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

Waste recovered by-products can increase growth of grass-clover mixtures in low fertility soils and alter botanical and mineral nutrient composition

The effectiveness of four by-products (biogas digestate, pot ale, rockdust and wood ash) as fertilisers of a perennial ryegrass (Lolium perenne)-red clover (Trifolium pratense) mixture in terms of biomass production, botanical composition and macro- and micronutrient concentrations was tested in an outdoor pot trial. This was carried out over two growing seasons using two inherently low-fertility soils used for forage production. Macro- and micronutrients (N, P, K, Ca, Mg, Co, Cu, Mn, Mo and Zn) relevant for crops and livestock were determined in soils and plants. All the by-products increased overall biomass production and affected nutrient concentrations of the individual plant species to varying degrees. In addition the competitive balance between grass and clover was altered leading to different botanical composition in the different treatments and consequently differences in the nutrient concentrations of the species mixture. Changes were due to the nutrients applied in the by-products per se and/or to changes in the soil chemistry caused by the by-products. The results suggest a potential to enhance agricultural productivity through improved production and quality of forage on less fertile land by matching of by-products and soil properties

Red clover increases micronutrient concentrations in red clover

Foraeg crops provide micronutrients as well as energy, protein and fibre to ruminants. However, the micronutrient concentrations of forage plant species differ, legumes generally having higher concentrations than grasses. In addition to that there are also strong effects of soil type. Typically, the concentrations of one or several micronutrients in forage are too low to meet the nutritional requirement of dairy cows. We hypothesized that the overall micronutrient (Co, Cu, Fe, Mn, Mo, Zn) concentrations of forage mixtures are affected by the red clover dry matter (DM) proportions and site effects. This hypothesis was tested at three contrasting sites. The results showed that increased red clover proportion increased the overall concentrations of several micronutrient in the mixtures at all sites. At the site with the widest range of red clover proportion (0-70 %) in the mixture, the Co, Cu and Fe concentrations more than doubled between the lowest and highest red clover DM proportions. At the other two sites a smaller increase in red clover proportion(from 10 % to 25 % or from 25 % to 50 %) also increased the overall concentrations of Co by up to 80 % but less for other micronutrients.  One of the sites generally had higher micronutrient concentrations in the crop and removed larger amounts of micronutrients with the harvested biomass compared to the other two sites.This could be explained by differences in pH and micronutrient concentrations of the soils at the sites. We conclude that increased red clover proportion in the sward has the potential to increase tha overall micronutrient concentrations but that the effect of the soil is also a controlling factor

Temporal and Spatial Positioning of Service Crops in Cereals Affects Yield and Weed Control

Leguminous service crops (SCs) can provide multiple services to cropping systems, reducing the reliance on external resources if sufficient biomass is produced. However, rapid light and temperature reductions limit post-harvest cultivation of SCs in Northern Europe. A novel practice of intercropping SCs in two consecutive crops (spring-winter cereal) to extend the period of SCs growth, and hence improve yield and reduce weeds, was tested. Three spatial and temporal arrangements of SCs and cash crops were investigated, as well as three SC mixtures, characterized by their longevity and frost sensitivity. Compared to no SC, the best performing mixture, frost-tolerant annuals, increased grain and N yield of winter wheat by 10% and 19%, respectively, and reduced weed biomass by 15% and 26% in oats and winter wheat, respectively. These effects were attributed to high biomass production and winter survival. However, this SC reduced oat yields by 15% compared to no SC. Furthermore, SC growth and service provision varied largely between experiments, driven by the weather conditions. Extending the SC's growth period by intercropping in two consecutive cereal crops has potential, but locally adapted species choices and establishment strategies are needed to ensure SC vitality until termination

Agroforestry with contour planting of grass contributes to terrace formation and conservation of soil and nutrients on sloping land

In hilly areas, agroforestry can be a more sustainable way of producing food and other products and services than agriculture based on sole-cropping. However, research is needed to evaluate and quantify formation of natural terraces in agroforestry and their contribution to soil conservation. This study quantified natural terrace formation and examined its role in reducing soil and nutrient losses during early stages of agroforestry with fruit trees, contour grass strips and maize or coffee in agroforestry systems on sloping land in northwest Vietnam. Two agroforestry systems, comprising longan (Dimocarpus longan L.)-mango (Mangifera indica L.)-maize (Zea mays L.)-guinea grass (Panicum maximum Jacq.) (fruit-maize-AF) and son tra (Docynia indica (Wall.) Decne.)-coffee (Coffea arabica L.)-guinea grass (fruit-coffee-AF) were compared with sole-cropped maize (sole-maize) and sole-cropped coffee (sole-coffee), respectively. Terrace formation was evaluated over five years using erosion pins placed above grass strips and the volume of terrace formed was estimated. Soil and nutrient losses were quantified using soil traps. The results showed that terraces formed as the systems developed, through gradual deposition of soil sediment above the living grass strips and trees. Accumulated soil sedimentation above the grass strips during the five-year study period raised the soil surface by 4.0 cm in fruit-maize-AF and 4.2 cm in fruit-coffee-AF, and the volume of terraces generated by the grass strips was 0.26 and 0.43 m3/m respectively. The fruit-maize-AF and fruit-coffee-AF systems significantly reduced losses of soil, soil organic carbon (SOC) and associated nutrients (N, P, K) compared with sole-maize and sole-coffee already in the first two years, while the reductions were greater from year 3 onwards. On average across experiments and years, the agroforestry systems reduced soil, SOC, N, P and K losses by 27–76%, 21–78%, 20–82%, 24–82% and 22–84%, respectively. These findings show that agroforestry with fruit trees, grass strips and crops could be a useful management practice and viable option for sustainable agricultural systems on sloping land, by reducing soil (and carbon and nutrient) losses through terrace formation

Grain legumes and dryland cereals contribute to carbon sequestration in the drylands of Africa and South Asia

Grain legumes and drylands cereals including chickpea (Cicer arietinum), common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), groundnut (Arachis hypogaea), lentil (Lens culinaris), pigeon pea (Cajanus cajan), soybean (Glycine max), finger millet (Eleusine coracana), pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor) are the leading sources of food grain in drylands of Africa and South Asia. These crops can help smallholder agriculture to become more resilient, productive, and profitable, but their quantitative impact on carbon sequestration is unknown. The aim of this review study was to quantify their contribution to carbon sequestration across the drylands of Africa and South Asia based on 437 publications with 1319 observations in studies conducted across 32 countries. Cropping systems with grain legumes showed the greatest increase in soil organic carbon (SOC) concentrations, while cereals (and pigeon pea) gave the largest amount of aboveground carbon stock (>2 Mg C ha 1). Estimated carbon stock in post-harvest residues of these crops was 1.51 +/- 0.05 Mg C ha 1 in Africa and 2.29 +/- 0.10 Mg C ha 1 in South Asia. These crops produced more aboveground carbon, and significantly increased SOC, when grown as intercrops. Soils with low initial SOC (32%) showed the greatest potential for carbon sequestration when cropped with grain legumes and dryland cereals. This study is the first of its kind to provide evidence that grain legumes and drylands cereals improve carbon sequestration across Africa and South Asia