Mixed-weed infestations: prediction of crop losses for economic weed management in rice.
Two experimental approaches, involving density series of Eclipta alba, Leptochloa filiformis, Eleusine indica, a 1:1 mixture of Echinochloa colona [Echinochloa colonum] and Echinochloa crus-galli, and a range of rice cv. Oryzica 1 densities, were used to estimate crop losses for late weed infestations and the benefit of increasing rice densities to reduce weed competition. The experiments were conducted at CIAT in Colombia, and current values of rice and farming inputs were used for economic analyses. For farmers who obtained av. rice yields (5600 kg/ha), only weed densities >20 plants/m2 justified herbicide applications after 30 d after emergence. Incentives to reduce herbicide use were even greater on low-yielding farms (3500 kg/ha), where herbicides were only economical at weed densities of >30 plants. Hand weeding was a viable alternative to herbicide use at weed densities of >25 plants. Using high rice densities, which resulted in large numbers of tillers/m2, was economical even at dense weed infestations (>100 plants). It was suggested that omitting late post-em. herbicide application could reduce weed control costs in rice in Colombia by 28%.