The influences of cropping system on weed communities of rice in Côte d'Ivoire, West Africa.
In West Africa, agricultural land use for rice production is changing rapidly with increased cropping intensity in some areas. Studies were conducted to examine how the different cropping systems are reflected in rice weed populations. Weed species were surveyed on 126 rice farms in the humid forest to the moist savannah zones of Côte d'Ivoire. Two additional surveys were undertaken in a peri-urban area of the savannah zone to examine the effects of water control and cropping diversification on weed species in lowland rice systems. Multivariate techniques were used to describe the weed communities encountered. Weed species in upland rice differed between forest and savannah zones with Chromolaena odorata being common in the former and Platostoma africanum and Mariscus cylindristachus were associated with the savannah. Bacopa decumbens, Fimbristylis littoralis, Sphenochlea zeylanica and Echinochloa colona were common in both forest and savannah lowlands and, the sedges, Cyperus difformis and Cyperus iria, were particularly abundant in the savannah. Dissimilarities in species composition were found between unimproved and improved lowland fields in the savannah zone. These differences can be explained largely by difference in water management and hydrology, rather than by cropping intensification. Weed biomass in irrigated systems was lower than in open or bunded rainfed fields, though there were no differences in grain yield. Sustained flooding, associated with land use intensification, favoured a shift towards Cyperus iria and Sphenochlea zeylanica. In crop rotations where vegetables are grown during the dry season, Cynodon dactylon and Cyperus rotundus were more common.