The influences of cropping system on weed communities of rice in Cote d'Ivoire, West Africa

In West Africa, agricultural land use for rice production is changing rapid ly with increased cropping intensity in some areas. Studies were conducted to examine how the different cropping systems are reflected in rice weed po pulations. Weed species were surveyed on 126 rice farms in the humid forest to the moist savannah zones of Me d'Ivoire. Two additional surveys were un dertaken in a peri-urban area of the savannah zone to examine the effects w ater control and cropping diversification on weed species in lowland rice s ystems. 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 Platosoma africanum and Mariscus cylindristachus were associated with the savannah. Bacopa decumbe ns, Fimbristylis littoralis, Sphenochlea zeylanica and Echinochloa colona w ere common. in both forest and savannah lowlands and, the sedges, Cyperus d ifformis and Cyperus iria, were particularly abundant in the savannah. Dissimilarities in species composition were found between unimproved and im proved lowland fields in the savannah zone. These differences can be explai ned 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 grai n yield. Sustained flooding, associated with land use intensification, favo ured a shift towards Cyperus iria and Sphenochlea zeylanica. In crop rotati ons where vegetables are grown during the dry season, Cynodon dactylon and Cyperus rotundus were more common. (C) 2001 Elsevier Science B.V. All right s reserved.