Stocking Rate and Carrying Capacity
Carrying capacity describes the average number of animals that can be placed on a pasture for a season without harming it. This is determined by estimating the maximum amount of forage that can be removed through grazing and still allow for the maintenance or improvement of the health of the pasture. The amount of forage or grass residue left behind after the grazing period must be enough to protect the plants and soil, conserve moisture and trap sediment, and to allow for healthy plant growth the following spring. The grass left behind must be alive and capable of regrowth.
If the pasture us overstocked, pasture quality will decline due to over-grazing. Forage production and soil health will drop. Also, weeds may invade and the desirable species will be replaced with less palatable and less productive species.
Below is a step-by-step explanation of how to determine the appropriate stocking rate for your pasture. You can also use the stocking rate calculator to assist you.
Finding the balance
Step 1. Estimate grazeable acres and forage production
First, a realistic estimate of how many acres of the pasture the livestock actually use is required. In most cases, the entire area of pasture will not be grazed. If the pasture contains dense bush, sloughs or steep slopes, for example, these acres need to be subtracted from the total area.
Next, estimate the amount of forage in pounds per acre of dry matter (DM) produced by the pasture during the growing season. Yeilds from previous hay production can be used to make an estimate. Clipping, drying and weighing forage samples from the pasture will also provide an estimate of forage production.
Of a total of 160 ac, a pasture has 20 ac of bush and 15 ac in sloughs, leaving 125 ac of grazeable pasture. The estimated forage production is 2500 lbs/ac. Therefore, the total production is:
- 125 ac X 2500 lbs/ac = 312,500 lbs of total forage production (DM).
Step 2. Determine the utilization rate
The utilization rate the total amount of forage that is removed from the stand. It includes all uses of the forage – the amount taken by the herd, plus the amounts lost to trampling, insects and other losses.
What is left behind at the end of the grazing season ultimately will determine what the yield potential will be for the next year.
Different forages can tolerate different levels of grazing. Soil and moisture conditions must also be taken into consideration.
- For pastures with native forages, a good rule of thumb is to take half and leave half – a 50% utilization rate.
- On lighter soils and in droughtier conditions, the utilization rate may only range from 50% down to 30% or lower, leaving 50-70% or more behind.
- Healthy stands of of tame forages can handle a 60% utlilization, but rates greater than 70% will decrease forage production in the long term.
The actual amount of available forage will be a percentage of the total dry matter yield of the pasture. For example, if the pasture referred to in the previous step is a good quality tame forage stand, only 187,500 lbs is available for use (60% of 312,500 lbs).
Step 3. Estimate the forage requirement for the herd
The forage requirement will vary based on weight, the make-up of the herd (cows, steers, etc) and the number of animals. These values are used to calculate the daily forage requirement for the herd.
Cattle will eat 1.5% to 3.5% of their body weight per day, depending on age and sex. For cows, 2.5% of the body weight is commonly used. Calves are not considered until they reach 600 lbs. For steers, 3% is commonly used.
- Example for a 1500 lb cow:
1500 lbs X 2.5% = 37.5 lbs of forage dry matter per day
- Example for a 750 lb steer:
750 lbs X 3% = 22.5 lbs of forage dry matter per day
Step 4. Determine the stocking rate
With the above information, the carrying capacity of the pasture can be estimated and an appropriate stocking rate selected.
Example calculation using the information from the previous steps:
What we know:
- 125 ac of grazeable pasture
- total forage dry matter yield is 312,500 lbs
- pasture is tame forage with a 60% utilization rate so yield available for the herd is 187,500 lbs
- herd consists of 50 animals, cows averaging 1,500 lbs with calves
- each cow requires 37.5 lbs of forage per day (calves don’t count)
Question: How long could this herd stay on this pasture?
Calculate how much forage is required for the herd.
- 37.5 lbs/day X 50 head
= 1,875 lbs/day for the entire herd
Now put the information together to get the total number of days.
- 187,500 lbs of available forage ÷ 1,875 lbs/day required
= 100 (99.7) days
Question: How many cows could be put on the pasture if the grazing period was 130 days?
Calculate how much forage each cow needs for 130 days.
- 37.5 lbs/cow/day X 130 days
= 4,875 lbs/cow for the duration of the 130 day period
Knowing the requirements of each animal and the amoung of forage available, calculate how many cows may be placed on the pasture.
- 187,500 lbs of available forage ÷ 4,875 lbs
= 38 head