Estimating Forage Availability
Almost every Kerr Center livestock workshop teaches the basics of estimating available forage and figuring where to place temporary fences to match the number of livestock with the amount of forage available. These basic skills are fundamental to successful managed grazing.
Forage estimation starts with a pasture transect. As with soil sampling, this is simply a line through a pasture that takes in a representative sample of soil type and moisture, forage species, etc.
Walk along the transect, stopping after a set number of paces. At each stop, randomly place a sampling frame. (This can be accomplished by simply tossing the frame and letting wherever it chances to fall be the sampling point.)
The sampling frame is a square 12 inches on a side. The Kerr Center’s frame is welded from rebar for durability, but an expedient version can be worked up from PVC or other materials.
Measure the height of the vegetation in the sampling frame with a grazing stick or falling forage plate meter.
Forage weights are usually expressed in terms of pounds of dry matter. So, after the clipped forage is weighed, it must be oven-dried and re-weighed. The dry weight is what is used in the calculations that follow.
At this point in a workshop, several participants usually start to get worried looks on their faces. They’re wondering whether they have to make these forage measurements every time they move livestock – and how much time that will take!
Fortunately, these measurements are only made initially, to calibrate a forage stick or falling plate forage meter, or simply to aid the manager in developing a visual “feel” for how much forage is available in a given pasture. However, a fresh set of measurements is necessary for different forage mixes, pastures, seasons, and so on.
In the example shown below, nine sampling points along the transect yielded a total wet weight of 14 ounces of forage. After drying, the clipped forage weighed 7 ounces, or 0.44 pounds.
Since there were 9 samples (each clipped from a one square-foot frame), this weight of forage is divided by nine to calculate the amount of dry matter per square foot: 0.048 lbs. dry matter per square foot.
That number is then multiplied by the number of square feet in an acre (43,560) to find the amount of dry matter per acre: 2,129 lbs. dry matter per acre.
The dry matter per acre figure is then divided by the measured forage height (five inches) to calibrate the grazing stick or falling plate forage meter. For this forage mix at this site and season, we’ve now established that every inch of forage height is equivalent to 426 lbs. of dry matter per acre. The next time we need to estimate forage availability in this pasture, we can just walk the transect measuring forage heights, and not have to clip, dry, or weigh any forage samples.