How Many Horses Can Your Pasture Maintain?

Equine Science Team 
Iowa State University

Stocking rates provide information on how many horses a pasture can carry in a month. In general, the approximate pasture needs per average-sized mature horse, with pasture providing most, if not all, of the nutrition is:

• 1 -  2 acres with an excellent, dense sod, permanent pasture
• 2 - 2.5 acres with an average permanent pasture (spring growth will be OK but summer forage is average)
• 3+ acres with a thin, poor sod that is unmanaged (supplemental forage will likely be needed)

Depending on the pasture and how it is managed supplemental hay and/or grain may be needed to meet a horse's requirements. To evaluate the current forage and how many horses one can keep on pasture the stocking rate takes into account the following:

• Forage species
• Season of year
• Environmental moisture
• Fertilization
• Length of time horses have access to pasture

Table 1 lists the forage needs for one animal unit factor or one horse. For example, a mature horse at maintenance requires 0.9 animal unit factors. Table 2 provides values for each month on how many animals a specific type of pasture will carry for one month. If forage is not listed, one can use a value for a forage that is similar and calculate the stocking rate. Table 2 is also divided into continuous grazing and rotational grazing. Continuous grazing is where the pasture is grazed for an extended amount of time with no or infrequent rest to the plants from grazing. Rotational grazing is moving grazing livestock between pastures or paddocks as needed or on a regular basis.

Table 1. Horse Needs Animal Unit Factors1

Table 2. Total Animal Unit Months of forage for selected Pasture Type and its Availability by Month during the Grazing Season

1N = Nitrogen
2Animal Unit Months For Each Forage Type

How to Calculate Stocking Rates

Example - You own 3 horses. The horses are used for light trail riding during the spring, summer, and fall months. The horses are housed on 10 acres.

1. List the type of horses in column 1. Enter mature horses, light exercise for kind of livestock.
2. List the AU Factor from Table 1. The AU factor for a mature horse light work = 1.0
3. Multiply column 2 (number of horses) * column 3 (AU Factor) = column 4 (Animal units). The number of horses is 3 and the AU factor from table 1 is 1.0. Multiply 3 x 1.0 = 3 animal units. Place 3.0 in the corresponding month’s column.
4. Add each column to give the total Animal Unit Months of forage needed. For example, in the month of May, the forage in the pasture needs to supply 3.0 AUM's
5. Choose the type of forage. In this example, unimproved bluegrass is compared to rotational improved orchard/brome paddocks. Place acreage owned under acres.
6. Calculate AUM for the amount of acreage one owns.
   1. Unimproved bluegrass 10 acres * 0.6 (AUM from table 2 for the month of May) = 6
   2. Improved orchard/brome 4 paddocks 10 acres * 0.5 (AUM from table 2 for the month of April) = 5
   3. Continue with acreage * AUM from table 2
   4. Add the total AUM the acreage provides for the growing season. Unimproved bluegrass = 21; Improved orchard/brome = 76sad
7. Compare
   1. Once the tables are complete compare what the horses need versus what the forage produces per grazing month. For example 3 horses grazing in the month of may requires 3 AUMs to solely feed them on pasture. An unimproved bluegrass pasture in the month of May supplies 6 AUMs which exceeds the needs of the 3 horses. For the month of September, only 2 AUMs are supplied with 10 acres of unimproved bluegrass indicating the horses will require supplemental feed to meet their requirements. For a rotational improved orchard/bromegrass pasture, 17 AUMs are provided in the month of May and 5 AUMs in the month of September. Therefore, if the pasture was divided into a rotational grazing system, the 10 acres would meet the 3 horses' nutritional requirements from April through November without supplemental feed. Excess pasture should be managed either by baling or mowing the pasture.

Table 3. Calculated Animal Unit Factors

1Enter number of each kind of livestock
2AU Factor = Value from table one for kind of livestock
3Animal units = Number in column 2 x AU factor in column 3
4Animal units to be carried each month, Enter the total animal units as shown in column 4 that will be on hand each month

Table 4. Calculated Animal Unit Months of forage for selected Pasture Type and its Availability by Month during the Grazing Season

1List type of forage
2List the number of acres
3Animal Unit Months Per Acre = AUM per acre = Acres in column 2 x AUM per acre for each month given in Table 2.
4AUM = Acres in column 2 x total AUMs for each kind of pasture as shown in the last column in Table 3

Resources

• Pasture_Calculation_Worksheet.pdf
• Equi-Analytical is an excellent company to have pasture, hay, and grain analyzed. The company also provides information on how to take samples including the pasture.
• Guide for Year-Round Forage Supply
• Pasture Management Guide for Livestock Producers