Matching Forage Supply and Animal Demand

A straightforward approach to determine the number of animals the management unit can support over a period of time is to divide the total forage biomass (ie., forage supply) by the total amount of forage consumed by a grazing animal during the grazing period (ie., forage demand). Calculations based on long term average forage production provide an appraisal of carrying capacity, whereas existing forage levels give an estimate of shorter term stocking rates.

When following this procedure, forage supply is derived from an estimate of biomass produced over the entire management unit. This estimate of biomass is the result of either sampling using a method to determine biomass, past experience, or professional judgement. Under sustainable management objectives, however, the actual amount of forage available to grazing animals must be considered to be less than total biomass. For example, an adjustment for allowable use must be incorporated into the calculation to ensure that some ungrazed residual biomass is maintained to protect soil and vegetation resources. Further adjustments should be made to discount the contribution of less accessible forage, such as areas that are distant from water (Table 1) or having steep, rugged terrain (Table 2).

Table 1. Suggested Reductions in Cattle Carrying Capacity with Distance from Water
Distance from Water
(km)
Reduction in Carrying Capacity
(%)
0 - 1.6 None
1.6 - 3.2 50
>2 100

Source: Holechek et al. (1995), p 197.

Table 2. Suggested Reductions in Cattle Carrying Capacity for Different Slopes
Slope
(%)
Reduction in Carrying Capacity
(%)
0 - 10 None
11 - 30 30
>21 - 60 60
>60 100

Source: Holechek et al. (1995), p 199.

Forage demand is usually a simple calculation that assumes that most rangeland ruminants have a daily consumption equivalent to about 2% of their body weight. However, attempts to allocate forage sources between various herbivores using animal unit equivalents create a more complicated situation, because plants rated as forage for one species may not be significant in the diet of another.

Obviously, this method to estimate carrying capacity involves many assumptions that are subject to a considerable amount of subjective interpretation. On the supply side of the equation, it is difficult to obtain accurate estimates of forage availability over extensive areas. Similarly, the willingness of animals to graze less accessible areas depends on the availability of alternative forage sources within the pasture, and the level of allowable use varies according to species composition of the vegetation, seasonal conditions and management objectives. On the demand side of the equation, animal intake is affected by breed, age, reproductive status and forage quality; and it is also difficult to quantify consumption by wildlife and invertebrate herbivores. Therefore, these calculations should be regarded only as initial guidelines for animal numbers in the management unit that will require further strategic adjustments depending upon specific management objectives, personal experience, immediate forage conditions, and inventory or monitoring that describes rangeland condition and rangeland trend assessments.

References and Further Reading

(Note: pdf files require Adobe Acrobat (free) to view)

Cordova, F.J., J.D. Wallace, and R.D. Pieper. 1978. Forage intake by grazing livestock: A review. Journal of Range Management 31:430-438. (pdf)

Holechek, J.L. 1988. An approach for setting the stocking rate. Rangelands 10:10-14.

Holechek, J.L., and R.D. Pieper. 1992. Estimation of stocking rate on New Mexico rangeland. Journal of Soil and Water Conservation. 47:116-119.

Holechek, J.L., R.D. Pieper, and C.H. Herbel. 1995. Range management principles and practices. Prentice Hall, Englewood Cliffs, NJ. 2nd ed. pp. 197-204.

Martin, S.C. 1975. Stocking strategies and net cattle sales on semi-desert range. US Department of Agriculture, Forest Service Research Paper. RM-146.

Troxel, T.R., and L.D. White. 1989. Balancing forage demand with forage supply. Texas A M University Extension Service Publication. B-1606.

Workman, J.P., and D.W. MacPherson. 1973. Calculating yearlong carrying capacity: An algebraic approach. Journal of Range Management 26:274-277. (pdf)