Economic modelling of semi-arid rangelands has received much attention in recent literature. A major outstanding issue is how stochastic rainfall and the feedback effect of heavy grazing pressures on vegetation productivity can be accounted for in these models. This paper presents a model for calculating the optimal livestock stocking rate in a semi-arid rangeland that accounts for stochastic rainfall, the ecological feedback effect and variable prices. The model is developed for rangelands dominated by annual rather than perennial grasses, such as the African Sahel. The feedback effect is modeled on the basis of an ecological study, conducted in northern Senegal, that analyzes the impact of different grazing pressures on vegetation productivity. The paper presents both a general model and an application of the model to the Ferlo, a semi-arid rangeland in northern Senegal.