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The Agricultural Policy/Environmental eXtender (APEX) model was developed for use in whole farm/small watershed management. The model was constructed to evaluate various land management strategies considering sustainability, erosion (wind, sheet, and channel), economics, water supply and quality, soil quality, plant competition, weather and pests. Management capabilities include irrigation, drainage, furrow diking, buffer strips, terraces, waterways, fertilization, manure management, lagoons, reservoirs, crop rotation and selection, pesticide application, grazing, and tillage. Manure may be applied as solid or liquid. Confined feeding areas may contain a lagoon to catch runoff from the feeding area plus wash water that is used in the barn. Effluent from the lagoon is applied automatically to a field designated for liquid manure application. Solid manure is scraped from the feeding area automatically at a user input interval in days and stockpiled for automatic application to designated fields. An owner may have any number of solid manure application fields. The model provides for application of manure from outside sources. These applications may be scheduled on certain dates or automatically at regular intervals. Manure is also supplied to fields by grazing animals. The manure management model is flexible enough to simulate a variety of systems from confined feeding (with or without lagoons) to grazing animals plus manure from outside sources. A variety of livestock including cattle, swine, and poultry may be considered because daily manure production in kg/head and it's ingredients (mineral and organic N and P) are inputs. APEX simulates runoff, soil erosion, and manure erosion. Routing mechanisms simulate soluble nutrient transport with water, organic nutrient transport by sediment, and manure transport by water.