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Comparative assessment of productivity gaps of major food crops across smallholder farming systems in the tropics - A modelling and data-mining approach

Tittonell P., Affholder F., Scopel E., Lafarge T., Corbeels M.. 2011. In : Yield gap assessment workshop. Pékin : China Agricultural University, 1 p.. Yield gap assessment workshop, 2011-08-31/2011-09-02, Pékin (Chine).

Tropical agriculture presents special challenges for the achievement of the potential yield of major food crops. Many tropical agroecosystems are characterised by strongly weathered, Oxidic and Kaolinitic soils of inherently poor fertility, acid and often young soils that are formed on resistant minerals present in coarse textures (Sanchez, 1977). Rainfall patterns vary from dry and semi-arid areas with unreliable distributions, to areas of excess rainfall interrupted only by a brief dry season. The latter, together with the absence of a cold season (frost) allows weeds, pests and diseases (and also natural control agents) to complete their cycle many times during the year. In the majority of cases, cropping systems in the tropics co-exists with greater biological diversity than in temperate regions. Agricultural production takes place to a large extent in smallholder family farming systems. Farmer priorities and objectives are not always to maximise yields but sometimes just to minimize risks. Tropical agriculture is often embedded in contexts of wide diversity of social traditions and institutions that govern the use of natural resources. Yield gap analyses are particularly challenging in tropical agriculture due to uncertainties with yield potential measures and the lack of experimental references. Under farmers' conditions strictly bio-physical yield-determining factors are often confounded with the impact of local practices, traditions and a multiplicity of production objectives. For instance, what would be the yield potential of cereals intercropped with legumes or other food crops, including perennial crops such as cassava or fruit trees, as practised in several regions of the tropics? The mere determination of average yields under smallholder conditions is poses challenges, du to high spatial variability across and within farms and wide inter-annual variability due to climate, especially in zones of savannah. In many tropical regions of the world, however, modern agricultural management by commercial farmers allow harvesting high yields of major food crops. Maximum yields of these farming systems may represent the (water-limited) yield potentials under tropical conditions. We propose a comparative analysis of yield gaps across farming systems that cover a wide diversity in terms of agroecological conditions and degree of intensification. We examine yield gaps of maize under commercial and smallholder farming in Brazil, Mexico, Togo, Kenya , Zimbabwe and Vietnam. We study the main causes behind yield gaps in rainfed cereal systems in the Sahel, and examples of fine-tuning practices to increase already high yields of irrigated rice in the Philippines or southern Europe. We look at long term changes in yield gaps as a consequence of soil degradation in savannah-derived agroecosystems, and at the relative impacts of biotic and abiotic factors in determining yield gaps. These analyses are done statistically, using classification and regression tree analysis, or through crop modelling. The latter include simulation of (water-limited) yield potentials and effects of management factors for possible yield gains through virtual factorial experimentation. We discuss and conclude on the advantages and shortcomings of these methods, and on the relevance of the yield gap analysis as a concept to guide agricultural research in the tropics. (Texte intégral)...

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