Stocking Rates and AUM

Carrying capacity is considered to be the average number of animals that a particular pasture or range can sustain over time, says this report by the Government of Alberta, Agriculture and Rural Development.
calendar icon 4 July 2009
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Alberta Agriculture, Food and Rural Development

Stocking rates are expressed as the number of animal unit months (AUM) supplied by one acre of land.

The greater the difference between current forage production and the potential forage production of a pasture, the greater the difference will be between carrying capacity and recommended stocking rate. Pastures that are in poor condition should be stocked well below its carrying capacity; this will help the pasture recover. Carrying capacity considers the livestock's forage needs, as well as adequate carryover. Carryover is the forage material that is left behind after grazing. It is important to leave enough carryover to help the plants regrow, restore plant food reserves, help conserve precious moisture and protect the soil from wind or water erosion.

What is an Animal Unit Month?

An Animal Unit Month (AUM) is the amount of forage required by an "animal unit" grazing for one month. The standard animal unit is defined as one mature 1000 lb cow with a calf, or equivalent, and is based upon the average daily forage intake of 26 lb dry matter per day. That consumption, combined with a factor for tramping and waste of about 25%, results in an estimate of about 1000 pounds of dry matter (DM) from forage to supply one AU each month.

Not all my cows are 1000 lbs, and how do I adjust other types of livestock?

Animal unit equivalents, are used to adjust for any animal type. For example, bigger cows and heavier calves graze more forage, so adjustments need to be made when matching livestock needs with available forage.

Class of Animal Animal Unit Equivalents
Cow, 1000 lb, with or without a calf
1.00
Bulls, 2 years and over
1.50
Yearling heifers and steers
0.67
Weaned calves
0.50
Horse, 2 year old
1.00
Horse, 3 years old and over
1.50
Horse, yearlings
0.75
5 Ewes or does, with or without lambs or kids
1.00
5 Rams or bucks
1.30
5 Weaned lambs or kids, up to 12 months
0.50
Bison cow
1.50
Bison bull
1.80
Bison yearling
0.75
5 Deer
1.00

How do I figure out how many AUMs I have?

Approximately 1000 lb of forage dry matter is needed for each AUM, you can estimate your stocking rates based on forage dry matter yield. Historical pasture yield, current season yield clippings or estimates using a grazing stick can help determine how many AUM's you have.

July 2009

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