Stocking Rate Calculator: Animal Units per Acre
Choose what to solve for. Forage yield (3,000 lb dry matter per acre), 50 % utilization and 26 lb a day per animal unit are typical planning values; replace them with figures for your pasture.
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Formula
- Usable forage per acre or hectare = forage production x utilization %
- Forage one animal unit eats = intake per day x grazing days
- Animal units supported = pasture size x usable forage per area / forage per animal unit
- Area needed = animal units x forage per animal unit / usable forage per area
- Stocking rate = animal units / area
Worked example
100 acres, 3,000 lb dry matter per acre, 50 % utilization, 26 lb per AU a day, 180 days of grazing.
- Usable forage: 100 x 3,000 x 0.5 = 150,000 lb
- One AU eats 26 x 180 = 4,680 lb
- 150,000 / 4,680 = 32.05 AU, so 32 whole animals at 1 AU each; stocking rate 0.32 AU per acre
- For 20 animals: 20 x 4,680 / 1,500 = 62.4 acres
- For 365 days of grazing, one AU needs about 6.33 acres.
All default figures are typical planning values.
When to use it
Use it to check whether a pasture can carry the animals you plan to graze, or how much land you need for a herd. The result depends strongly on forage yield, which varies with rainfall, soil and season, so use measured or locally published figures where you can.
Common mistakes
- Using total forage production without a utilization percentage. Animals trample and leave residue.
- Counting every animal as one AU. A sheep, a calf and a horse have different factors.
- Mixing units: pounds with hectares, or kilograms with acres.
- Treating a planning estimate as a guarantee in a dry year.
How it differs from nearby calculators
The hay requirement calculator works out bales when animals are fed hay instead of grazing, while this page works out how much land supports them on pasture. The chicken feed calculator covers bagged feed for poultry.
Frequently asked questions
What is an animal unit?
An animal unit (AU) is a planning measure: one mature 1,000 lb (454 kg) cow with or without a calf, eating about 26 lb (11.8 kg) of dry matter a day. A typical sheep is often counted as about 0.2 AU, but use the factor for your own animals.
How do I calculate how many animals a pasture supports?
Usable forage = acres x yield x utilization. For 100 acres at 3,000 lb per acre and 50 %, that is 150,000 lb. One AU eats 26 lb x 180 days = 4,680 lb, so 150,000 / 4,680 = 32 AU.
How many acres do I need for 20 animals?
Divide the forage they eat by the usable forage per acre. 20 AU x 4,680 lb = 93,600 lb, and each acre gives 3,000 x 0.5 = 1,500 lb, so you need 62.4 acres.
Why is utilization only 25 to 50 percent?
Grazing animals trample, foul and leave residue that the plants need to regrow, so only part of the production is harvested. Lower utilization is typical for rangeland and higher for managed rotational grazing. This is a planning estimate, not a guarantee.
How many acres does a cow need per year?
With the default typical figures of 3,000 lb of forage per acre and 50 percent utilization, an animal unit eating 26 lb a day for 365 days needs 9,490 lb. Each acre gives 1,500 lb, so that is about 6.33 acres per animal unit.
How many sheep can graze 10 acres?
Use 0.2 animal units per sheep, a typical planning figure. With the defaults, 10 acres supports 3.2 AU for 180 days, which is 16 sheep.
How much pasture do two 1,200 lb horses need?
Two horses at 1.2 AU each (1,200 lb divided by 1,000 lb) are 2.4 AU. With the typical defaults they eat 2.4 x 4,680 = 11,232 lb over 180 days, and at 1,500 lb usable per acre that needs 7.49 acres.