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).
| 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.