A framework to improve fertilizer recommendations for irrigated rice in West Africa

A framework to optimize soil fertility management in irrigated rice, based on soil and climate variability, fertilizer and paddy prices and farmers' objectives is presented. The framework uses three models in succession. The dynamic ecophysiological ORYZAS model simulates potential rice yields under irrigation, based on weather conditions, cultivar choice and sowing date. This yield potential is used in the static FERRIZ model, together with site specific information on recovery efficiency of applied N, P and K, indigenous soil N, P and K supply, and maximum N, P and K accumulation and dilution in rice dry matter. Resulting outputs are required fertilizer doses to obtain different target yields depending on yield potential and soil nutrient supply. The framework allows sensitivity analysis and agro-economic evaluation of different fertilizer options. In a last step, the dynamic decision tool RIDEV is used to simulate optimal timing of different management actions such as fertilizer application, weeding, and harvest. Resulting integrated crop management recommendations ensure high resource use efficiency adapted to local socio-economic and bio-physical conditions. Provided necessary input data are available, this framework can be applied to field, perimeter or regional scale. The framework was applied to the Office du Niger in Mali, using field data of 58 farmers. In the wet season, fertilizer doses adjusted to three soil fertility classes out-performed current uniform recommendations only slightly, except on soils with very low K supply (10 cases). Only on these soils was application of K profitable. Profit-optimizing fertilizer doses had high costs and low value/cost ratios. Target yields close to potential yield increased farmers' risk. Adjusting fertilizer doses to the lower yield potential in the dry season reduced costs and risk without reducing profit, resulting in better value/cost ratios. New simple recommendations for both wet and dry seasons were formulated.