Cropping intensity effects on upland rice yield and sustainability in WestAfrica

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 approp riate management practices to the intensified land use, which may threaten sustainability. Diagnostic field trials were conducted at four sites in dur ing 1994 and 1997, comparing 228 farmers' fields with diverse cropping hist ories. 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 qual ity, and the potential soil N supplying capacity. Weed species and dry biom ass and rice grain yield were determined under farmers' management and in t hree 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 t o researchers' management in intensified systems. Increased cropping intens ity 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 b imodal forest zones (1.55 vs. 1.02 ). Intensification-induced yield loss wa s about 25% and appeared to be related mainly to increased weed infestation (72% more weed biomass) and declining soil quality (about 20% less soil or ganic C content and N supply). Weeds were the dominant factor responsible f or 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 organ ic carbon and N supplying capacity was strongest in the derived savanna zon e where N supply explained 35% of the yield gap. In the two savanna environ ments, a large share of the intensifiaction-related yield reduction could n ot be ascribed to either weeds or N (probably related to changes in soil ph ysical parameters or pests). Long-term upland rice productivity can thus no t be sustained at current intensification practices. Improved management st rategies should aim primarily at reducing weed pressure and improving soil organic matter content and N supply. Technical options are likely to vary w ith agro-ecological zones.