When extension centers and other information sources discuss stocking rates, they usually make their recommendations in acres per animal (or animals per acre, depending on the climate and the type of animal). For instance, you might read a factsheet that advocates 5 cow-calf pairs to the acre.
There’s one problem with this method of calculating stocking rate—how big are those cows? A 1,400-pound cow eats considerably more than a 900-pound cow.
And what if you want to do mixed-species grazing? How do you figure out how much forage is required to feed both cattle and sheep, for instance?
Enter the animal unit.
Animal Units and Equivalents
A 1,000-pound beef cow with an unweaned calf commonly serves as a standard of measure in grazing management. That standard is the animal unit (AU). The hypothetical beef cow is considered 1 AU.
Because we know roughly how much other types of livestock eat compared to the 1,000-pound beef cow, we can easily make comparisons. We can take the basic AU and modify it to reflect a bigger cow, or a dry cow, or even a goat instead of a cow. The result is the animal unit equivalent (AUE).
Compare the following AUEs:
- 1,000-pound beef cow with calf: 1.0 AUE.
- 1,200-pound beef cow with calf: 1.2 AUE.
- 1,500-pound beef cow with calf: 1.5 AUE.
- Dry beef cow: 1.0 AUE.
- Mature bull under 2,000 pounds: 1.5 AUE.
- Mature bull over 2,000 pounds: 2.0 AUE.
- Weaned calf: 0.75 AUE.
- 2-year-old steer or heifer around 700 pounds: 0.8 AUE.
- Ewe-lamb pair: 0.3 AUE.
- Mature dry sheep: 0.2 AUE.
- Yearling sheep: 0.15 AUE.
- Mature goat: 0.17 AUE.
- Yearling goat: 0.1 AUE.
- Mature light horse: 1.25 AUE.
- Mature heavy horse: 2.0 AUE.
- Bison: 1.0 AUE.
- Mature elk: 0.65 AUE.
Determining how many animals you can have on a given piece of land then becomes a matter of matching forage production to livestock needs.
Animal Unit Days and Months
So how long can a pasture feed a given type of animal? A 1,000-pound beef cow with an unweaned calf requires an average of 26 pounds of forage dry matter per day. This gives us the animal unit day (AUD).
Now we come to the animal unit month (AUM). The animal unit month reflects how much forage it takes to feed an animal unit for 30 days. Remember, the animal unit is a 1,000-pound beef cow with a calf, and she consumes 26 pounds of dry-weight forage per day. That gives us 780 pounds of forage for a 30-day period.
The AUM can be used to help you determine the stocking rate for a pasture. Let’s say you have a 40-acre pasture estimated at 0.2 AUM/acre. That means that every acre of the pasture can support 0.2 AUE for 30 days—the entire pasture can support 8 AUE for a month. So for one month you could graze your choice of:
- 8 beef cow-calf pairs at 1,000 pounds each.
- 5 beef cow-calf pairs at 1,500 pounds each.
- 10 steers at 700 pounds each.
- 26 ewe-lamb pairs.
- 47 mature goats.
- 6 light horses.
- 4 heavy horses.
- 8 bison.
- 12 mature elk.
You can also use the AUM to figure out how to mix species in your grazing plan. Just to give you an idea of a few possible combinations, in this scenario you could graze:
- 4 beef cow-calf pairs at 1,000 pounds each + 13 ewe-lamb pairs.
- 6 beef steers at 700 pounds each + 2 light horses.
- 10 ewe-lamb pairs + 29 mature goats.
How do you determine the AUMs for your pasture? Check online for average AUM data in your area. If you can’t find anything specific, you may have to keep your own forage production records. If you determine how much dry-weight forage your pastures produce over a 30-day period, you will have no problem calculating the AUMs, as one AUM uses 780 pounds of forage.