Soil fertility in mixed farming systems relies on the manure produced by livestock and its recycling in the entire system. In the particular case of crop-livestock system with grazing area, the proper functioning of the system also depends on the presence of nitrogen-fixing plants in the area where livestock grazes (the grazing land). In this paper, we study the impact of biological nitrogen fixation (BNF) and livestock management on the flux of manure exported outside the grazing land. We address this issue using a modeling approach. We consider a plant-soil model composed of a set of nonlinear ordinary differential equations that represents the grazing land. We assume that the manure produced by the grazing livestock can be partially exported as a fertilizer outside of this area. Through the mathematical analysis of the model, and analytical and numerical optimization, we then determine the optimal livestock management in terms of grazing rate and manure recycling percentage that lead to the maximal flux of exported manure. We focus more precisely on the role of nitrogen-fixing plants and their impact on the optimal livestock management. When grazing rate is high and the capacity of plants to fix nitrogen is important, we showed that it is necessary to recycle some of the manure produced by the livestock in the grazing land to maximize the flux of exported manure. On the contrary, if we can optimize both the grazing rate and the manure recycling percentage, then it is better to transfer all the produced manure and to adapt the grazing rate accordingly to minimize nitrogen losses from the soil. Finally, to maximize the flux of exported manure, it is also necessary to bring the system to a state in which the plants fix nitrogen. In this way, we can benefit from the nitrogen fixation which provides an additional input of nitrogen in the system