Grazing Period Stock Density
(Lesson 5. Grazier’s Arithmetic continued)
After making basic farm stocking decisions, the time comes for every grazier to make the actual decision of where to place a break fence or how many animals to place in a particular paddock. That decision is based on the same principles used in the carrying capacity equation discussed above but modified to represent single grazing period conditions rather than seasonal values.
The carrying capacity equation (Equation 1 previous page) becomes the stock density equation (Equation 2 below) with the following modifications:
Equation 2: Stock Density
| Available Forage X Grazing Period Utilization Rate | |
| Stock Density = |
|
| Average Daily Intake X Length of Grazing Period |
Available forage is the quantity of forage dry matter that is actually allotted to the animals for a grazing period. Accurately measuring forage availability is time consuming and expensive so we tend to rely on estimations of yield. The simplest method is to look at a pasture and make an educated guess as to what the forage availability is likely to be. With practice, a good grazier can consistently estimate within 20 percent ± the actual yield. A second method relates height and condition of the pasture to dry matter yield. Height X dry matter yield relationships for several types of pasture are given in Table 1.
Table 1. Estimated dry matter yield in pounds per acre-inch for several pasture types and stand conditions.
| Pasture Species | Stand Condition, (lb./acre/inch)* | ||
|---|---|---|---|
| Fair | Good | Excellent | |
| Bluegrass/Clover | 150-200 | 250-300 | 350-400 |
| Perennial Ryegrass/Clover | 150-200 | 250-300 | 350-400 |
| Orchardgrass/Legume | 150-200 | 250-300 | 300-350 |
| Timothy/Legume | 150-200 | 250-300 | 300-350 |
| Tall Fescue + Nitrogen | 100-150 | 200-250 | 350-400 |
| Mixed Pasture | 150-200 | 250-300 | 300-350 |
*Values from Pasture Stick developed by Cornell University and NRCS
The stand condition is determined based on visual estimate of green plant ground cover after the paddock has been grazed to a 2-4 in. residual. An excellent stand has at least 90 percent of the ground covered by green plant material or less than 10 percent exposed soil. The good condition has 75 – 90 percent ground cover or 10 – 25 percent bare ground. Fair condition has less than 75 percent ground cover or greater than 25 percent bare ground exposed. In all cases, moderate soil fertility is assumed.
The following example illustrates how to determine where to place a temporary fence to create a paddock to feed a herd of 100 steers weighing 600 lb./hd for 3 days with a rate of gain objective of 2.25 lb./hd/day. The pasture is orchardgrass-red clover 10 inch tall (including the stubble; or 7 inches of usable forage with a 3″ stubble assumed) and the area where the steers have just finished grazing has about 20 percent bare ground. The pasture is 40 acres and is 660 ft wide. To use the stock density equation we must first determine the appropriate values.
Forage availability can be estimated from Table 1 using the average sward height of 7 inches and the stand condition as good. The corresponding value for an orchardgrass-legume pasture is approximately 250 lbs./acre-inch, so the available forage is 1750 lbs./acre (7 inches X 250 lbs./acre-inch).
Figure 1 can be used to estimate the appropriate utilization rate for a 3 day grazing period. As an average daily gain of 2.25 lbs./hd/day is a high performance objective, utilization cannot be excessive or else intake will be limited. To maintain an intake rate of 3.5 percent of bodyweight, a 50 percent utilization rate appears to be appropriate to use in the calculation. Assuming the 3 day grazing period, we can make the following calculation:
| 1750 lbs. forage/acre X .5 utilization rate | |
|
|
= 8333 lbs. liveweight/acrey |
| 0.035 lbs. forage/lb. liveweight X 3 days |
The steers weigh 600 lb/head and each acre will support 8333 lbs. liveweight, so the pasture can be stocked at the rate of 14 steers/acre/3 day period (8333 lbs. liveweight/acre ÷ 600 lb. liveweight/steer). The herd of 100 steers will require 7.2 acres/paddock (100 steers ÷ 14 steers/acre).
For ease of figuring, assume 8 acres per feed strip. It is better to give a little more and waste a little feed than to allow too little and limit intake. To determine where to place the fence, calculate the total square footage in the 8 acres (8 acres X 43,560 ft2 /acre = 348,480 ft2) and divide by the known width ( 348,480 ft2 ÷ 660 ft = 528 ft). Placing the temporary fence at approximately 530 ft will give adequate forage for the 100 steers for the 3 day grazing period.
It is very important that values used for the parameters in the equation are realistic in how they relate to one another. All of the parameters are interrelated and inserting an inappropriate value for any one parameter will result in erroneous conclusions. For example, if available forage is below 1500 lbs./acre, an intake of 3.5 percent would be impossible to achieve. For this reason, the equation cannot be used as most mathematical formulas where if all but one value is known the remaining value can be calculated. A calculation can be made, but the result may be biologically meaningless.