Carrying Capacity and Stocking Rate

Written by Rachel Frost, Montana State University

The most important decision for successful range management is determining carrying capacity and setting a proper stocking rate. Matching the number of grazing animals with the forage resource is an important management decision regardless of grazing system. Too many animals in a management unit or pasture will reduce livestock weight gain, conception rates, and body condition and cause undesired changes in the soil and vegetation.

Carrying Capacity

The number of grazing animals a piece of land can support long term while maintaining or improving the rangeland resources (vegetation, soils, and water) is called carrying capacity. The characteristics of the land, vegetation, and soil determine the carrying capacity, not the land manager. Proper carrying capacity attempts to balance between long-term forage supply and forage consumption by all grazing animals, both livestock and wildlife. Determining carrying capacity is an important goal of any rangeland inventory or monitoring program and forms the basis of stocking rate decisions. Furthermore, an assessment of carrying capacity can provide information on potential economic returns from ranch developments and forms one basis of ranch value on the real estate market. The actual carrying capacity for any management unit varies across years because annual forage production fluctuates due to variability in both the annual and growing season precipitation and temperature.

Determining carrying capacity The simplest and most reliable way to determine carrying capacity is to obtain past stocking rates and grazing management information for a piece of land and then assess the ecological status or condition of the rangeland. If the condition is good or improving, the current stocking rates are below carrying capacity. If the condition of the rangeland is declining, carrying capacity has been exceeded, and grazing management practices or stocking rate may have to be adjusted.

What if there are no historical stocking rate data available? One can estimate carrying capacity for a parcel of land in several other ways, even without historical stocking rate information. The first way is to measure annual forage production on the land and calculate an estimate of carrying capacity.

This method is useful but is based on a series of estimates for annual forage supply and animal demand. Continued monitoring of range condition and adjustments may be necessary to determine a final carrying capacity. Your local Cooperative Extension Service or Natural Resources Conservation Service office can assist you in this process.

Another way to estimate carrying capacity is to compare the land to similar rangeland with a successful history of setting stocking rates. Find out the carrying capacity estimates of the area and use those as a guideline to begin the process of determining an appropriate carrying capacity for your particular piece of land.

Trial and error is an inherent part of estimating carrying capacity. Additionally, carrying capacity is complicated by the fact that both forage production and animal intake are dynamic factors that vary with ecological site, topography, time of sampling, and plant species composition. The diets of grazing animals also vary according to animal nutritive requirements and the unique dietary preferences of species, breeds, and individuals. Therefore, carrying capacity estimates should be treated as an initial starting point for the management unit that will almost certainly be revised with continued monitoring and current environmental conditions.

Adapted from: Determining Carrying Capacity by Ken Sanders

Stocking Rate

As stated above, the most important decision for successful range management is setting a proper stocking rate. Stocking rate is defined as the number of animals grazing on a given amount of land for a specified time. Stocking rates are often expressed in AUs/unit area (links to Animal Unit).

4 Steps to Setting a Basic Stocking Rate:

1. How much forage do you have? The first step toward determining how many animals the grazeable portion of a management unit can support is to measure how much forage the area produces. The amount of forage produced can vary greatly from year to year depending on weather; therefore, estimated forage production for determination of stocking rate should not be based on one year of forage production data. Managers can estimate the forage production of their land by mapping the management area into units of land that produce similar kinds and amounts of forage. These units are termed ecological sites or habitat types and most often will differ in soil type and plant productivity. Each unit should be surveyed to determine biomass production by 1) mowing or clipping small areas, 2) referring to picture guides to visually estimate amount of forage available, or 3) referring to site guides to estimate average productivity of the site. Only a certain portion of the biomass can be safely removed, and this depends on the vegetation and other management objectives. A common starting point is that 50% of the annual production that is potential forage can be harvested each year. Once a biomass estimate for each site has been obtained, multiply the number of acres within each site by its estimated biomass production per acre and sum these values for a total amount of potential forage produced. Finally, multiply the total amount of biomass by the percent of allowable use to obtain the total amount of forage available for livestock use.

2. How much of that forage can be used by grazing animals? (Usable Forage) Land characteristics such as topography and distance from water influence the amount of forage available to grazing animals. Forage in areas with very rough or steep topography may be inaccessible to grazing animals or used much less than forage on level ground close to water. For example, grazeable plants located two miles or more from water are essentially unusable for livestock. They represent potential forage for livestock if the animals could be enticed to use the area. All management units should be stratified into areas of available and potential forage. Areas of potential forage identify management opportunities to improve livestock distribution across a greater portion of the landscape.

3. How much forage do your grazing animals need? (Forage Demand) To calculate how much forage your grazing animals will need, you must determine the average weight of the animals in a herd or flock and the number of days the herd will graze the management unit. The average daily forage consumption of an animal — a combination of eating, trampling, and spoilage from urine or feces — will vary with the nutritive quality of the forage; however, an annual average of 2.6% is usually acceptable. For example, a 1,200-pound range cow that consumes 2.6% of her body weight requires 31 pounds of forage per day (1,200 pounds * 2.6%). When you multiply the animal's daily need by the number of days the management unit will be grazed, you obtain forage demand for that period. In the example above, if the cattle grazed the unit year round, then each cow would require 11,315 pounds (31 pounds * 365 days) of forage for the year.

4. What is your appropriate stocking rate? Now that you have answered the above questions — how many animals can you graze and for how long — to determine the number of animals that can be grazed on a management unit, you must divide the pounds of usable forage by forage demand. For example, if a management unit that is grazed year long has 465,000 pounds of usable forage, and annual forage demand per animal is 11,315 pounds of forage, then 41 cows can graze the unit for 12 months. If the grazing period is less than 12 months, then more animals can graze the area for a shorter period. A three-month grazing period would have a stocking rate of about 164 cows [465,000 pounds of available forage /(31 pounds forage/head/day * 91 days) = 164.8 head].

 Adapted from: Launchbaugh, K.L. Forage Production and Carrying Capacity: Guidelines for Setting a Proper Stocking Rate

For additional help on calculating stocking rates, see the following resources: Pratt, M. and A. Rasmussen. 2001. Determining Your Stocking Rate. Utah State University Extension Publication NR/RM/04