THRESHOLD LEVEL OF WEED UMBRELLA SEDGE (CYPERUS-IRIA) IN UPLAND RICE (ORYZA-SATIVA) UNDER RAIN-FED DIRECT-SEEDED CONDITION

A field experiment was carried out during the wet season of 1993 and 1 994 to find out the minimum density of the problem weed umbrella sedge or flatsedge (Cyperus iria L.), which causses the least yield reducti on in rainfed direct-seeded upland rice ('Annada'), and above which we ed control measures could be contemplated. Different densities of this weed from 40 to 400 plants/m(2) producing dry matter of 0.3-2.34 tonn es/ha reduced the grain yield of rice by 11-40%. Thus a density of 40 plants of Cyperus iria/m(2) with a dry-matter accumulation of 0.3 tonn e/ha could be considered as the threshold level in upland rice.