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.).
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.
(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.)
[1] 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.