Weed Problems in Rice and Possibilities of Chemical Weed Control

Unlike insect pest and disease outbreaks, crop losses due to weed competition often fail to create a sensation because of the fact that the damage is not clearly of a visual nature. The spread of modern dwarf rice varieties coupled with an increase in fertilizer use and poor land preparation often lead to serious decline in grain yields. Field experi-ments conducted at the headquarters of the All India Coordinated Rice Improvement Project at Hyderabad and at the cooperating centres all over the country have shown that the predominant weed species under lowland rice culture were mostly grasses and sedges like EchinoMoa crusgaUi, E. clanun, Panicum sp., Fimbristylis miliaeta and Cyperus sp. However, under the upland conditions perennial sedges like Cyperus rotundas and grasses like Cynodon daetylon were posing major problems. Herbicide screening trials have indica-ted that butachlor, C 19490 and USB 3584 (dinitramine) alone or in combination with the isopropyl ester of 2, 4-D, and another triazine compound C-288 applied at 4 to 6 days after planting or sowing under lowland rice culture recorded excellent weed control and superior grain yields of rice comparable to handweeded plots. However, in the uplands the choice of herbicides at present is limited only to butachlor, USB 3564, oxadiazon, and propanil although none of them are sufficiently effective in controlling all the weed flora under such heavily weed infested conditions. Management of weeds by adopting suitable crop rotations and cropping patterns to bring about possible alterations in the weed community, adoption of improved cultural and water management practices and herbicide application to the extent possible should form the basis of an integrated approach to meet the future challenges in weed control.