Phalaris minor. In: Brown, H., Cussans, G.W., Devine, M.D., Duke, S.O., Fernandez-Quintanilla, C., Helweg, A., Labrada, R.E., Landers, M., Kudsk, P., Streibig, J. C., Proc. 2nd Int. Weed Control Congress, Copenhagen (Denmark), 25—28 June 1996. 2, 529—534. Singh, S., Kirkwood, R.C., Marshall, G., 1997. Effects of isoproturon on photosynthesis in susceptible and resistant biotypes of Phalaris minor and wheat. Weed Res. 37, 315—324. Singh, S., Kirkwood, R.C., Marshall, G. 1998a. Effect of ABT on the activity and rate of degradation of isoproturon in resistant biotype of Phalaris minor and wheat. Pest. Sci. 53, 123—132. Singh, S., Kirkwood, R.C., Marshall, G., 1998b. Effect of monooxygenase inhibitor piperonyl butoxide on herbicidal activity and metabolism of isoproturon in resistant biotypes of Phalaris minor. Pest. Biochem. Physiol. 59, 143—153. Singh, S., Kirkwood, R.C., Marshall, G., 1998c. Control of isoproturon resistant biotypes of Phalaris minor by chlorotoluron and clodinafoppropargyl. Resistant Pest Manage. (in press). Singh, S., Malik, R.K., Balyan, R.S., 1990. Weed management in wheat. Haryana Farming (India) 19, 15—16. Singh, S., Malik, R.K. Balyan, R.S., Singh, S., 1995a. Distribution of weed flora of wheat in Haryana. Indian J. Weed Sci. 27, 114—121.

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