Maize yield response, nitrogen use efficiency and financial returns to fertilizer on smallholder farms in southern Africa

Maize is considered as “life” in southern Africa because it is the staple food crop and the main component of food aid interventions. However, its productivity is very low, partly because of the limited use of external inputs. Although maize response to fertilizer has been the focus for many years of studies on research stations, information is scanty on the level of crop response and profitability on smallholder farms in most parts of southern Africa. Therefore, the objective of the present analysis was to determine yield responses, nitrogen use efficiency and returns to investment in fertilizer in the unimodal rainfall region of southern Africa. This analysis compared yield responses to various rates of nitrogen (N) fertilizer with maize grown without external inputs (absolute control) on a total of 940 demonstration sites in 47 districts across Malawi, Mozambique, Zambia and the southern highlands of Tanzania. Across the sites, average yields were 1.6 t ha⁻¹ in the control, 2.8 t ha⁻¹ with ≤50% of the recommended N rate and 4 t ha⁻¹ where 100% or more of the recommended N rate was applied. Except in Zambia, agronomic efficiency and partial factor productivity of N were higher with 50% of the recommended N rate compared to 100% of the recommended N rate. Net present values (NPV) were also positive in over 50% of the cases, indicating that investments in N fertilizer will generate profits over time. In contrast, for maize grown without N fertilizer, NPVs were negative in over 68% of the cases across the four countries. The value-cost ratio was >2 with the various N rates at the aggregate level, but it was <2 on sites in Mozambique and Zambia. We conclude that growing maize without N inputs results in loss of land productivity and profitability while application of 50% or more of the recommended N fertilizer ensures substantial increases in yields and profitability.