Forage Demand in Animal Unit Months

An animal unit month is the forage one animal unit eats in a month, and it is the currency in which grazing leases, public range permits and pasture rents are written across North America. The conventional figure is 780 lb — about 354 kg — of dry matter, arrived at as 26 lb a day for 30 days, for a 1,000 lb (454 kg) beef cow with or without a calf.

Every one of those numbers is a convention rather than a physical constant, and it is worth saying plainly because they are so often quoted as though they were measured. The 1,000 lb cow is a committee's choice made when cows were smaller than they are now; commercial cows today commonly weigh 1,300–1,500 lb and eat proportionally more. The 26 lb daily intake is about 2.6% of body weight, which is reasonable for a dry cow and low for one nursing a calf. The 30-day month is arithmetic convenience. Different jurisdictions and different agencies use different bases, dairy and sheep systems use others again, and the AUM figure itself is actively disputed where grazing fees are set from it. So the intake per animal unit is an input on this page, not a constant buried in the code — put your own figure in.

The calculation multiplies animal units by intake by days, and the result is the demand side of the grazing balance. Fifty animal units for a five-month season at 11.8 kg a day need $50 \times 11.8 \times 150 = 88{,}500$ kg of dry matter. Set that against what the land will actually supply, at a realistic utilisation rate, and the arithmetic says whether the herd fits or whether something has to give.

Demand computed this way is a floor, not a budget. It counts what goes into the animal and nothing else, so it must be divided by the utilisation rate before it can be compared with standing forage — 88,500 kg of demand at 50% utilisation requires 177,000 kg of forage to be standing there. It also takes no account of the year: a dry summer can halve production on the same acres, which is why a herd sized to an average year is a herd that will be selling cows in a bad one, and why conservative stocking with a plan for surplus is the more durable strategy.