Animal Unit Month (AUM) Grazing Calculator
Convert a mixed herd to animal units, calculate the AUMs a pasture supplies from forage yield and utilisation, and find the stocking rate, grazing days available or acres needed — with animal-unit equivalents for cattle, horses, sheep and goats.
How it works
AU = Σ head × AU equivalent; 1 AUM = 790 lb air-dry forage (26 lb/day × 30.4); AUMs available = acres × usable yield × proper-use factor ÷ 790; months of grazing = AUMs ÷ AU
An animal unit is a 1,000-pound cow with her calf eating about 26 pounds of air-dry forage a day; an animal unit month is what she eats in a month, about 790 pounds. Converting a mixed herd into animal units lets you compare its demand with the forage a pasture can spare — its production times a proper-use factor of 25–50% that leaves enough leaf to keep plants growing and soil covered. Range managers lease and plan in AUMs; graziers use the same arithmetic to set stocking rates, decide how long a paddock will last and see whether hay must fill a gap. Because forage production swings with rainfall, plan conservatively and adjust with a mid-season forage check; overstocking in a dry year does damage that takes several years to repair.
Worked example
40 cow-calf pairs, 2 bulls, 15 yearlings and 2 horses are 56 AU and need 336 AUMs for six months. 320 acres yielding 2,400 lb with 50% use supply 384,000 lb, or 486 AUMs — a 150-AUM surplus, so the herd could graze about 8.7 months or the pasture could carry 25 more AU.
Frequently asked questions
What proper-use factor should I use?
50% is the classic “take half, leave half” for managed tame pasture; native rangeland is often planned at 25–35%, and riparian or degraded areas lower still.
Do heavier cows change the AU?
Yes — intake scales roughly with metabolic weight; a 1,400 lb cow is about 1.3 AU. Adjust the equivalents to your animals.
Related calculators
Farm planning estimate. Breeds, climate, feed quality and local extension guidance change the numbers — use this to plan, and your own records to manage.