Technical Abstract: The Integrated Farm System Model simulates the major biological and physical processes of a crop, beef, or dairy farm. Crop production, feed use, and the return of manure nutrients back to the land are simulated over each of 25 years of weather. Growth and development of alfalfa, grass, corn, soybean, and small grain crops are predicted on a daily time step based upon soil water and nitrogen availability, ambient temperature, and solar radiation. Tillage, planting, harvest, storage, and feeding operations are simulated to predict resource use, timeliness of operations, crop losses, and nutritive changes in feeds. Feed allocation and animal response are related to the nutritive value of available feeds and the nutrient requirements of the animal groups making up the dairy herd. The quantity and nutrient content of the manure produced is a function of the quantity and nutrient content of the feeds consumed. Nitrogen volatilization occurs in the barn, during storage, following field application, and during grazing. Denitrification and leaching losses from the soil are related to the rate of moisture movement and drainage from the soil profile as influenced by soil properties, rainfall, and the amount and timing of manure and fertilizer applications. Edge-of-field runoff losses of sediment, sediment-bound phosphorus, and soluble phosphorus are predicted as influenced by manure and tillage management as well as daily soil and weather conditions. Following the prediction of losses, whole-farm mass balances of nitrogen, phosphorus, and potassium are determined as the sum of all nutrient imports in feed, fertilizer, deposition, and legume fixation minus the exports in milk, excess feed, animals, manure, and losses leaving the farm. Simulated performance is used to determine production costs, incomes, and economic return for each year of weather. A whole-farm budget is used, which includes fixed and variable production costs. This total production cost is subtracted from the total income received for milk, animal, and excess feed sales to determine a net return to the herd and management. The distribution of simulated annual values quantifies weather-related risk experienced by simulated production systems. By comparing simulation results for different production systems, the effects of system differences are determined, including resource use, production efficiency, environmental impact, production costs, and net return.