Cowpea residue management effect on productivity of subsequent millet in a legume-cereal crop rotation

Growing cowpea (Vigna unguiculate (L) Walp) in rotation with millet (Pennisetum glaucum (L.) R. Br.) can increase millet productivity through biological nitrogen fixation. However, in Senegal, legume crop residues are generally removed for livestock feed, and little information exists on how burying the residue instead of removal affects the subsequent crop yield. This study assessed the productivity of cowpea varieties and evaluated prior cowpea residue management effect on grain and biomass yield of millet (Pennisetum glaucum (L.) R. Br) in rotation. Experiments were conducted in the National Center for Agronomic Research (CNRA) of Bambey in Senegal from 2020 to 2022. Treatments include 20 cowpea varieties, two fertilizer rates, and cowpea residue management practices (residue removal vs. burying) in a split-split plot arrangement. Results showed that fertilizer and variety had significant effect on grain and fodder yields of cowpea. Cowpea fodder nutrient concentrations differed by variety, and the superior cowpea variety, 66-35F, in nutrient concentration in this study had a 2.8-0.45-0.82% N-P-K. Applying fertilizer to previous cowpea crop increased millet grain yield by 63% compared to millet yield following previous non-fertilized cowpea plots. Furthermore, incorporation of cowpea residue prior to millet planting increased millet yield by 40% compared with the removal of cowpea biomass in 2020 but had no significant effect in 2021. In general, the amount of cowpea biomass buried had significant positive relationship with millet grain yield. Across years, millet grain yield increased by 0.16 kg ha(-1) for each kilogram per hectare increase in cowpea biomass buried prior to millet planting. Results showed that a minimum of 1500 kg ha(-1) cowpea buried biomass was required to increase the probability of millet grain yield above 1000 kg ha(-1). We concluded that the previous year's cowpea residue burial and fertilizer management improved productivity of the rotational millet crop.