Response of lowland rice to agronomic management under different hydrological regimes in an inland valley of Ivory Coast

A large proportion of the rice in West Africa is produced in rainfed lowland ecosystems, mainly in inland valleys. The hydrological conditions (duration and intensity of flooding) vary with the toposequence position between the fringe and the centre of the valley. Production methods tend to evolve from the currently predominant unbunded plots without external input use, to input-intensive production in bunded plots. Agronomic management interventions co-evolve and may include varietal choice, herbicide use, and mineral N fertilizer application. The response of rice and the associated weeds to such interventions is likely to vary with the prevailing hydrological regime. A 2-year field experiment was conducted in northern Ivory Coast to determine the impact of water regime (plot position in the valley, presence of bunds) and input use (mineral N fertilizer and herbicide) on the productivity (yield and N use efficiency) of traditional and modern rainfed lowland rice cultivars and the biomass and composition of the associated weeds. Installing field bunds reduced seasonal variations in ponded water depth and resulted in a mean increase in rice grain yield of 30–40% (p < 0.005). This increase was associated with a 25% lower cumulative weed biomass and a several-fold increase in the agronomic use efficiency of applied mineral N in bunded than unbunded plots. Under low input management, traditional varieties tended to out-yield modern varieties in unbunded plots. Improved crop management such as herbicide and fertilizer application, and the construction of field bunds was more effective to increase the yield and N use efficiency in the flooded valley centre than in the drought-prone valley fringes. There is a need for site-specific targeting of modern cultivars, land development and improved production methods in the inland valleys of the West African savanna zone.