One of the first questions the new owner or potential buyer of a unit of pasture or rangeland asks is "How many cattle, sheep, horses, etc. can I graze on this land?" In other words, what is its carrying capacity?
CARRYING CAPACITY is defined as the maximum stocking rate possible which is consistent with maintaining or improving vegetation or related resources. It may vary from year to year on the same area due to fluctuating forage production.
STOCKING RATE is defined as the number of specific kinds and classes of animals grazing or utilizing a unit of land for a specified time period. It may be expressed as animal unit months or animal unit days per acre, hectare, or section, or the reciprocal (area of land/animal unit month or day).
GRAZING CAPACITY, although sometimes used synonymously with carrying capacity, is defined as the total number of animals which may be sustained on a given area based on total forage resources available, including harvested roughages and concentrates. It is the relationship between number of animals and area of land at any instant of time, expressed as animal-units per acre, animal-units per section or AU/ha. For definitions of other terms used in this discussion, see the Glossary of Terms Used in Range Management.
Recommended Method to Determine Carrying Capacity
By far the easiest and most accurate method of determining the carrying capacity of a unit of land is to obtain past stocking rates and grazing management history from the previous owner/grazer and then assess the ecological status (range condition) and range trend of the land.
If range trend has been stable or upward the past few years, then the stocking rates have been within carrying capacity limits and past management practices have been effective. If trend is downward, then an adjustment in management or stocking rate is needed.
Caution! Downward trend does not necessarily mean numbers of animals should be reduced. In fact, stocking rate is the last thing to consider. A more likely cause of downward trend, especially on Idaho rangelands, is poor livestock distribution. This can be easily assessed by doing some simple utilization mapping. If there are areas of a pasture that are under used and others over used, then what can be done to encourage the livestock to make more use of the under utilized areas?
Other factors to consider before reducing stocking rate are:
- Is a change in grazing season warranted?
- Is the grazing system being used working or are changes needed? Would a shorter period of grazing or a season of rest improve the range?
- Is the appropriate kind of animal being used (i.e. is the rangeland better suited for sheep than cattle, etc.)?
- Is there an alternative source of forage available? Or is brush or weed control warranted?
- Has there been a recent extended drought?
- Have other uses increased (i.e. numbers of wildlife) or caused the downward trend?
If it is determined that one or more of these factors is not the cause of downward trend, then an adjustment in stocking rate may be warranted. Make the adjustments you feel may be necessary, monitor trend, and readjust upward or downward as conditions warrant.
Another easy and fairly accurate method of determining carrying capacity is to look at comparable pasture or rangeland in the area and find out what their carrying capacity estimates are. The local Cooperative Extension Service or Natural Resources Conservation Service offices may also be able to assist you in determining carrying capacity. If the unit of land you are interested in is public land, the administrating agency should already have an estimate of carrying capacity.
What if no historical stocking rate is available?
If there is no historical stocking rate information available or the local Extension Service or Natural Resources Conservation Service offices can not provide such information, they may be able to assist you in measuring annual forage production on the land in question and calculate an estimate of carrying capacity.
This may very well be the case if you have irrigated pasture that is seeded to a forage species or mix of species that is not commonly grown in your area. Caution! This method works well in theory, but is based on a series of estimates. The final result is only as good as the estimates. Contact your local Cooperative Extension Service or Natural Resources Conservation Service office for assistance.