Maize-grain legume intercropping is an attractive option for ecological intensification that reduces climatic risk for smallholder farmers in central Mozambique

The increasing demand for food from limited available land, in light of declining soil fertility and future threats of climate variability and change have increased the need for more sustainable crop management systems. Conservation agriculture (CA) is based on the three principles of minimum soil disturbance, surface crop residue retention and crop rotations, and is one of the available options. In Southern Africa, CA has been intensively promoted for more than a decade to combat declining soil fertility and to stabilize crop yields. The objective of this review is to summarize recent advances in knowledge about the benefits of CA and highlight constraints to its widespread adoption within Southern Africa. Research results from Southern Africa showed that CA generally increased water infiltration, reduced soil erosion and run-off, thereby increasing available soil moisture and deeper drainage. Physical, chemical and biological soil parameters were also improved under CA in the medium to long term. CA increased crop productivity and also reduced on-farm labor, especially when direct seeding techniques and herbicides were used. As with other cropping systems, CA has constraints at both the field and farm level. Challenges to adoption in Southern Africa include the retention of sufficient crop residues, crop rotations, weed control, pest and diseases, farmer perception and economic limitations, including poorly developed markets. It was concluded that CA is not a ‘one-size-fits-all’ solution and often needs significant adaptation and flexibility when implementing it across farming systems. However, CA may potentially reduce future soil fertility decline, the effects of seasonal dry-spells and may have a large impact on food security and farmers’ livelihoods if the challenges can be overcome.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1742170513000550

Conservation agriculture in Southern Africa: Advances in knowledge

A comparative analysis of conservation agriculture systems: Benefits and challenges of rotations and intercropping in Zimbabwe

Sustainable and resilient cropping systems are required in southern Africa to arrest declining soil fertility and offset the future negative effects of climate change. Conservation agriculture (CA) has been proposed as a potential system for improving soil quality and providing stable yields through minimum soil disturbance, surface crop residue retention (mulching) and crop rotations or associations. However, concerns have been raised about the lack of evidence of the benefits of CA for small-scale farmers in southern Africa. This research was carried out in two communities and one on-station site in Zambia to provide more scientific evidence about the effects of CA on soil quality, infiltration, soil moisture and crop performance. Results from Kayowozi showed that maize yields in a direct seeded CA treatment, using cowpea seeded with a dibble stick in full rotation, increased by up to 78% after four cropping seasons in comparison to a conventional control using a ridge and furrow system. At Malende, maize yields for animal traction rip-line seeded and direct seeded plots were, on average, 75% and 91% higher than a conventionally tilled control plot after six cropping seasons. Detailed studies undertaken at the Monze Farmer Training Centre revealed that CA treatments, especially that using cotton in rotation, increased water infiltration and soil moisture. In some years, infiltration was five times higher on CA fields than on those using conventional tillage. Carbon increases were only found at the on-station long-term trial, where, over time, CA plots outperformed conventional practice leading to an overall increase of 12% carbon in the first 30 cm, compared with decreases of 15% in the conventional control. Comparative analyses between the on-farm and on-station trials point to a lack of adequate mulching, which might be the reason for lower carbon at the on-farm sites. We conclude that the effects of CA can build up on different soil types in most systems, but that scaling up and out requires the whole community to be targeted, rather than relying on individual farmers to overcome constraints related to the set-up in rural communities.

Conservation agriculture in eastern and southern provinces of Zambia: Long-term effects on soil quality and maize productivity

Sustainable intensification through rotations with grain legumes in Sub-Saharan Africa : A review

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.

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

Carbon and nitrogen mineralisation potential of mixed cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) crop residues produced under intercropping, as well as a reddish-brown soil classified by FAO as Ferralic Cambisol previously under intercrops, were studied over a 10-week incubation period under controlled conditions (25°C and moisture content of 70% field capacity, 125 mm) in the laboratory. Treatments consisted of cotton residues (100 : 0), cowpea residues (0 : 100), and cotton–cowpea residues (50 : 50, 70 : 30, and 30 : 70). These ratios were based on yields obtained in different cotton–cowpea intercrop treatments from a field study. Cowpea residues (0 : 100) released the highest amount of mineral N of 36.4 mg/kg soil, and cotton residues (100 : 0) least, 19.2 mg/kg soil, while the other mixtures were in between. All treatments except for cowpea residues (0 : 100) and the 30 : 70 mixture showed immobilisation of soil N during the first 2 weeks of incubation. The trend for C mineralisation was similar to that of N, and cowpea residues (0 : 100) released the highest amount, 492 mg C/kg soil, while cotton residues (100 : 0) recorded the least, 315 mg C/kg soil. The C mineralisation patterns of cowpea residues (0 : 100) and 30 : 70 treatments were exponential and were well described by the equation: where CE is exponentially mineralisable C fraction, k is the rate constant, and t is time in days. The mineralisation patterns for other treatments were sigmoidal and were well described by the equation: where CS is sigmoidally mineralisable C fraction; t 					0 is time in days required for complete mineralisation of CS 					, while k is rate constant. The amount of N released from soil previously under cotton–cowpea intercrops and sole crops was approximately one-third of the amount released when the residues were incorporated. The highest amount of N released (12.2 mg/kg soil) was from soil previously under sole cowpea, while soil from the 1 : 1 cotton–cowpea intercrop released 9.9 mg/kg soil and soil from sole cotton released 5.9 mg/kg soil. There was no significant effect (P > 0.05) of previous crop on C mineralisation patterns of the soil. Mixtures slow down N losses and increase nutrient use efficiency of legume residues, especially in the short-term. When cotton is grown as a sole crop, starter N to offset negative effects of initial N-immobilisation at the start of season is required. A better understanding of controlling parameters of decomposition can make it possible to predict C and N mineralisation patterns in mixtures. Reduced C mineralisation in cotton–cowpea mixtures may result in more C sequestration and, hence, SOM build-up and improved sustainability in the long term in intercropping systems.

H. K. Murwira

Papers

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

Effect of cotton-cowpea intercropping on C and N mineralisation patterns of residue mixtures and soil.