# Animal Unit Equivalent for Beef Cattle Based on Metabolic Weight

Llewellyn L. Manske PhD, Associate Range Scientist

North Dakota State University

Dickinson Research Extension Center

Should producers allocate more grass to large cows than to cows of average size during planning of grazing management strategies? Large cows eat more forage than do cows of average size. Body size affects the quantity of dry matter intake. Numerous methods have been devised to predict or plan for livestock demand for forage. As a rough guideline, daily dry matter intake can be estimated relatively quickly by using 2% of body weight (Holechak et al. 1989). This technique is useful for general decisions, but when used to estimate forage needs in a grazing system, it tends to underestimate the forage needs for lighter animals and overestimate the forage needs for heavier animals.

A more accurate estimate of daily or monthly forage demand of livestock on a grazing system can be reached by using the metabolic weight of the livestock rather than the live weight of the animals. It has been found that metabolic weight accounts for significant variation in dry matter intake among animals of different size (NRC 1996). Metabolic weight is the live weight to the 0.75 power. Beef cattle animal unit equivalents can be determined for animals of different sizes by calculating their metabolic weight as a percentage of the metabolic weight of a 1000 pound cow. A 1000 pound cow with or without a calf is defined as 1.00 animal unit, which has a daily dry matter allocation of 26 pounds of forage (20 pounds for the cow, 6 pounds for the calf).

Method to calculate Animal Unit Equivalent (AUE) for beef animals of different weights based on metabolic weight (live animal weight0.75) with a 1000 pound cow equal to 1.00 Animal Unit (AU).  This method requires a calculator with (yx) function and memory.

Table 1 lists the calculated animal unit equivalents based on metabolic weight for a wide range of live animal weights. Calculating the animal unit equivalent for each cow in a herd would yield an accurate estimate of the quantity of forage required by grazing livestock; however, this does not seem practical or necessary to manage a grazing system properly. But increasing the accuracy of the forage demand estimate by grouping similarly sized animals of a herd into a few size categories and assigning appropriate animal unit equivalents to each group does seem practical and beneficial. This would enable the manager to allocate the pasture forage resources more accurately. Table 2 suggests a few beef cattle size categories and corresponding animal unit equivalents that could be used for planning grazing management strategies.

### Literature Cited

Holechek, J.L., R.D. Pieper, and C.H. Herbel. 1989. Range management principles
and practices. Prentice Hall, N.J.

Jacoby, P.W. 1989. A glossary of terms used in range management. Soc. For
Range Manage., Denver, CO.

National Research Council. 1996. Nutrient requirements of beef cattle. Seventh
revised edition. National Academy Press, Washington DC.

 Table 1. Animal Unit Equivalent (AUE) based on metabolic weight (live animal weight 0.75). Animal Live Weight (lbs) Animal Unit Equivalent yx 0.75 (% of 1000 lbs) 600 0.682 650 0.724 700 0.765 750 0.806 800 0.846 850 0.885 900 0.924 950 0.962 1000 1.000 1100 1.074 1200 1.147 1300 1.217 1400 1.287 1500 1.355 1600 1.423 1700 1.489 1800 1.554 1900 1.618 2000 1.682 2200 1.806 2400 1.928 2600 2.048 2800 2.165 3000 2.280

 Table 2. Suggested practical application of Animal Unit Equivalent based on metabolic weight (live animal weight 0.75). Beef Animal Category Animal Unit Equivalent Weaned animal lighter than 800 lbs 0.75 Young animal 800-900 lbs 0.85 Cow 900-1100 lbs with calf 1.00 Cow 1100-1300 lbs with calf 1.15 Cow heavier than 1300 lbs with calf 1.25 Bull lighter than 2000 lbs 1.50 Bull heavier than 2000 lbs 2.00

 Definitions from Society for Range Management Glossary, Jacoby, Chair., 1989. Animal-unit. Considered to be one mature cow of approximately 1,000 pounds, either dry or with calf up to 6 months of age, or their equivalent, based on a standardized amount of forage consumed. Animal-unit-month. The amount of dry forage required by one animal unit for one month based on a forage allowance of 26 pounds per day.