Weed population and biomass in direct-seeded rice (Oryza sativa) as influenced by irrigation

The response of lowland weeds and rice (Oryza sativa L.) to various water-m anagement practices was determined at the wetland farms of Coimbatore, duri ng the rainy and winter seasons (1997-98 and 1998-99). Weed population and growth were generally higher during the rainy season than winter season. Va riation in irrigation regimes brought about distinct changes in weed growth and establishment in rice fields. Irrigating 5 cm depth 1 day after disapp earance of ponded water in transplanted rice recorded the lowest number of weeds, viz. grasses, sedges and broad-leaved weeds and the lowest dry weigh t of weeds. However, irrigating 5 cm depth 1 day after disappearance of pon ded water in direct-seeded rice and continuous submergence of 2.5 cm depth were at par with transplanted rice. Lower water regimes recorded the maximu m weed population and biomass due to lack of flooding. Transplanted rice recorded more water use than the direct-seeded rice. The highest grain yield was recorded with direct-seeded rice during the rainy a nd by transplanted, rice during winter season. Benefit : cost ratio was mor e for the higher water regimes of direct-seeded rice than transplanted rice as transplanted rice required more labour for raining nursery and for tran splanting.