Integrated weed management in green gram (Vigna radiata (L.) Wilczek)

Abstract

Weeds are known to cause substantial yield losses in green gram, owing to the slow initial growth of the crop and severe competition by weeds. Although hand weeding has been proved effective in reducing weed infestation, unavailability of timely labour and high labour costs have created practical difficulties in adopting manual weeding on a large scale, thus increasing the dependency on chemical weed control using herbicides. However, continuous use of herbicides results in problems like environmental contamination and resistance development. The present study entitled “Integrated weed management in green gram [Vigna radiata (L.) Wilczek]” was undertaken with the objective of developing an economically viable integrated weed management strategy that provided broad spectrum weed control in green gram. The field experiment was carried out from December 2020 to March 2021 at Kotteppadam rice field under the Department of Agronomy, College of Agriculture, Vellanikkara. The experiment included eight treatments replicated thrice in a randomized block design (RBD). The treatments were: stale seed bed for 14 days followed by (fb) shallow digging (T 1 ), stale seed bed for 14 days fb shallow digging fb oxyfluorfen at 0-3 DAS (T 2 ), oxyfluorfen at 0-3 DAS fb hand weeding at 25 DAS (T 3 ), imazethapyr at 0-3 DAS fb hand weeding at 25 DAS (T 4 ), imazethapyr + imazamox at 0-3 DAS fb hand weeding at 25 DAS (T 5 ), diclosulam at 0-3 DAS fb hand weeding at 25 DAS, hand weeding at 20 DAS and 40 DAS (T 7 ) and unweeded control (T 8 ). The major broad leaved weeds observed were Melochia corchorifolia, Aeschynomene indica, Grangea maderaspatana, Phyllanthus amara, Heliotropium indicum, Mimosa invisa and Mimosa pudica, whereas Brachiaria mutica, Digitaria ciliaris, Echinochloa colona, Oryza sativa and Cynodon dactylon were the predominant grasses. Hand weeding resulted in the lowest weed density as well as weed dry matter production at all stages of observation. All the pre-emergence herbicidesintegrated with hand weeding were also equally effective, among which imazethapyr+ imazamox fb hand weeding and diclosulam fb hand weeding recorded the highest weed control efficiency of 97 per cent. Similarly, nutrient removal by weeds was the lowest in hand weeding, which was statistically similar to all the treatments where herbicide application was integrated with hand weeding. Stale seed bed provided effective control of weeds only during the early stages of crop growth. Weed density, weed dry matter production, nutrient removal by weeds and weed index were invariably the highest in unweeded control. Number of branches/plant, number of pods/plant, grain yield and haulm yield of green gram were higher in hand weeding and the treatments where herbicide application was integrated with hand weeding. Uncontrolled weed growth resulted in yield losses up to 70 per cent in green gram, which was reduced to 5.82 per cent and 8.22 per cent in imazethapyr + imazamox fb hand weeding and diclosulam fb hand weeding respectively. These treatments registered high grain and haulm yields of 549 kg and 1296 kg, and 535 kg and 1271 kg respectively. Although the application of pre-emergence herbicides resulted in a reduction in the soil microbial count at flowering as compared to the untreated plots, the population recovered by harvest stage in the plots treated with oxyfluorfen, imazethapyr and imazethapyr + imazamox. However, diclosulam was found to have a pronounced inhibitory effect on the soil microbial population at harvest as well. Available N, P and K contents in soil were found the lowest in unweeded control. Although the gross returns were the highest in hand weeding, imazethapyr + imazamox fb hand weeding and diclosulam fb hand weeding registered the highest net returns of Rs. 9,126/ha and Rs. 8,733/ha, respectively. Accordingly, B:C ratio was also the highest in imazethapyr + imazamox fb hand weeding (1.28) and diclosulam fb hand weeding (1.27). Thus, application of these two pre-emergence herbicides followed by hand weeding at 25 DAS proved highly successful in achieving season- long weed control in green gram, thereby enhancing the yields and fetching higher monetary returns.