Cropping intensity effects on upland rice yield and sustainability in West Africa

Increased demand for land is forcing many farmers to intensify their upland rice-based systems with a gradual shift from bush fallow rotation towards sedentary agricultural production. However, farmers are not adapting appropriate management practices to the intensified land use, which may threaten sustainability. Diagnostic field trials were conducted at four sites in during 1994 and 1997, comparing 228 farmers' fields with diverse cropping histories. The objective was to improve future research priority setting and to guide site and systems-specific targeting of available technical options. Soil samples (0-20 ) were analyzed for pH, organic matter quantity and quality, and the potential soil N supplying capacity. Weed species and dry biomass and rice grain yield were determined under farmers' management and in three super-imposed researcher-managed subplots (hand weeding at 28, 56, and 84 days, 30 mineral fertilizer N application, and a combination of both). Yield gaps were attributed to weeds and nitrogen, based on yield response to researchers' management in intensified systems. Increased cropping intensity and reduced fallow duration were associated with yield reduction, which was largest at the sites in the derived savanna (1.48 vs. 1.15 ) and the bimodal forest zones (1.55 vs. 1.02 ). Intensification-induced yield loss was about 25% and appeared to be related mainly to increased weed infestation (72% more weed biomass) and declining soil quality (about 20% less soil organic C content and N supply). Weeds were the dominant factor responsible for rice yield loss in the forest area (explaining 65% of the yield gap) and appeared to play a lesser role in the savanna. The reduction in soil organic carbon and N supplying capacity was strongest in the derived savanna zone where N supply explained 35% of the yield gap. In the two savanna environments, a large share of the intensifiaction-related yield reduction could not be ascribed to either weeds or N (probably related to changes in soil physical parameters or pests). Long-term upland rice productivity can thus not be sustained at current intensification practices. Improved management strategies should aim primarily at reducing weed pressure and improving soil organic matter content and N supply. Technical options are likely to vary with agro-ecological zones.