Evaluation of Weed Control Efficacy and Seed Cotton Yield in Transgenic Cotton

The research was conducted with transgenic cotton hybrid during winter irrigated season of 2009-10 and 2010-11 at the experimental site of Tamil Nadu Agricultural University, Coimbatore, with seven weed management methods viz., pendimethalin at 1.0 kg ha-1 + hand weeding, pendimethalin at 1.0 kg ha-1 + power weeder weeding, hand weeding on 25 and 45 DAS, power weeder weeding on 25 and 45 DAS, hand weeding on 25 + power weeder weeding 45 DAS,power weeder weeding on 25 + hand weeding 45 DAS and unweeded check. In both the years, higher seed cotton yield was recorded in hand weeding twice at 25 and 45 DAS and pendimethalin at 1.0 kg ha-1 + hand weeding also efficiently suppressed the weeds and recorded higher seed cotton yield in transgenic cotton. Introduction Cotton is one of the important crop that has been genetically altered to address challenges with insect control. 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). Cotton hybrids are cultivated under wider plant spacing and heavily fertilized, which inturn invite multiple weed species infestation. Due to increased scarcity of labourers, manual weeding is not economical and the available pre emergence herbicide has lesser weed control efficiency in controlling major problematic weeds.Mechanical weed control method was partially effective because most of the weeds growing in intra rows escaped weeding and incessant rains make the manual weeding impossible which resulted in an inefficient weed control situation and low seed cotton yield (Rajeswari and Charyulu, 1996). With these in view, the study was conducted with integration of one or two methods for the effective control of weeds with better economic returns in transgenic cotton hybrids. Materials and Methods Experimental Site The research was conducted with an experimental site of Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu. The farm is situated in Western Agro climatic zone of Tamil Nadu with 11o N longitude, 77o E latitude and at an altitude of 426.7 m above mean sea level and the farm receives the normal total annual rainfall of 674.2 mm in 45.8 rainy days. Trial was conducted in soil with sandy clay loam type of soil and the soil was medium in organic carbon content and the available nutrient status was low in nitrogen, medium in phosphorus and high in potassium. Experimental Design, Selection of Cultivar and Sowing The experiment was laid out in randomized complete block design (RBD) with seven treatments and replicated thrice. The gross plot size adopted was 48.6 Sq. meter (9.0 m × 5.4 m). Transgenic cotton hybrid Bollgard II was used for the study. The cotton was sown manually keeping the distance of 90 × 90 cm at 2.5 kg·ha−1 of delinted seeds during winter irrigated season of Tamil Nadu. Treatment Details Treatments consisted of seven weed management methods viz., pendimethalin at 1.0 kg ha-1 + hand weeding (T1), pendimethalin at 1.0 kg ha-1 + power weeder weeding (T2), hand weeding on 25 and 45 DAS (T3), power weeder weeding on 25 and 45 DAS (T4), hand weeding on 25 + power weeder weeding 45 DAS (T5), power weeder weeding on 25 + hand weeding 45 DAS (T6) and unweeded check (T7) in transgenic cotton. As per the treatment schedule pendimethalin was applied as pre-emergence. Hand operated knapsack sprayer fitted with a flat fan type nozzle (WFN 40) was used for spraying the herbicides adopting a spray volume of 500 litres ha−1. Observations on Weeds Weed Density The weed count was recorded species wise using 0.5 m × 0.5 m quadrat from four randomly fixed places in each plot and the weeds falling within the frames of the quadrat were counted and the mean values were expressed in number m−2. Weed Dry Weight The weeds falling within the frames of the quadrat were collected, categorised into grasses, sedges and broad-leaved weeds, shade dried and later dried in hot-air oven at 80°C for 72 hrs. Weed Control Efficiency Weed control efficiency (WCE) was calculated as per the procedure given by Mani et al. (1973). Where, WCE weed control efficiency (per cent) WDc weed biomass (g m -2) in control plot. WDtweed biomass (g m -2) in treated plot. Seed cotton yield The seed cotton yield obtained from the net plot area at each picking was recorded and expressed in kg ha-1. Statistical Analysis The data were statistically analysed following the procedure given by Gomez and Gomez (2010) for randomised block