The higher the animal density on a farm, the more likely there will be an excess of nutrients on the farm. Animal density is often quantified as animal units (AU)/acre. An animal unit is 1000 lb of live weight of any animal. The optimal animal density depends greatly on the balance between home-grown and imported feed. In Pennsylvania, for confined dairies, if the density is less than 1 AU/A, the farm will likely have less manure nitrogen than crops require. If the animal density is between 1 to 2 AU/A, the farm will be near to balance between manure nitrogen and crop requirements. If the density is greater than 2 AU/A, the farm will likely have more manure nitrogen than the crops require. For P balance on dairy farms, the animal densities would be approximately one-half those given above for N. These animal densities are a general indicator of farm nutrient balance which can be useful as a preliminary assessment in nutrient management planning. The actual balance on an individual farm will depend on many farm-specific factors and should be evaluated carefully as part of the nutrient management planning process.
For more detailed information, see PSU Nutrient Balance Fact Sheet #38B developed by Penn State University.
Author: Doug Beegle, Penn State University
