Northern Prairie Wildlife Research Center
Today, the majority of the SNG is managed under twice-over deferred rotation systems (Table 2). Most of these are 3-pasture systems, although 4-pasture is also common, with a smaller number of 2-pasture and 5-pasture systems. One allotment is managed as a 6-pasture deferred rotation system. Several allotments, or a pasture within an allotment, are managed as season-long systems. Grazing usually begins about 15 May and lasts for 5.5 to 6 months, ending about mid-November.
Stocking on the SNG is measured in terms of animal units (AU) and animal months (AM). An animal unit is considered to be a cow/calf pair. Thirty days of grazing by 1 cow/calf pair constitutes one animal month. Appendix 2 gives some history of the stocking of the 51 allotments of the North Unit. The 1995 stocking was: 9,391 cow/calf pairs utilizing 50,076 AM's, 424 yearlings utilizing 2,255 AM's and 342 bulls utilizing 1,819 AM's (USFS, personal commun.). An animal unit month (AUM) is the basic unit of grazing capacity and is defined as potential forage intake (animal demand) of one animal unit for one month or 30 days (Valentine 1990). As a quantitative measure of carrying capacity, the AUM is further described as 750 pounds of air dry forage (2.5% of body weight X 1000 pounds X 30 days). The AUM's available on the SNG can be estimated from the stocking values given for 1995, although there is not complete agreement on how this should be translated. The USFS gives the following estimates: 1 cow/calf pair AM = 1.32 AUM's; 1 yearling AM = 1 AUM; 1 bull AM = 1.5 AUM. The total AUM's available for 1995 using these values was 71,084. Given the total acreage of the SNG of 70,180, this translates into about 1 acre per AUM. The value of accurate stocking rate data cannot be overstated. The stocking rate will greatly affect the quantity and/or quality of the available and residual forage, which will directly affect livestock production as well as many wildlife species.
Twice-over, Deferred Rotation
------------------Number of Patures--------------------
South S West
South S East
1Yearling pasture in Ekre allotment is grazed season long. East-West
pastures are managed as twice-over, deferred rotation system.
2The West pasture is grazed season-long by yearlings. The other four are grazed as a thrice-over, deferred rotation system. One pasture is grazed twice over.
3East pasture grazed season long with yearlings. The remaining three pastures are grazed as a twice-over, deferred rotation system.
4One of the pastures is grazed thrice-over.
5One of the pastures is grazed only once.
6Two pastures of the three are grazed thrice-over.
7Two pastures are grazed four times and one three times.
8One of the pastures is grazed only once.
9Two of the pastures are grazed only once.
10All four pastures are grazed thrice-over.
11The West S 1/3 pasture is grazed only once at a reduced rate.
12The West pasture is grazed only once, and one of the four pastures is private land.
13All four pastures are grazed thrice-over.
14One of the pastures is grazed only once.
15Both pastures grazed thrice-over.
16This allotment is managed as a two-pasture deferred rotation with private land.
17The five pastures are managed as two three-pasture deferred rotation systems with one pasture being used both ways.
18Operated as a six-pasture deferred rotation system by using private land.