Animal Unit Months
Animal unit months (AUM) is commonly adopted in rangeland situations as the unit used to express carrying capacity. AUM is a measure of forage supply within the management unit, based on the amount required to support an animal unit for 1 month. Animal unit days (AUD) and animal unit years (AUY) can also be adopted, but AUM better accommodates flexible or season-long grazing schedules.
Attempts have been made to convert AUM into a constant value that quantifies that amount of forage needed to support an animal unit for a month. For example, 1AU (~450 kg animal) x 2% intake/day x 30 days = 270 kg usable forage will support a 450 kg animal for 30 days. However, the actual weight assigned to an animal unit or for daily intake remains an issue of contention. As it is, AUM provides a useful rule-of-thumb to describe the carrying capacity within a management unit, and to serve as a common measure to compare sites or years.
References and Further Reading
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Scarnecchia, D.L. 1985. The animal-unit and animal-unit-equivalent concepts in range science. Journal of Range Management 38:346-349. (pdf)
Scarnecchia, D.L., and M.M. Kothmann. 1982. A dynamic approach to grazing management terminology. Journal of Range Management 35:262-264. (pdf)
Society for Range Management. 1989. A glossary of terms used in range management. Society for Range Management. Denver, CO. 3rd ed. p. 3.
Vallentine, J.F. 1990. Grazing management. Academic Press. San Diego, CA. pp 280-284.