The Integrated Farm System Model (IFSM) simulates at the whole-farm scale the management procedures, biophysical processes, and economics involved in crop, dairy, or beef production. The IFSM was modified to simulate growth and nutritive value of multiple-species forage mixtures planted in temperate pastures. When used to predict yield and quality of multiple-species pastures monitored near State College, PA, the calibrated model predicted neutral detergent fiber content within 24%, crude protein content within 25%, and annual yield within 7%. The model had greater difficulty, however, predicting observed seasonal distribution of yield and shifts in botanical composition. The model was applied to contrast the economics and nutrient dynamics of single-species vs. multiple-species pastures on hypothetical farms. Over 25 years of weather, multiple-species pastures had a similar mean yield (4% higher) but resulted in higher annual profit (by 16%) than grass monocultures, primarily due to reduced feed costs. Model predictions may be improved by adding explicit representation of root biomass or better representation of plant phenology.