Range Management Terminology
- Animal Unit
- Animal Unit Month
- Range Condition
- Carrying Capacity
- Stocking Rate
- Sample Calculations:
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The information below outlines some of the concepts of range management and defines the related terminology. In addition, sample calculations are included for determining the grazing period, stocking rate and carrying capacity.
Animal Unit
An Animal Unit (AU) is one mature cow weighing approximately 1,000 pounds, that is either dry (not nursing) or has a calf up to six months old.
Animal Unit Month
An Animal Unit Month (AUM) is the amount of forage required by one AU for one month. An AU requires about 1,000 pounds of forage dry matter each month.
Some animals require more or less forage because of their age, size and species. For example, a horse or bull is considered equivalent to 1.5 AUMs, whereas a weaned calf is equivalent to 0.5 AUM. Adjustments can also be made to account for heavier beef cattle breeds.
Range Condition
Range condition is the present state of the vegetation compared with that of the potential plant community for the range site. Range sites represent combinations of soil, vegetation, landscape and climate, with different potential for growing forage.
Range condition is used to measure changes that have taken place in the plant community. It provides a basis for predicting the degree of improvement that is possible under various management strategies and measures any deterioration of the range site.
Carrying Capacity
One of the basic principles of range management is the need to balance the amount of forage consumed by livestock with the amount of forage available on the land. In order to accomplish this, the carrying capacity of the land must be established.
Carrying capacity is generally expressed as the number of AUMs that can be maintained on a specific parcel of land, and is based on the average amount of forage produced annually. Because livestock often prefer certain areas, while leaving other areas unused, adjustments to the carrying capacity may be required.
Clipping studies or established benchmark sites can be used to determine the annual forage production of the various sites (range communities). Once land managers know the total amount of forage produced, a “safe use” factor of 50 per cent is applied, which becomes the amount of vegetation carried over to the next grazing year. This carry-over helps to maintain plant vigor, prevent soil erosion, and provide food for wildlife.
Lands under grazing disposition are inventoried to determine the carrying capacity, thereby ensuring the resource can be sustained and the range maintained on a long-term basis.
Stocking Rate
Once the carrying capacity of the area has been determined, the stocking rate can be set. The stocking rate is the actual number of animals that can graze on the land during the grazing season. Under normal growing conditions, and with proper management, the stocking rate and the carrying capacity are the same. If weather conditions are more favorable (higher rainfall) and plant growth is greater, the stocking rate may be increased. In times of drought, however, the stocking rate should be reduced to maintain adequate carry-over for the next growing season.
Sample Calculation of a Grazing Period
| Grazing lease size | 640 acres |
| Carrying capacity (available forage) |
210 AUMs |
Rancher's herd
| Mature cows | 75 |
| Bulls | 3 |
Calculation of the available grazing period
| 75 mature cows x 1.0 AUM = | 75.0 AUs |
| 3 bulls x 1.5 AUMs = | 4.5 AUs |
| Total AUs = | 79.5 |
Sample Calculation of a Stocking Rate
The following example shows how many AUs can graze for four months within the carrying capacity of the range.
| Grazing lease size | 640 acres |
| Carrying capacity (available forage) |
210 AUMs |
The 52 AUs could be any mix of livestock, such as 2 bulls and 49 cows.
Sample Calculation of Carrying Capacity
The following example illustrates how the vegetation community-type affects the number of AUMs.
| Community-type | Area |
Production |
|---|---|---|
| Rough fescue-Idaho fescue-Parry oatgrass | 315 ac. | 1929 lb./ac. |
| Bluebunch wheatgrass-Sedge | 55 ac. | 1623 lb./ac. |
| White Spruce-Willow-Sedge | 270 ac. | 750 lb./ac. |
Carrying Capacity
Rough fescue-Idaho fescue-Parry oatgrass
Bluebunch wheatgrass-Sedge
White Spruce-Willow-Sedge
Total AUMs = 304 + 45 + 101 = 450 AUM
Recognizing that the White Spruce-Willow-Sedge community-type is not preferred by livestock, the recommended carrying capacity should be set at 349 AUMs (450 - 101).
For updated information
Contact any of the offices below:
Alberta Sustainable Resource Development
Lands Division
Rangeland Management Branch
9920 - 108 Street NW
Edmonton, Alberta T5K 2M4
Telephone: (780) 427 3595
Forestry Division
Forest Management Branch
9920 - 108 Street NW
Edmonton, Alberta T5K 2M4
Telephone: (780) 422 4590
OR call the Lands Division office in your area.
