Tailoring conservation agriculture technologies to West Africa semi-arid zones: Building on traditional local practices for soil restoration

Low inherent fertility of tropical soils and degradation, nutrient deficiency and water stress are the key factors that hamper rainfed agriculture in semi-arid West Africa. Conservation Agriculture (CA) is currently promoted in the region as a technology to reduce soil degradation, mitigate the effect of droughts and increase crop productivity while reducing production costs. CA relies on the simultaneous use of three practices: (1) minimum or zero-tillage; (2) maintenance of a permanent soil cover and; (3) diversified profitable crop rotation. The most prominent aspect of CA for degraded lands in the semi-arid tropics would be the organic soil cover that impacts on the soil water balance, biological activity, soil organic matter build-up and fertility replenishment. Yet, the organic resources are the most limiting factor in Sahelian agroecosystems due to low biomass productivity and the multiple uses of crop residues, chiefly to feed the livestock. Hence, CA as such may hardly succeed in the current Sahelian context unless alternative sources of biomass are identified. Alternatively, we propose: (1) to gradually rehabilitate the biomass production function of the soil through increased nutrient input and traditional water harvesting measures that have been promoted as "soil and water conservation" technologies in the Sahel, e.g. zai, in order to restore soil hydrological properties as prerequisite to boosting biomass production; (2) to encourage during this restorative phase the regeneration of native evergreen multipurpose woody shrubs (NEWS) traditionally and deliberately associated to crops and managed the year around and; (3) to shift to classical, less labour intensive CA practices once appropriate levels of soil fertility and water capture are enough to allow increased agroecosystem primary productivity (i.e., an active 'aggradation' phase followed by one of conservation). The CA systems we propose for the Sahelian context are based on intercropping cereal crops and NEWS building on traditional technologies practiced by local farmers. Traditionally. NEWS are allowed to grow in croplands during the dry season; they reduce wind erosion, trap organic residues and capture the Harmattan dust, influence the soil hydraulics and favour soil biological activity under their canopies. They are coppiced at the end of the dry season, leaves and twigs remain as mulch while branches are collected for domestic fuel and other uses. Shoots re-sprouting during the rainy season are suppressed as weeds. Such CA systems have limited competition with livestock due to the poor palatability of the shrub green biomass, which may increase their acceptance by smallholders. Such aggradation-conservation strategy is not free of challenges, as it may imply initial soil disturbance that entail important labour investments, substantially change the structure and management of the cropping system (annual crop-perennial plant), and lead to emerging tradeoffs in the use of resources at different scales. This paper offers a state of the art around NEWS and their integration in relay intercropping CA systems, discusses the above mentioned challenges and the main research needs to address them. (C) 2011 Elsevier B.V. All rights reserved.