Farmers' agronomic and social evaluation of productivity yield and N2-fixation in different cowpea varieties and their subsequent residual N effects on a succeeding maize crop

Adjei-Nsiah, S.; Kuyper, T.W.; Leeuwis, C.; Abekoe, M.K.; Cobbinah, J.; Sakyi-Dawson, O.; Giller, K.E.


Cowpea-maize rotations form an important component of the farming systems of smallholder farmers in the forest/savannah transitional agro-ecological zone of Ghana. We evaluated five cowpea varieties for grain yield, N-2-fixation, biomass production, and contribution to productivity of subsequent maize grown in rotation. We further analyzed the interrelationship between these technical dimensions and the social acceptability of these cowpea varieties for farmers. Cowpea grain yield ranged between 1.1 and 1.4 t ha(-1) with no significant yield differences among the different varieties. Using the N-15 natural abundance technique, the average proportion of N-2 fixed ranged between 61% for Ayiyi and 77% for Legon prolific. This resulted in average amounts of N-2 fixed in above-ground biomass ranging between 32 and 67 kg N ha(-1), respectively. Variation in estimates due to differences in delta N-15 among reference plants were larger than differences between cowpea varieties. The amount of soil-derived N ranged from 15 to 20 kg N ha(-1). The above-ground net N contribution of the cowpea varieties to the soil (after adjusting for N export in grains) was highest for Legon Prolific (31 kg N ha(-1)) due to high N-2-fixation and high leaf biomass production. Maize grain yield after cowpea without application of mineral N fertilizer ranged between 0.4 t ha(-1) with maize after maize to 1.5 t ha(-1) with Legon Prolific. The N fertilizer equivalence values for the cowpea varieties ranged between 18 and 60 kg N ha(-1). IT810D-1010 was ranked by the farmers as the most preferred cowpea variety due to its white seed type, short-duration, ease of harvesting and good market value. Despite the high leaf biomass production and high amount of N-2 fixed by Legon Prolific, it was generally the least preferred variety due to lower market price, late maturity, least potential cash income (due to the red mottled seed type) and difficulty in harvesting. Although farmers recognized the contribution of cowpea to soil fertility and yields of subsequent maize, they did not consider this as an important criterion for varietal selection. Soil fertility improvement must be considered as an additional benefit rather than a direct selection criterion when designing more sustainable smallholder farming systems.