Animal unit

The concept of an animal unit (AU) has traditionally been used in North America to facilitate planning, analysis and administration of forage use by grazing livestock, but the term has also had other applications (in relation to odor control regulation, feedlot size, manure management, etc.). The term has been variously defined by regulation in different jurisdictions, and by livestock management specialists, rangeland resource managers and others. Consequently, when using or interpreting the term, care is needed to ensure that a definition appropriate for the purpose is being used. Most (but not all) definitions are based on the concept that a 1000-pound (454 kg) cow, with or without an unweaned calf, is one animal unit, with such a cow being assumed to consume 26 pounds (about 12 kg) of forage dry matter per day.
Animal unit months (AUMs) in a grazing area (calculated by multiplying the number of animal units by the number of months of grazing) provide a useful indicator of the amount of forage consumed. On public lands in various jurisdictions, authorized use of forage for grazing is commonly expressed in animal unit months.
In the USA, the federal grazing fee, which applies to Federal lands in 16 Western states on public lands managed by the BLM and the U.S. Forest Service, is adjusted annually and is calculated by using a formula originally set by Congress in the Public Rangelands Improvement Act of 1978. Under this formula, as modified and extended by a presidential Executive Order issued in 1986, the grazing fee cannot fall below $1.35 per animal unit month (AUM); also, any fee increase or decrease cannot exceed 25 percent of the previous year's level. (On these federal grazing lands, an AUM is the amount of forage needed to sustain one cow and her calf, one horse, or five sheep or goats for a month.) The grazing fee for 2012 is $1.35 per AUM, the same level as it was in 2011. For a grazing license or permit on Crown land in British Columbia, the grazing fee per authorized AUM is "93% of the average gross sales revenue per kilogram for live beef cattle marketed during the immediately preceding 3 years through the B.C. Livestock Producers Cooperative Association."
For grazing livestock, most differences in definition relate to how animal unit equivalents should be calculated for weights and classes of livestock other than 1000-pound cows. Different approaches have included:
Federal land management agencies, such as the Bureau of Land Management and the National Forest Service, may use different standards for setting grazing fees. The Natural Resources Conservation Service uses animal units to estimate manure production and manure nutrient content when designing projects under the Environmental Quality Incentives Program (EQIP). The Environmental Protection Agency does not use the term animal units to define size classes for purposes of compliance with effluent limitations for Concentrated Animal Feeding Operations (CAFOs). Instead it sets thresholds by specifying the actual number of animals.
Regulatory definitions of animal unit equivalents also vary according to purpose and also vary among jurisdictions. For various species, they are often based on age and sex categories, rather than precise weights. Some examples are:
Special definitions exist for various other applications. For example, for purposes of comparing manure nitrogen and phosphorus contents and manure production for various animal species, the US Natural Resources Conservation Service has used 1000-pound animal units, regardless of animal species.