Improving maize productivity in northern Benin through localized placement of amendments and fertilizers

Throughout much of Sub-Saharan Africa, maize production in smallholder farms is characterized by low productivity due to low fertility soils, the scarce availability and use of external inputs and recurrent droughts exacerbated by climate variability. This situation calls for exploring fertilizer and amendment management practices that are efficient and can concurrently improve soil fertility, yields and economic returns. One such technology is the localized application of small quantities of manure and/or mineral fertilizers in the planting holes either at sowing or shortly after planting. Unlike for sorghum and millet, few studies have evaluated the agronomic and economic performances of this technology on maize. Besides, few studies have quantified the risk associated with this technology due to variability in crop management, soil and climatic factors. The general objective of this study was therefore to assess the agronomic and economic potential of localized application of manure and fertilizers in maize-based cropping system in northern Benin, with a long-term goal of developing recommendations. For this purpose, we combined three different approaches: (1) two on-station experiments to assess the agronomic potential and economic profitability of hill-placed manure and mineral fertilizer (or fertilizer microdosing), (2) farmer-field trials to quantify the variability in yield response, economic profitability and risk associated with some of the most promising treatments under real-world conditions and (3) modeling the response of maize across a range of rainfall conditions to further evaluate the sustainability of these practices in northern Bénin. All experiments were carried out in Ina district (Bembèrèkè, northern Benin) from 2012 to 2015. The on-station experiments showed that localized application of manure and fertilizers significantly increased grain and stover yields by 64-132% and 28-131%, respectively, across years. Combining hill-placed manure and fertilizers further increased grain yields by 31-55% on average. The increases in yields under fertilizer microdosing were accompanied by marked increases in nutrient uptake and negative nutrient balances. Nutrient balances were equally or more negative on microdosing fertilized plots than on the unfertilized controls. This was particularly the case for P and K and suggests that microdosing may enhance nutrient mining and should probably not be used for extended periods. The on-farm demonstrations showed a large variability in maize yield responses to fertilizer microdosing which could be partly explained by some measured soil parameters (clay and /or silt, total carbon, exch-Mg, pH) and weed pressure. Overall, absolute yield response tended to decrease with increasing yields in the control plots. Based on the value-cost ratio (VCR) the economic performance of the recommended fertilizer rate was less than that of the microdosing treatments (alone or combined with manure) despite the higher labor cost associated with the latter treatments. Despite the greater variability compared to the control, the risk of no return on investment was nearly nil for microdosing treatments (alone or combined with manure). The long-term scenario analysis using the specifically-parameterized DSSAT model revealed that the application of 2 g of N-P-K15-15-15 fertilizer + 1 g urea per hill (equivalent to 23.8 kg N ha-1) improved both the long-term average and the minimum guaranteed yield without increasing inter-annual variability and the economic risk compared to unfertilized plots. Even though combining microdosing with manure (at least at 1 t ha-1) was economically slightly riskier than microdosing alone, this risk remained low since a VCR of 2 could be achieved in almost 100% of the years. This makes the latter more sustainable and appropriate for smallholder farmers than the current recommendation.