Rangeland Ecology & Management

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Animal Units

Animal units (AU) are used as a basis to standardize and express stocking rates among different kinds and classes of livestock with similar dietary preferences. An AU is essentially a measure of forage demand by a theoretical animal unit that can then be converted according to the relative demands of different animal classes (based primarily on metabolic bodyweight) (Table 1).

Table 1.
Suggested Animal Unit Conversion Factors for Various Classes of Cattle
Class of Cattle Relative Animal Unit
Mature Bull (>24 months) 1.5
Young Bull (18-24 months) 1.15
Cow and Calf Pair 1.35
Mature Cow, Non-Lactating 1.0
Pregnant Heifer, Non-Lactating (>18 months) 1.0
Yearlings (18-24 months) 0.9
Yearlings (15-18 months) 0.8
Yearlings (12-15 months) 0.7
Calves (weaning -12 months) 0.6
Calves (weaning at 8 months) 0.5

Source: Vallentine (1990), p 279.

However, inconsistencies persist among land management agencies and within published literature regarding approaches to quantify AU. For example, some interpretations loosely define an AU as the forage demanded by a mature cow, although this value could vary widely depending upon breed and reproductive status of the cow. Others advocate that an AU represents the forage demanded by a standard livestock unit weighing 450 kg. Another straightforward approach is to simply estimate daily intake as 2%-3% of the animal's body weight.

References and Further Reading

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Cordova, F.J., J.D. Wallace, and R.D. Pieper. 1978. Forage intake by grazing livestock: A review. Journal of Range Management 31:430-438. (pdf)

Forero, L., L.R. Rittenhouse, and J.E. Mitchell. 1989. A cow-calf vs. yearling substitution ratio for shortgrass steppe. Journal of Range Management 42:343-345. (pdf)

Hinnant, R.T. 1994. What is an animal unit? A time to conform. Rangelands 16:33-35.

Holechek, J.L., R.D. Pieper, and C.H. Herbel. 1995. Range Management Principles and Practices. Prentice Hall, Englewood Cliffs, NJ. 2nd ed. pp. 268-272.

Perrier, G.K. 1996. Viewpoint: The animal unit as an ecological concept. Rangelands 18:30-31.

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 276-280.