Cowpea (Vigna unguiculata (L.) Walp) is important in semi-arid West Africa. Yields are low due to inadequate water and nutrient availability and other constraints. Grain and fodder yield responses to nutrient application were determined from 21 site-years of research conducted in the Sahel and Sudan Savanna. The incomplete factorial treatment arrangement varied by country but included: Four levels each of P and K in 7.5 and 10.0 kg ha-1 increments, respectively; Mg-S-Zn-B package (Mg-S-Zn-B); and comparable with and without manure treatments. Yield increases due to P application always occurred with curvilinear to plateau or linear responses. The overall mean grain yield increase was 0.35 Mg ha-1 and 47% due to application of 22.5 kg ha-1 P. Application of K resulted in a linear negative effect in 2014 and positive effect in 2015 for on-station trials in Niger, but had no effect in Burkina Faso and for on-farm trials in Niger. Yield was not affected by Mg-S-Zn-B in Burkina Faso but was increased by a mean of 0.085 Mg ha-1 in Niger. Manure application resulted in a mean yield increase of 0.1 Mg ha-1 in Niger but only with fertilizer P applied, and had no effect in Burkina Faso. Cowpea grain and fodder yields were responsive to fertilizer P up to 22.5 kg ha-1 but little affected by other applied nutrients.
Key words: Fertilizer, fodder, grain, manure, micronutrient, phosphorus, secondary nutrient, Sahel, Sudan Savanna.
AE, Agronomic efficiency of applied nutrient use; CP, cost of nutrient use relative to cowpea grain value or kilogram of cowpea grain required to equal the cost of one kilogram of nutrient applied; EOR, the economically optimal rate of nutrient application or the rate expected to maximize net return to nutrient application; PCR, profit cost ratio or the net returns divided by the costs of a nutrient application.
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