Millet growth in windbreak-shielded fields in the Sahel : experiment and model

In the Sahelian zone, future food supply is insecure due to increasing land degradation. Wind erosion contributes significantly to impoverishment of the sandy soils, which are often loose and sparsely covered by vegetation for most of the year. At the onset of the growing season (May - July), strong winds often precede rains and may cause damage to the young seedlings by abrasion and burial. A possible control measure is the implementation of windbreaks (WBs) that reduce the wind velocity near the soil surface, thus protecting soil and crops. Moreover, the change in air flow may lead to a more favourable crop microclimate. Higher crop yields and increases in the long-term sustainability of crop production may be the result of effective windbreaks integrated in crops. However, windbreaks also compete with crops for limited resources, which may outweigh their potential benefits.Windbreak experiments, reported in literature, illustrate various benefits as well as negative impacts of windbreaks on crop growth (Chapter 1). In addition to a more favourable microclimate and reduced wind erosion, increased soil fertility, and complementary use of resources by trees and crops are possible agroforestry benefits. On the other hand, competition and allelopathic interactions between species, as well as attraction of pests and diseases by trees may reduce crop production. Whether the overall windbreak effect results in yield increases or losses depends on many interacting factors, e.g. climate, soil properties, crop species and WB-design. The few studies performed in semi-arid regions give insufficient insights to formulate generally applicable rules that allow extrapolation of experimental results to other locations or from one WB-design to another. Understanding of the tree-crop interactions is required for the design of optimum windbreak-cropping systems that can provide an option for sustainable land use in the Sahel.This research aimed at enhancing the understanding of posit