Responses of a feed intake model for grazing beef cattle to changes in model parameters, forage composition, and supplementation programs with energy and protein were evaluated. Without supplements, the model systematically underpredicted intake of low-quality (low digestibility) forages and subsequent overprediction was observed for high-quality diets. In general, for a reference diet of Italian ryegrass, the model was relatively insensitive to microbial growth parameters, highly sensitive to the microbial carbohydrate composition constant, and moderately sensitive to the microbial N composition constant. Intake prediction was sensitive to changes in the microbial use rate constant for fiber but insensitive to those for protein and starch. Model predictions were highly sensitive to the amount of nondegradable fiber in each of the forages tested. Supplementation effects on forage intake were quantified by supplementing all forage diets with chemical components equivalent to that provided by 1 kg of corn grain or 1 kg of cottonseed meal. Supplementation of the forage diet with the concentrate source resulted in substitution ratios of forage to supplement intake consistent with in vivo results. As forage quality increased, substitution of concentrate for the forage increased. However, the model failed to predict the increased forage intake typically observed with protein supplementation, suggesting that it is insufficient for intake prediction in protein-limiting situations. Nevertheless, the model correctly predicted effects of energy supplementation and forage composition on forage intake, suggesting that different controls must regulate intake responses to supplemental protein.