Cotton–cowpea intercropping and its N₂ fixation capacity improves yield of a subsequent maize crop under Zimbabwean rain-fed conditions

Abstract

Intercropping cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) is one of the ways to improve food security and soil fertility whilst generating cash income of the rural poor. A study was carried out to find out the effect of cotton–cowpea intercropping on cowpea N₂-fixation capacity, nitrogen balance and yield of a subsequent maize crop. Results showed that cowpea suppressed cotton yields but the reduction in yield was compensated for by cowpea grain yield. Cowpea grain yield was significantly different across treatments and the yields were as follows: sole cowpea (1.6 Mg ha⁻¹), 1:1 intercrop (1.1 Mg ha⁻¹), and 2:1 intercrop (0.7 Mg ha⁻¹). Cotton lint yield was also significantly different across treatments and was sole cotton (2.5 Mg ha⁻¹), 1:1 intercrop (0.9 Mg ha⁻¹) and 2:1 intercrop (1.5 Mg ha⁻¹). Intercropping cotton and cowpea increased the productivity with land equivalence ratios (LER) of 1.4 and 1.3 for 1:1 and 2:1 intercrop treatments, respectively. There was an increase in percentage of N fixation (%Ndfa) by cowpea in intercrops as compared to sole crops though the absolute amount fixed (Ndfa) was lower due to reduced plant population. Sole cowpea had %Ndfa of 73%, 1:1 intercrop had 85% and 2:1 intercrop had 77% while Ndfa was 138 kg ha⁻¹ for sole cowpea, 128 kg ha⁻¹ for 1:1 intercrop and 68 kg ha⁻¹ for 2:1 intercrop and these were significantly different. Sole cowpea and the intercrops all showed positive N balances of 92 kg ha⁻¹ for sole cowpea and 1:1 intercrop, and 48 kg ha⁻¹ for 2:1 intercrop. Cowpea fixed N transferred to the companion cotton crop was very low with 1:1 intercrop recording 3.5 kg N ha⁻¹ and 2:1 intercrop recording 0.5 kg N ha⁻¹. Crop residues from intercrops and sole cowpea increased maize yields more than residues from sole cotton. Maize grain yield was, after sole cotton (1.4 Mg ha⁻¹), sole cowpea (4.6 Mg ha⁻¹), 1:1 intercrops (4.4 Mg ha⁻¹) and 2:1 intercrops (3.9 Mg ha⁻¹) and these were significantly different from each other. The LER, crop yields, %N fixation and, N balance and residual fertility showed that cotton–cowpea intercropping could be a potentially productive system that can easily fit into the current smallholder farming systems under rain-fed conditions. The fertilizer equivalency values show that substantial benefits do accrue and effort should be directed at maximizing the dry matter yield of the legume in the intercrop system while maintaining or improving the economic yield of the companion cash crop.