Variability in maize yield and profitability following hill-placement of reduced mineral fertilizer and manure rates under smallholder farm conditions in northern Benin

Whereas the decision to promote a given agricultural intensification technology has hitherto been largely based on its average agronomic or economic performance, it is increasingly being recognized that the variability in the performance must also be taken into account in order to develop more meaningful and flexible recommendations. This is true in particular for microdose fertilization which is being actively promoted in sub-Saharan Africa as a means to increase crop productivity, profitability and fertilizer use efficiency. To this end, a total of 51 onfarm maize trials were carried out in northern Benin in 2014 and 2015. The performance of two microdose fertilization options (MD1=23.8 kg N, 4.1 kg P, and 7.8 kg K ha−1; MD2=33.1 kg N, 8.2 kg P, and 15.6 kg K ha−1) applied alone or combined with hill-placed manure (FYM) at 3 t ha−1 was compared to an unfertilized control and a broadcast fertilizer treatment at the recommended rate (RR; 76 kg N, 13.1 kg P, and 24.9 kg K ha−1). On average, microdose fertilization alone increased maize grain yields by 1145 kg ha−1 (+105%), compared to the unfertilized control (1096 kg ha−1). There was no significant difference in yields between MD1, MD2 and RR in both years. Combining microdose fertilization with manure further increased yields by 848 kg ha−1 (+40%) on average. There was a large variability in yields among farmers, from 420 to 1687 kg ha−1, 1419 to 3418 kg ha−1 and 1834 to 4475 kg ha−1 for the control, sole microdose (MD1 and MD2) and microdose+FYM treatments, respectively. Variability tended to be lowest in the control treatment. Absolute yield response to microdose fertilization tended to decrease with increasing yields in the control plots and was well explained by the combination of some measured soil parameters (clay and/or silt, total carbon, exch-Mg, pH) and weed pressure. Based on the value-cost ratio (VCR), the economic performance of the RR treatment was less than that of the microdose treatments (alone or combined with manure) despite the higher labor cost associated with the latter treatments. MD1 should be favored over MD2 because yields were not significantly different yet the risk of achieving low VCRs was lower in MD1. Despite the greater variability compared to the control, the risk of no return on investment was nearly nil for MD1 (6%) and MD1+FYM (2%) as a result of the strong increase in yield. Despite the overall good performance of fertilizer microdosing, more effort is needed to better understand crop response to microdose fertilization for a broader range of environmental conditions in Benin in order to fine tune recommendation domains