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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.