Cathie Erichsen Arychuk,
Bison Production Specialist, Alberta Agriculture
Reprinted from The Tracker; volume 4, issue 6, June 2000, pages 72-73
Pasture forage production depends on soil conditions and climate. Pasture production also depends on stand condition and plant species present. Average stocking rates are a good starting point, but you should determine a stocking rate for each pasture since pastures can vary greatly.
Stocking rates are expressed as the number of Animal Unit Months (AUM) supplied by one acre of pasture for one year. An animal unit (AU) is defined as one mature 1000 lb. cow with or without a calf. The animal unit is based on an average daily forage consumption of 26 lbs. of dry matter (forage dried until no moisture remains). Since the original animal unit concept was based on beef cattle, we use animal unit equivalents to deal with other animal sizes and species.
|Animal Unit Equivalents||AU|
|Bison, mature cow - with or without calf||1.0|
|Bison, mature bull||1.5|
|Bison, two year old heifer||0.7|
|Elk, mature cow - with or without calf||0.5|
|Elk, mature bull||0.7|
The next table gives recommended stocking rates in AUMs/acre for seeded tame pastures, in four condition classes, based on annual precipitation. This table assumes average pasture inputs and a continuous grazing system. When determining stocking rates for pastures with effective rotational grazing systems and improved fertility in zones with over 14 inches of annual precipitation, use the stocking rates from the next higher precipitation zone. These stocking rates are based on average conditions, and can be used as a starting point in determining stocking rates. Over time you can adjust the stocking rates for each of your pastures.
|Stocking Rates for seeded tame pastures in Alberta (in AUMs/acre)|
|Annual Precipitation Zones||
Pasture Condition Class
A pasture in excellent condition should produce 75 to 100% of the top yield for the area, with less than 5% of the total production coming from weeds or undesirable species. At least 95% of forage production should come from adapted grasses and legumes. A pasture in good condition will produce 60 to 75% of the top area yield, with 90% of this production from adapted species. A pasture in fair condition will only produce 50 to 60% of the top yields for the area, with 60% of production coming from adapted forage species. A poor condition pasture will produce less than 50% of the area's top yield, and have less than 50% of the production from adapted species.
Example 1: Calculating the number of pasture acres needed for your herd
-18 to 22 inch annual precipitation zone
-Excellent pasture condition class (2.00 AUM/acre)
-Grazing season 165 days (5.5 months)
-80 bison cow/calf pairs (80 AU)
= AU x Months Grazing/AUM/acre
= 80 x 5.5/2.0
= 220 acres for the season
Example 2: Calculating Pasture Capacity
-Annual precipitation zone 18-22 inches
-200 acre grass-legume pasture (excellent condition = 2.00 AUM/acre)
-Want 120 days of grazing (4 months)
= Acres x AUM/acre ) months grazing
= 200 x 2.00/4
100 AU equivalents are 100 bison cow/calf pairs or 200 elk cow/calf pairs.