Review of interventions and technologies for sustainable intensification of smallholder crop production in sub-humid sub-Saharan Africa

Besides low soil fertility, climate variability has often been identified as the major constraint to agricultural productivity in Sub-Saharan Africa (SSA), with rainfall variability (both within and across seasons) being the most critical. Traditionally, reasonable yields could be achieved in spite of constant or smaller resource inputs by expanding the cropped area, but this is no longer a viable option. Crop production intensification is required to produce more food per unit of input and land, while maintaining or rebuilding soil fertility. However, most smallholder farmers lack access to resources such cash, fertiliser and technological expertise to address constraints caused by the biophysical environments in which they operate. The objective of this review was to collate and appraise the range of crop production intensification options that have been developed for smallholder farmers in SSA. A case study from central Mozambique was included to illustrate the impact and relevance of locally feasible options to farmers who own different resources. The study has revealed that sustainable intensification of crop production requires that multiple constraints are addressed simultaneously, in this case primarily soil fertility/plant nutrient supply and weed management. Success of crop intensification options will also depend on proper targeting to different farm types as well as field soil fertility gradients. Although smallholder farmers in SSA have limited assets, the case study revealed the occurrence of local opportunities to increase current crop productivity which in some cases do not need substantial capital inputs by the farmers, but more efficient use and targeting

Yield and labor relations of sustainable intensification options for smallholder farmers in sub-Saharan Africa. A meta-analysis

Sustainable intensification of agricultural production is needed to ensure increased productivity relative to inputs. Short-term yield returns and labor input are major determinants of the fate of sustainable intensification options on smallholder farms in sub-Saharan Africa because labor shortage is often acute, and most farmers lack access to labor-saving technologies. We assessed the relationship between maize grain yield change and labor input from a total of 28 published papers (631 data pairs) including subsets of data pairs within specific sustainable intensification practices. Among the reviewed technologies, manually dug planting basins showed ratios between the change in yield and change in labor inputs (Y/L) below 1, suggesting that labor demand increased more than yield. In contrast, ridging showed average Y/L values 2. No-till showed high Y/L (average1.7) when combined with herbicides but average Y/L 1 (total labor) when manually weeded. Manually weeded rotations showed average Y/L 1 and manually weeded intercropping systems average Y/L around 1. The relations revealed four scenarios: high yield returns but low labor demand, high yield returns and labor demand, low yield returns and labor demand, and low yield returns but high labor demand. High yield with high labor demand requires mostly investments in machinery and/or herbicides to reduce labor input. Low yield with low labor demand requires improved crop management, whereas low yield with high labor demand requires a combination of improved crop management and investments to reduce labor. This is the first comprehensive assessment showing that the sustainable intensification options being considered for smallholder farmers may increase crop yield but also labor demand. Options that include mechanization and herbicides at low cost are likely to be adopted due to their reduction effect on drudgery and total labor input

A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions

Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact

Does labour invested in sustainable intensification practices give sufficient yield returns?

The increasing demands for nutritious food, feed, fibre and fuel of a growing world population with changing consumption patterns cannot be satisfied through increasing the arable land area. Sustainable intensification of crop production is needed, as in producing more without environmental harm but with positive effects on natural resources, profits and social capital. However, smallholder farming systems in e.g. sub-Saharan Africa (SSA) face many barriers to increased crop productivity, one being labour shortages during key periods that strongly influence yields. For example, insufficient labour often leads to late planting and a mismatch with the growing season, and to poor weed control leading to high competition for nutrients, water and light and thus to low yields

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

Living within their means: Reallocation of farm resources can help smallholder farmers improve crop yields and soil fertility

Slash and burn agriculture is no longer feasible due to the rising population pressure and a dwindling resource base. Crop production intensification is required to produce more food per unit area of land, while rebuilding soil fertility. We explored the impact of reallocation of limited resources on crop productivity across farms of different resource endowment in central Mozambique. The results suggested that decreasing the cropped area and concentrating resources (fertiliser, manure and labour within the farmers' means) to smaller fields can increase total farm maize production for poor as well as relatively richer farmers in central Mozambique, but that the fertility of the retained and abandoned land is important for the overall outcome. Concentrating resources to smaller areas would in most cases also lead to increased concentrations of soil organic carbon (SOC), indicating positive effects on soil fertility and crop productivity in the long term. However, this apparently does not apply when only fertiliser is used and if harvest residues and weeds are removed or burnt. Organic inputs are thus also required, such as retention of crop residues, and applications of manure if available can further increase SOC. The study has revealed that sustainable intensification or crop production requires that multiple constraints be addressed simultaneously. Success of crop intensification options will also depend on proper targeting to different farm types as well as variability in soil fertility. Differences in resource ownership and bio-physical circumstances lead to different opportunities for individual farmers even within the same area. While targeted interventions for individual farmers might not be feasible, targeting defined farmer/endowment groups may be a sustainable pathway to increase productivity. Although smallholder farmers in sub-Saharan Africa (SSA) have limited assets, the study revealed opportunities to increase crop productivity without substantial capital inputs, but more efficient use and targeting