How to measure pasture productivity and capacity

An AUM, short for animal unit month, is the amount of forage required by one animal unit (AU) for one month. The estimated AUM of a pasture can be used as a measure of potential forage production in pounds or tons per acre. AUM is based on an animal intake of 2.5% to 3% of body weight per day on a dry matter basis, that is adjusting for moisture content of feed. For example:

  • A 1,000-pound cow eating 2.5% of its body weight eats about 25 pounds per day (1,000 pounds x 0.025).
  • An AUM is equal to 750 pounds of forage (25 pounds/day x 30 days/month).

The NRCS Web Soil Survey has data for the different soil types in the United States. Potential productivity in AUM per acre or tons of hay per acre for different crops are listed for defined areas of interest (property or management units of a farm). Read step-by-step instructions for how this works.

Estimating stocking rate

AUM is also used to estimate the stocking rate of a pasture, the number of AU that can be grazed per acre of pasture over the growing season. Stocking rate needs to be adjusted down to account for residual forage for plant health and grazing done by wildlife. The AUM remaining is called the carrying capacity of the pasture that leads to sustainable forage production over an extended time.

  • One AUM is feed for one AU for one month during the growing season.
  • Five AUM is feed for one AU for five months during the growing season.
  • Five AUM is feed for five AU for one month during the growing season.

Animal unit equivalents

One AU is equivalent to a 1,000-pound cow. This can be converted to different sizes and types of animals (Table 1).

Table 1. Animal-unit equivalents guide (from National Range and Pasture Handbook)
Kinds / classes of animals Animal unit equivalent pounds of forage - day pounds of forage - month pounds of forage - year
Cow, dry 0.92 24 727 8,730
Cow, with calf 1.00 26 790 9,490
Bull, mature 1.35 35 1,067 12,811
Cattle, 1 year old 0.60 15.6 474 5,694
Cattle, 2 years old 0.80 20.8 632 7,592
Horse, mature 1.25 32.5 988 11,862
Sheep, mature 0.20 5.2 158 1,898
Lamb, 1 year old 0.15 3.9 118 1,423
Goat, mature 0.15 3.9 118 1,423
Kid, 1 year old 0.10 2.6 79 949
Deer, white-tailed, mature 0.15 3.9 118 1,423
Deer, mule, mature 0.20 5.2 158 1,898
Elk, mature 0.60 15.6 474 5,694
Antelope, mature 0.20 52 158 1,898
Bison, mature 1.00 26 790 9,490
Sheep, bighorn, mature 0.20 5.2 158 1,898
Previously titled
Animal unit month for pastures

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