In three field trials, three varieties of cow pea (Prima, Ife Brown and Vita-5) grown as mono-crops or interplanted with maize (in one trial) were sprayed with a combination of PKS or NPKS in solutions containing 100, 100, 100 and 41 mg/1 of N, P, K and S respectively at the onset of flowering or 2 weeks later, in order to evaluate the effect of foliar spray of mineral nutrients on the performance of the crop.
Early season sprayings of NPKS and PKS increased vegetative development and dry-matter accumulation in both mono- and mixed-croppings of Prima and Vita-5, but had little effect on Ife Brown. Seed yield was increased by 80–225 kg/ha in Prima and 160–218 kg/ha in Vita-5. In all cases of enhanced growth and yield, spraying at flowering was more beneficial than spraying 2 weeks later whilst NPKS evoked greater responses than PKS.
In two late-season trials, all treatment combinations enhanced vegetative development and dry-matter accumulation in Ife Brown and Vita-5. Seed yield of Ife Brown was increased by 206 kg/ha in the first late season and by 550 kg/ha in the second late season whilst that of Vita-5 was increased by 226 kg/ha in the first late season, during which it was planted. In the first late season of Ife Brown, late spraying was more beneficial than early spraying, whilst plants sprayed with PKS out-yielded those sprayed with NPKS. On the other hand, in the second late season, Ife Brown responded more to early spraying than late spraying and NPKS was more beneficial than PKS: Vita-5 grown in the first late season responded similarly.
In all cases of enhanced seed yield during the seasons, the pathway of response was through increased pod production and/or retention on the plants. The results therefore suggest that foliar spray of nutrients during the post-flowering period enhance seed yield in cow pea by ensuring prompt delivery of mineral nutrients to the site of photosynthesis at a time when nutrient depletion in the leaves and reduced efficiency of mineral nutrient uptake by the roots limit the supply of photosynthates to developing reproductive organs.