Studies on crop weed competition and chemical weed control for rice in Sikkim.
In field trials conducted at Tadong, Sikkim, in India during 1981-1982 the effects were evaluated of pre-em. applied oxyfluorfen (2.5 litres/ha), butachlor granules (7 kg/ha) and oxadiazon (1.5 litres), and post-em. applied thiobencarb (3 litres) and 2,4-D (1 kg) compared to hand weeding twice for the control of weeds in rice cv. DR-92. The effects of weed competition on rice yields and yield components were also evaluated. The most problematic weeds were Echinochloa crus-galli, Echinochloa colonum, Cyperus rotundus, Ageratum conyzoides, Cyperus eragrostis, Hydrocotyle hepalensis and Paspalum pasplodes [Paspalum distichum]. All herbicide treatments increased rice grain yields and decreased weed DW from unweeded control values of 1100 and 69.4 kg/ha, resp., to 1658-2119 and 29.3-44.6 kg, resp., with thiobencarb resulting in the greatest rice yields and the least weed DW. Thiobencarb resulted in the greatest net returns (Rs 1513.80 per ha) compared to the unweeded control. In the crop-weed competition studies, the removal of all weeds (by hand weeding) for up to 40 d after planting resulted in the greatest rice grain yields of 6051 kg, compared to unweeded control values of 3312 kg.