Productivity and biological N2-fixation in cereal-cowpea intercropping systems in sub-Saharan Africa. A review

Abstract

Intercropping is a common practice among farmers in sub-Saharan Africa, regarded as a sustainable way of improving land productivity to meet food and nutritional requirements for a growing population, especially in marginal areas. Cowpea (Vigna unguiculata L. Walp) is often intercropped with major cereal crops, maize (Zea mays L.), sorghum (Sorghum bicolor L. Moench) and pearl millet (Pennisetum glaucum L.R.Br ). Here we conducted a systematic literature review on cowpea intercropped with maize, sorghum or pearl millet reported in sub-Saharan Africa with the objectives (i) to determine yield and productivity of component crops and (ii) to quantify biological N2-fixation in sole or intercrops. We retrieved 60 unique publications combining 1196, 998 and 25 observations of yields, land productivity and N2-fixation, respectively, for crops grown as intercrops and monocrops. The major results are as follows: (1) land productivity of cowpea intercropped with maize, sorghum and pearl millet is favourable, with average land equivalent ratios of 1.42 ± 0.47, 1.26 ± 0.35 and 1.30 ± 0.32, respectively; (2) no significant differences between the proportion of nitrogen derived from the atmosphere (%Ndfa) for sole or intercropped cowpea were found, with average values of 56.00 ± 4.89 and 46.62 ± 7.05, respectively; (3) however, the total amount of fixed nitrogen was higher in cowpea monocropping systems due to higher biomass production; nitrogen fixation was 57 kg N ha−1 and 36 kg N ha−1 in monocrops and intercrops respectively. We conclude that cereal-cowpea intercropping is a pathway for intensification for the low nutrient input systems of smallholder farmers in sub-Saharan Africa. Our review also suggests potential for improvement of these systems, based on the choice of the associated varieties, planting patterns and sowing time, cowpea leaf harvesting as a vegetable, and fertilization.