Example to Calculate Carrying Capacity by Matching Forage Supply and Animal Demand
A 4,000 ha ranch in the black grama (Bouteloua eriopoda) semi-desert ranges of south-eastern Arizona produces 300 kg/ha/year of key forage species in an average year. Because range condition across the ranch is currently fair, allowable use is set at 30% with the objective of improving range condition.
The terrain is generally flat, so grazing distribution is unrestricted by slope. However, inadequate drinking water development on the ranch results in 60% of the area being less than 1.6 km from water, another 25% is 1.6-3.2 km from water, and the remaining 15% of the ranch is rarely grazed because it is further than 3.2 km from water.
What is the grazing capacity of the ranch, assuming it is stocked with cows having an average weight of 400 kg, and the area is grazed for an entire year?
a. Calculate total forage on the ranch that is accessible to livestock:
Total forage = forage production x area = 300 kg/ha x 4,000 ha = 1,200,000 kg.
However, not all of this forage is accessible to livestock because grazing access is restricted by distance from water:
- 60% of the ranch is 1.6 km from water (i.e., 100% accessible),
4,000 ha x 60% x 300 kg/ha x 100% accessible = 720,000 kg forage available in this zone.
- 25% of the ranch is 1.6 - 3.2 km from water (i.e., 50% accessible),
4,000 x 25% x 300 kg/ha x 50% = 150,000 kg forage available in this zone.
- 15% of the ranch is > 3.2 km from water (i.e., 100% unaccessible),
4,000 x 15% x 300 kg/ha x 0% accessible = 0 kg forage available in this zone.
Therefore, there is
- 720,000 kg + 150,000 kg + 0 kg
= 870,000 kg forage on the ranch which is accessible for grazing.
b. Calculate usable forage by adjusting for allowable use:
- Usable forage = total accessible forage x allowable use
= 870,000 kg x 30% = 261,000 kg forage available for use by livestock.
c. Calculate the forage demanded by each cow for a year:
- Forage demand = 2% x bodyweight x grazing days = 2% x 400 kg x 365 days
= 2,920 kg forage is needed to feed each cow for one year.
d. Calculate grazing capacity:
- Grazing capacity = forage supply ÷ forage demand/cow = 261,000 kg ÷ 2,920 kg/cow
= 89 cows/year.
The grazing capacity of the ranch is estimated to be 89 cows/year, while meeting the concurrent objective of improving range condition. Adjustments to this stocking rate may be required according to seasonal conditions and information from on-going monitoring.
References and Further Reading
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.