Technical Abstract: The Integrated Farm System Model (IFSM) simulates production, harvest, and storage of crops and forage; cattle grazing and feeding; dairy or beef production; and nutrient cycling over many years of weather at a whole-farm scale. The pasture submodel of IFSM was modified to allow simulation of forage mixtures with up to four species. The model was used to predict whole-farm effects of utilizing multiple-species pastures. Model predictions were calibrated and evaluated with observations from a multiple-species pasture experiment conducted near State College, PA. Once calibrated, the model was used to simulate two hypothetical dairy farms: one applying 150 lb/acre of nitrogen fertilizer each year on a grass-only pasture and one applying no nitrogen fertilizer on a 3-species pasture. Model predictions of average yearly pasture yield and quality, nutrient cycling, and farm-level economics were compared over 25 years of weather. On average, the model predicted 4% higher yield for the 3-species pasture and 16% higher annual profit, due mainly to lower feed costs. The farm with the 3-species pasture also leached less soil nitrogen to groundwater. The model provides a useful tool for comparing the whole-farm effects of various forage mixtures over many years of weather.