Effects of cotton-cowpea intercropping on crop yields and soil nutrient status under Zimbabwean rain-fed conditions

Intercropping of cotton and cowpea is one of the ways to improve food security and soil fertility while generating and maintaining cash income of the rural poor. A study was carried out to find out the effects of cotton-cowpea intercropping strategies on crop yields and soil nutrient status under rain-fed conditions. The study was carried out at Kadoma Cotton Research Institute (CRI), Ntini and Mukosi sites which are all in Kadoma District. The treatments were sole cotton, sole cowpea, and 2 rows of cotton alternating with 1 row of cowpea (2:1), 1 row of cotton alternating with 1 row of cowpea (1:1). The intercrops were either planted at the same time (simultaneously), or cowpea was planted 4 weeks after cotton (4WAC). Results showed that cowpea suppressed cotton yields but the reduction in yield was compensated for by the yield of cowpea and also the residual fertility from cowpea residues. The reduction in cotton yield was less when cowpea was planted 4 weeks after cotton and when the row configuration was 2:1 (cotton: cowpea). Cowpea grain yield across the sites was as follows, sole cowpea (1.6 t ha-1), 1:1 sim (1.1 t ha-1), 2:1 (0.7 t ha-1), 1:1 relay (0.8 t ha-1) and 2:1 relay (0.3 t ha-1). Cotton lint yield across the sites was as follows, sole cotton (2.0 t ha-1), 1:1 sim (0.7 t ha-1), 2:1 sim (1.2 t ha-1), 1:1 relay (1.5 t ha-1) and 2:1 relay (1.8 t ha-1). Comparable intercrops had higher cowpea grain yields in the simultaneous than in the relay intercrops but cotton lint yields were higher in relay than simultaneously planted intercrops. All the intercrops were productive as compared to the sole crops with an average land equivalence ratio (LER) of 1.3 for both dry matter and grain yield across all the sites. There was an increase in N2-fixation by cowpea in intercrops as compared to sole crops though the amount fixed was lower due to reduced plant population. Sole cowpea had N2-fixation of 73%, 2:1 simultaneous had 77% and 1:1 simultaneous had 85% while the total amount derived from N2-fixation was, sole cowpea (104 kg ha-1), 2:1 simultaneous (51 kg ha-1) and 1:1 simultaneous (96 kg ha-1 ). Sole cowpea and the intercrops contributed to positive N balances in the soil of 42.5 kg ha-1 for sole cowpea, 25.7 kg ha-1 for 2:1 simultaneous and 60.0 kg ha-1 for 1:1 simultaneous. Cowpea fixed N which was transferred to the companion cotton crop was very low with 1:1 simultaneous recording 3.6% and 2:1 simultaneous 0.9%. Soil mineral N and plant-available P generally increased after the intercrops with sole cowpea recording the highest and sole cotton the lowest and the intercrops recorded values were between those of sole cowpea and sole cotton. There was a slight change in pH and bases decreased but there was an increase in CEC. Microbial biomass C and N, and particulate organic matter C and N all increased especially after intercrops as compared to sole crops. Nitrogen release from sole crop residue and mixtures was in the order 36.4 mg kg-1 soil for cowpea residues, 33.4 mg kg-1 for 30:70 mixture, 27.1 mg kg-1 for 50:50 mixture, 21.6 mg kg-1 for 70:30 mixture and 19.2 mg kg-1 for cotton residues. The ratios given are for cotton: cowpea dry matter proportions obtained in the intercrop. The trend for C mineralization was the reverse of N mineralization and there was more C release from cotton residues. Grain yield after intercrops was substantial even without fertilizer (N) and was as follows, after sole cotton (1.1 t ha-1), sole cowpea (3.0 t ha-1), 1:1 intercrops (2.8 t ha-1) and 2:1 intercrops (2.5 t ha-1). Relay intercropping of cotton and cowpea is a good strategy to address issues of food security, income and soil fertility depletion. However issues of cotton pesticides effect on humans and livestock need to be understood in order to provide the correct recommendations. Markets and marketing infrastructure for cowpea also need to be improved in order to increase adoption of this strategy by farmers