International Journal of Agricultural Science and Research (IJASR) ISSN 2250-0057 Vol. 3, Issue 2, Jun 2013, 277-284 © TJPRC Pvt. Ltd.
WEED MANAGEMENT IN HERBICIDE TOLERANT TRANSGENIC COTTON (GOSSYPIUM HISRSUTUM L.) - A REVIEW C. NITHYA, C. CHINNUSAMY & D. RAVISANKAR DWSRC, Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India
ABSTRACT Cotton hybrids are cultivated under wider plant spacing and heavy fertilizer nutrients, which inturn invite multiple weed species infestation. Critically viewing, the manual and mechanical methods of weed control are less effective, costly, time consuming and to be repeated at frequent intervals. Many pre-emergence herbicides presently used in cotton for weed control take care of weeds only for a limited period. The available post-emergence herbicides are mostly non-selective and even directed spray of some herbicides cause considerable crop damage. This necessitates the development and testing of selective early post emergence herbicides for weed control in cotton. Hence, a brief review is presented on the type of weeds in cotton, competition between crops and weeds and their effect on seed cotton yield.
KEYWORDS: Weed Management, Roundup Ready Flex (RRF), Bollgard II, Seed Cotton Yield INTRODUCTION Cotton has retained its unique name and fame as the “King of fibres” and “White gold” because of its higher economic value among cultivable crops for quite a longer period. Globally cotton is cultivated in 34 million hectares spread over more than 70 countries with a production of 26 million tonnes. It provides employment to about 60 million people and contributes nearly 75 per cent of total raw material needs of the textile industry in India. Presently, India is the second largest producer of cotton in the world next to China. India ranks first in terms of area but its productivity is lower (502 kg lint ha -1). Among the ten major cotton growing countries, hybrid cotton cultivation occupies about 60 per cent of area which is a significant milestone in Indian cotton scenario. Transgenic Bt cotton technology has been widely accepted by Indian farmers across the country since its first commercialization in 2002. Apart from likelihood of reduction in insecticide usage by atleast 50 to 75 per cent in Bt cotton, it is also expected to ensure favourable ecological, economical and sociological returns in contrast to the harmful effects due to large scale use of insecticides (Kranti, 2002). Bt cotton has literally revolutionized cotton production in India. In a short span of eight years, 2002 to 2009, Bt cotton has generated economic benefits for farmers with halved insecticide requirements, contributed to the doubling of yield and transformed India from a cotton importer to a major exporter (Choudhary and Gaur, 2010). The introduction of herbicide-resistant crops has dramatically changed weed management in crop production systems (Owen, 2000). Following the registration of glyphosate-resistant cotton during 1997, this technology has been readily adopted by producers. Roundup Ready Flex cotton can provide producers with acceptable weed control without compromising cotton yield.
PREDOMINANT WEED FLORA IN COTTON FIELD Weed species in cotton field differ widely due to soil and environmental conditions. To evolve a successful weed management practice, identification of weeds associated with cotton crop is very much important. Balasubramanian (1985)
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observed that Trianthema portulacastrum (L.), Dactyloctenium aegyptium (Beauv.), Echinochloa colonum (L.) and Cyperus rotundus (L.) were the predominant weed species of cotton fields in sandy loam soils of Coimbatore and the weed ratio was 60 : 29 : 11 of broadleaved weeds : grasses : sedges, respectively. There were nearly 14 broadleaved weeds, five species of grasses and one sedge weed commonly associated with cotton crop (Balasubramanian, 1992). Cheema et al. (2008) reported that cotton field was infested with three different types of weeds such as broad leaf weeds, grasses and sedges. Broad leaf weeds comprising six species viz., Trianthema portulacastrum (L.), Digera arvensis. Forssk, Amaranthus viridis (L.), Cucumis prophetarum (L.), Portulaca oleracea (L.) and Tribulus terrestris (L.) was the dominant group with 54 per cent mean population followed by grasses like Cynodon dactylon (L.) Pers, Dactyloctenum aegyptium (L.), Echinochloa colonum (L.) Link. occupying 31 per cent. The lowest infestation of 15 per cent was that of sedges comprising a single species viz., Cyperus rotundus (L.) Among all the species Trianthema portulacastrum (L.) with 40 per cent density was the most serious weed infesting the trial field.
CRITICAL PERIOD OF CROP WEED COMPETITION Weed control, especially during the first eight weeks of cotton growth is essential due to the vulnerability of cotton to early season weed competition (Buchanan and Burns, 1970). Panwar and Malik (1991) reported that the competition of Trianthema portulacastrum was higher up to 50 DAS, whereas, competition of Echinochloa crus-galli was up to 100 DAS. Hence, first 60 DAS was the most critical period for crop-weed competition. Wn Jian Rong et al. (1999) found that the period of weed interference, crop damage and the critical time of crop-weed competition were 30 to 90 days which shared 50 per cent of the whole cotton growing period. Papamichail et al. (2002) reported that presence of weeds for more than three weeks after crop emergence caused significant reduction in crop growth and lint yields. Cotton is very sensitive to weed competition in the first 60 days of crop growth (Sivakumar and Subbian, 2002). Webster (2005) stated that, the requirement of early season weed control and the large number of problem species resulted in significant potential for cotton yield reduction.
EFFECT OF WEEDS ON COTTON PRODUCTIVITY Cotton being a wide spaced and relatively slow growing crop during its initial growth stages, suffers from severe weed competition and causing substantial reduction in seed cotton yields upto an extent of 69 per cent (Srinivasulu and Rao, 2000). Velayutham et al. (2002) reported that unweeded check reduced the boll number plant -1 and boll weight of cotton. Reduction in cotton yield varied from 40-85 per cent due to weed competition (Bhan and Mishra, 1993). Losses caused by weeds in cotton ranged from 45 to 75 per cent depending upon the nature and intensity of weeds (Sandhu et al., 1996). Mofett and Mcclosky (1998) also observed that seed cotton yield was reduced upto 34 per cent due to yellow nutsedge (Cyperus rotundus) infestation. Cotton being a wide spaced and relatively slow growing crop during its initial stages suffers from severe weed competition and causing substantial reduction in seed cotton yields upto an extent of 69 per cent (Srinivasulu and Rao, 2000). Mushtaq and Cheema (2008) observed that, weeds play a significant role in reducing the seed cotton yield on an average by 30 per cent.
GLYPHOSATE-RESISTANT COTTON Enhanced glyphosate resistant (Round up Ready Flex - RRF) cotton was introduced in 2006. RRF cotton exhibits both vegetative and reproductive tolerance to glyphosate to be applied POST over the top at any growth stage, without risk
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of boll abortion. RR Flex technology involves a new transformation event (MON 88913) with a different promoter with same CP4 EPSPS gene providing both vegetative and reproductive tolerance in cotton (Main et al., 2007). Topical applications of glyphosate over RR cotton were limited to 3.15 kg h a-1 from ground cracking through the fourth true leaf, while up to 5.04 kg ha -1 may be applied from ground cracking until seven days before harvest on RRF cotton (Burke et al., 2005). Increased dosages and an extended application time are beneficial since glyphosate provides broad-spectrum control of many annual and perennial grasses, sedges and broadleaf weeds (Corbett et al., 2004). Data indicated that glyphosate applied topically to RRF cotton after the 4 leaf stage did not reduce lint yield (Main et al., 2007).
WEED CONTROL IN GLYPHOSATE RESISTANT TRANSGENIC COTTON Glyphosate controls a broad spectrum of annual and perennial grasses, sedges, and broadleaf weeds and may be a viable alternative to other commonly used herbicides. Glyphosate applied twice controlled all species at least 93 per cent late in the season, and glyphosate applied three times was no more effective than glyphosate applied twice (Wilcut et al., 1996). Rotational crop flexibility because of the lack of glyphosate soil residues (Batts et al., 1998) was one of the greatest strengths of a glyphosate based weed control program. Culpepper and York (1998) reported that the popularity of glyphosate-resistant cotton cultivars reflected the broad-spectrum weed control possible with glyphosate, plus capability to produce cotton on more hectares compared with traditional weed management approaches. It must be noted that although many of the weeds commonly found in cotton (Dowler, 1995), no weeds tolerant of glyphosate or particularly difficult to control by glyphosate were present. Glyphosate can provide effective and economical post emergence control of weeds in glyphosate resistant (Roundup Ready) cotton cultivars, its application over the top are limited to the 4 leaf stage only. Planting second generation glyphosate resistant (Roundup Ready Flex) cotton cultivars would allow glyphosate applications over-the-top beyond the 4-leaf stage and up to 60 per cent open bolls (Clewis and Wilcut, 2007). Reddy and Boykin (2010) reported that, nine predominant summer annual weed species were effectively controlled by glyphosate as POST application with or without PRE herbicides with 95 per cent or more compared with no herbicidal treatments. Johnson et al. (2000) also described superior control of broadleaf weeds with treatments that contained atrazine applied POST in combination with glyphosate compared to glyphosate applied alone as POST. Similarly, Thomas et al. (2006) reported that, glyphosate formulations controlled annual grass populations at the time of treatment and was not influenced by trifloxysulfuron in mixture.
EFFECT OF GLYPHOSATE ON PRODUCTIVITY OF HERBICIDE TOLERANT TRANSGENIC COTTON Culpepper and York (1998) showed that glyphosate applied to glyphosate-tolerant cotton was an effective alternative to traditional herbicide programs. Yields and net returns were no greater with the glyphosate systems than with the more effective traditional systems. However, fewer herbicide applications and less total herbicide were required to produce equivalent yields and net returns in the glyphosate systems. Additionally, glyphosate systems gave the added benefit of crop rotational flexibility (Batts et al., 1998). According to Asher et al. (1997) glyphosate systems have provided higher yields than traditional herbicide programs. Glyphosate-only herbicide programs have resulted in equal yields and higher net returns in glyphosate-tolerant cotton (Vencill, 1998). Askew and Wilcut (1999) observed that glyphosatetolerant cotton yields were the best when soil-applied herbicides are used along with glyphosate to control weeds.
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Field studies indicated that glyphosate (1.12 kg ai ha -1) applied as PD (Post Directing) at the 6, 10 and 14 leaf growth stages did not affect the number of nodes, plant height, number of bolls or squares, total fruit retention, boll retention, or yield (File et al., 1998). Similarly, boll location on main stem branches was not affected by PD application of glyphosate (Murdock, 1999). According to May et al. (2003), transgenic pest-management-enhanced cultivars can be viewed as dual-purpose products, intended to control certain insects, facilitate weed management through herbicide tolerance, or combine the two, and to also produce yield. The main of herbicide system in their studies revealed that, glyphosate-resistant cultivars yielded 20 to 30 per cent more under glyphosate- only herbicide regimes compared to regimes containing only soil-applied herbicides. May and Murdock, (2002) also reported a 7 per cent yield increase at one of six locations when glyphosateresistant cultivars were cultivated in a glyphosate-only herbicide system compared with a herbicide regime containing soilapplied herbicides. Sanders et al. (2000) found under weed free conditions that glyphosate-resistant cotton cultivars produced with soil-applied herbicides yielded less compared with glyphosate only herbicide systems. Cotton lint yields for all glyphosate treatments were similar to yields obtained in the untreated check. In addition, other studies Viator et al. (2004) have indicated that no yield loss occurs from a post direct application.
ECONOMICS OF WEED MANAGEMENT IN HERBICIDE TOLERANT COTTON Weir et al. (1998) conducted economic analysis of Bollgard (Bt) cotton compared to conventional cotton in Mississippi and concluded that Bollgard is superior in enhancing monetary returns than conventional cotton. Production of transgenic varieties having greater net return and efficiency ranking than conventional cotton. The commercialization of glyphosate-resistant (GR) technology in soybean provided producers a highly efficacious and cost effective weed management option. Acceptable weed control and yield with consistent net income have been realized with a total post-emergence GR soybean glyphosate program when compared with conventional weed control programs (Shaw et al., 2001). A net return of Rs. 20,350 ha-1 was observed with a sequential glyphosate program in GR soybean compared to Rs. 13,550 ha-1 in conventional soybean with a standard pre-emergence herbicide application followed by a post emergence herbicide program (Reddy and Whiting, 2000).
CONCLUSIONS Post emergence spraying of potassium salt of glyphosate at 2700 g a.e ha-1 twice on 25 and 45 DAS for complete control of broad spectrum weeds with higher seed cotton yield and net income in herbicide tolerant transgenic cotton during winter irrigated seasons of Tamil Nadu.
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