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Animal Unit Equivalent for Beef Cattle Based on Metabolic Weight

Llewellyn L. Manske PhD, Associate Range Scientist

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.

animal2.gif (7401 bytes)

 

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.

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