Conservation Agriculture (CA) is being advocated to enhance soil health and sustain long term crop productivity in the developing world. One of CA's key principles is the maintenance of soil cover often by retaining a proportion of crop residues on the field as mulch. Yet smallholder crop-livestock systems across Africa and Asia face trade-offs among various options for crop residue use. Knowledge of the potential trade-offs of leaving more residues as mulch is only partial and the objective of this research is to address some of these knowledge gaps by assessing the trade-offs in contrasting settings with mixed crop-livestock systems. The paper draws from village surveys in 12 sites in 9 different countries across Sub-Sahara Africa and South Asia. Sites were clustered into 3 groups along the combined population and livestock density gradients to assess current crop residue management practices and explore potential challenges to adopting mulching practices in different circumstances. Results show that although highdensity sites face higher potential pressure on resources on an area basis, biomass production tends to be more substantial in these sites covering demands for livestock feed and allowing part of the residues to be used as mulch. In medium-density sites, although population and livestock densities are relatively lower, biomass is scarce and pressure on land and feed are high, increasing the pressure on crop residues and their opportunity cost as mulch. In low-density areas, population and livestock densities are relatively low and communal feed and fuel resources exist, resulting in lower potential pressure on residues on an area basis. Yet, biomass production is low and farmers largely rely on crop residues to feed livestock during the long dry season, implying substantial opportunity costs to their use as mulch. Despite its potential benefit for smallholder farmers across the density gradient, the introduction of CA-based mulching practices appears potentially easier in sites where biomass production is high enough to fulfil existing demands for feed and fuel. In sites with relatively high feed and fuel pressure, the eventual introduction of CA needs complementary research and development efforts to increase biomass production and/or develop alternative sources to alleviate the opportunity costs of leaving some crop residues as mulch.

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Agricultural production in sub-Saharan Africa has, in recent times, remained lower than the rest of the world. Many attribute this to factors inherent to Africa and its people, such as climate, soil quality, slavery and disease. This article traces the role of agriculture through history and argues that these are not the main reasons. Before the arrival of European traders, complex agricultural systems existed, which supported food security, manufacturing and trade. External interference manipulated these systems in pursuit of export crops. Independence has not fundamentally changed this; resource and wealth extraction continue to inhibit economic development for Africans in Africa.

a International crop research Institute for the semi-arid tropics, Bulawayo, Zimbabwe; b school of commerce, university of south australia, adelaide, australia ABSTRACT Productivity barriers and opportunities influencing smallholder irrigation sustainability in Zimbabwe were identified using case studies of the Silalatshani and Mkoba irrigation schemes. The major barriers were poor infrastructure and soil fertility, and poor access to farm inputs, farm implements, functioning markets and agricultural knowledge, which resulted in low yields, food insecurity and negative farm income. Most irrigated land remains unused, and marketing of produce is uncoordinated. Mobile technologies provide opportunities for market information dissemination. Institutions are needed to continuously encourage dialogue among agricultural value chain stakeholders to allow irrigators to align their operations to market demands and improve the viability of irrigation systems.

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