The concept of an animal unit 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. 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 definitions are based on the concept that a 1000-pound cow, with or without an unweaned calf, is one animal unit, with such a cow being assumed to consume 26 pounds of forage dry matter per day. Animal unit months in a grazing area 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.
Grazing livestock
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 ; also, any fee increase or decrease cannot exceed 25 percent of the previous year's level. 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:
Estimation based on forage consumption assumptions.
In British Columbia, the Range Regulation defines "animal unit month" for purposes of the Range Act. Effectively, the regulation assigns animal unit equivalents of 1 for a cow, 0.7 for a yearling of the genus Bos, 1.5 for a bull, 1.25 for a horse, 0.2 for a sheep, 0.2 for a llama, and 0.1 for an alpaca.
With regard to grazing permits on Navajo partitioned lands, "Animal Unit means one adult cow and her 6-month-old calf or the equivalent thereof based on comparable forage consumption. Thus as defined in the following: One adult sheep or goat is equivalent to one-fifth of an AU; One adult horse, mule, or burro is equivalent to one and one quarter AU; or One adult llama is equivalent to three-fifths of an AU."
In relation to environmental assessments for certain actions under US Agriculture regulations, "The term animal unit means a unit of measurement for any animal feeding operation calculated by adding the following numbers: the number of slaughter and feeder cattlemultiplied by 1.0, plus the number of mature dairy cattle multiplied by 1.4, plus the number of swine weighing over 25 kilograms multiplied by 0.4, plus the number of sheep multiplied by 0.1, plus the number of horses multiplied by 2.0."
With regard to swine in the State of Idaho, "animal unit" is "a unit of measurement equaling two and one-half swine, each weighing over twenty-five kilograms, or ten weaned swine, each weighing under twenty-five kilograms. Total animal units are calculated by adding the number of swine weighing over twenty-five kilograms multiplied by four-tenths, plus the number of weaned swine weighing under twenty-five kilograms multiplied by one-tenth."
According to an ordinance in the Township of Brady, Michigan, an "animal unit shall be construed as a unit of measure used to compare relative differences in the odor producing characteristics of animal wastes, with the following equivalencies applicable to various animals: A. Cattle: 1.00, B. Horses: 1.00, C. Swine: 1.00, D. Sheep/Goats 0.50, E. Poultry/Fowl 0.10. The equivalency for types of livestock not specifically listed above shall be the stated equivalency for the type of animal which is most similar in terms of odor producing characteristics of animal wastes, as determined, if necessary, by the Zoning Board of Appeals."
Other
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.
