Timing of weed management and yield losses due to weeds in irrigated rice in the Sahel

The effects of differing periods of weed management were studied in experiments in five farmers' direct-seeded, irrigated rice (Oryza sativa L.) fields during the 1999 wet season (1999WS) and seven fields during the 2000 dry season (2000DS) in the Senegal River delta. Ten weed management treatments were used to identify critical periods of competition and to enable the development of more precise management recommendations. Rice yields in 1999WS ranged from 3.2 t ha(-1) with no weed control to 6.3 t ha(-1) in weed-free plots; a yield loss of 49%. Major weed species encountered in 1999WS were Bolboschoenus maritimus, Oryza longistaminata, Cyperus difformis and Echinochloa colona. In 2000DS, C difformis dominated the weed flora, and rice yields ranged from 3.7 t ha(-1) with no weed control to 7.9 t ha(-1) in weed-free plots; a yield loss of 47%. To assess the effects of weed biomass on the rice crop over time a linear mixed model was used. This highlighted the importance of E. colona and C. difformis in the wet and dry seasons, respectively, and of O. longistaminata in both seasons. The presence of E. colona early in the crop seemed related to greater levels of rice biomass in 1999WS, while O. longistaminata always had a negative effect on crop growth even at relatively low levels of infestation. Gompertz and logistic equations were fitted to data representing increasing periods of weed-free growth and weed interference, respectively. Critical periods for weed control, obtaining 95% of a weed-free yield, were estimated as between 29 to 32 days after sowing (DAS) in the WS and 4 to 83 DAS in the DS. The lower temperatures in the early DS resulted in slower growth of rice and weeds, delaying the effects of competition. Weed competition either before or after these critical periods had negligible effects on crop yield. (C) 2003 Elsevier B.V. All rights reserved.