Long-term yield sustainability and financial returns from grain legume-maize intercrops on a sandy soil in subhumid north central Zimbabwe

To measure the yield and financial returns from five grain legume-maize intercrop combinations over 12 years of cropping, a field experiment was conducted on a loamy sand soil in the subhumid unimodal rainfall environment of Domboshava in north-central Zimbabwe. Inputs and management followed smallholder practice, including partial grazing of crop residues and a zero mineral fertilizer treatment. The intercropped legumes grew moderately well most years. Cowpea averaged the highest grain yield (0.244 t ha⁻¹) and haulm yield (1.54 t ha⁻¹) over the 12 years, followed by pigeonpea and sugar bean. Intercropped pigeonpea yield was the least variable of the legumes over the years. Maize grain yield was highly variable across years with or without fertilizer and was reduced in years of low (533 mm) and high (1313 mm) rainfall. The pigeonpea-maize intercrop grown without fertilizer produced 0.11 t ha⁻¹ (6.25%) more maize grain yield per year than sole crop maize, in addition to pigeonpea grain and haulms. Intercropped cowpea (which yielded more than double the above-ground non-grain biomass of pigeonpea) had less effect on maize grain yield. There was no trend to greater benefits from the legumes on maize yield after more years of intercropping. Net present values of annual margins accumulated over the 12 years for sole maize with fertilizer (US$1719 ha⁻¹) and without fertilizer (US$935 ha⁻¹) were higher than the fertilized and unfertilized intercropping options (US$1017 and US$745 ha⁻¹). Pigeonpea or cowpea-unfertilized maize generated more financial returns than the other intercrops, but the low yields and high labour costs for the legumes made the intercrops financially unattractive. We conclude that regularly intercropped pigeonpea or cowpea can to a small extent help to maintain maize yield when maize is grown without mineral fertilizer on sandy soils in sub humid zones of Zimbabwe, and simultaneously provide some nutritious food, but that financial considerations will encourage smallholder farmers to persist with growing low input sole crop maize.