Seasonal Carrying Capacity
(Lesson 5. Grazier’s Arithmetic continued)
Carrying capacity is the stocking rate that is economically and environmentally sustainable for a particular grazing unit throughout the grazing season. Carrying capacity is largely determined by four factors: 1) annual forage production, 2) seasonal utilization rate, 3) average daily intake, and, 4) length of the grazing season. These terms can be expressed in the mathematical formula below:
|Annual Carrying Capacity =||Seasonal Forage Production X Utilization Rate|
|Average Daily Intake X Length of Grazing Season|
Annual forage production is the total amount of forage dry matter produced per acre on an annual basis (see Lesson 1 for measurement techniques). This would include both hay and pasture harvested from grazed acres. In the formula, this term should be expressed as pounds of forage per acre.
Seasonal utilization rate is the percentage of the annual forage production that will actually be harvested by the grazing livestock. This will be very dependent upon rotation frequency and expected level of animal performance. Figure 1 can be used to estimate approximate seasonal utilization rate based on average grazing period length. For example on a 3-day rotation, a reasonable seasonal utilization rate would be 70 percent. Utilization rate is expressed as a unitless decimal fraction in the formula (i.e., 0.70).
Figure 1: Seasonal and grazing period utilization rates relative to length of grazing
Average daily intake should be set at the level that will be required to yield the desired animal performance level. This may well be the most difficult part of the entire process. To accurately determine the appropriate intake value, some estimate of forage digestibility and energy is required. These values cannot be reliably determined without careful forage sampling and laboratory analysis. For this reason we tend to insert arbitrary values in this space and error on the side of overestimating intake. Average forage dry matter intake values for high, medium, and low performance of either steers or cow-calf pairs would be 3.5 percent, 3.0 percent, or 2.5 percent as a percentage of the animal’s bodyweight. (0.035, 0.03, 0.025). Dry matter is calculated as the lbs of forage consumed as fed X the percent dry matter of the forage. If a cow consumes 150 lbs of pasture that is 20% dry matter, the cow consumes 150 lbs X .20 = 30.0 lbs dry matter.
For example, a 1200 lb. cow of medium milking ability would consume about 36 lbs. of forage dry matter on a daily basis (or 0.03 times body weight). In the calculation, intake is expressed as lbs. of forage/lb. of liveweight.
Length of the grazing period is a function of how many paddocks are available and the required rest period. Rest period requirements are going to vary for different species and environmental conditions, such as rainfall, temperature, etc. The choice for length of grazing period must be compatible with the utilization rate used in the calculation as indicated by Figure 1.
When the appropriate values have been entered into the equation and calculations made, the resulting answer is the pounds of animal liveweight that each acre of the grazing unit will support for the indicated grazing season.
As an example, we will assume that an average acre of improved pasture and hay land will produce 7600 lbs. of forage dry matter annually. If we plan to use an average 3-day grazing period, we find by referring to Figure 1 that the corresponding seasonal utilization rate is approximately 68 percent. The livestock will be steers that we hope to have gain 1.5 to 2 lbs./head/day. This would be a moderate performance level, so intake is entered at 3 percent of bodyweight, which is .03 lb. of forage/lb. of liveweight. It is important to enter intake in this format, not as 3 percent so that units cancel out. We will anticipate grazing the steers from May 1 to October 20 or a total of 164 days.
We make the following calculation:
|7600 lb forage/acre X .68|
||= 1050 lb liveweight/acre = Carrying Capacity|
|.03 lb forage/lb X 164 days liveweight|
The 1050 lbs. liveweight/acre is an indication of the carrying capacity of this unit. If we purchase 525 lb. steers, can we stock the unit at 2 steers (1050 lb. liveweight/acre ÷ 525 lb./steer) to the acre? Only on the first day of the season! Why? Because the animals are, hopefully, gaining weight every day and quite likely the average forage availability in August is lower than that in May. If expected average daily gain is 1-3/4 lb./hd/day, the average weight of steers at mid-season ( 1/2 of 164 day grazing season) will be 668 lb. (525 lb. + (82days X 1-3/4 lb./day)). Initial stocking rate could be set at 1.6 steers/acre (1050 lbs. liveweight/acre÷ 668 lbs. liveweight/steer). Remember this is a guideline to help make initial stocking decisions, and not a magical recipe for universal financial success.