Abstract A deterministic simulation model (FORAGE) has been developed as an interface between the plant and animal components of a general rangeland ecosystem model (SPUR). The model predicts forage intake and diet selection of grazing beef cattle by simulating the mechanistic components of grazing behavior. Bite size, rate of biting, rate of intake and grazing time are modified by changes in sward characteristics. Sensitivity analyses revealed that the model is sensitive to underestimation of critical parameters. The model suggests that, when forage availability is limited, intake is limited by rate of intake and grazing time.