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RHD Method Can Help Producers Assess Fall Forage Supplies

Posted Nov. 1, 2002

It's early November - how much forage do you have available in the pasture? How do you estimate how long your summer grasses will last before hay is needed? How many stockers will your winter pasture handle through the fall phase of production, or if you have your stockers on hand, will your pasture handle your needs?

These are questions many producers are asking during this time of year. This article will cover ways of estimating the amount of forage available at any given time and how to estimate the amount of time it will last. The process of estimating the forage supply relative to the forage demand by livestock at a given time is referred to as assessing reserve herd days (RHDs). There are three methods of assessing RHDs: visual, calculation and measured estimate.

The visual method is the simplest and quickest, but is less accurate than the other two. Its usefulness is limited to rotational grazing schemes with existing groups of livestock (covered in the July 2002 NF Ag News and Views). The calculation and measured estimate methods are more complicated, but more accurate, and both are easily adaptable to all grazing programs and groups of livestock. The calculation method is easier to perform than the measured estimate method, and will be the focus of this article.

Using the calculation method, "known pounds per acre inch" of grazeable forage are determined from sample field harvests that are then used to extrapolate total production based on forage height estimates. This requires clipping plots of a known area to a predetermined residual height, i.e., 4 inches for wheat pasture. To keep it simple, use clipping frames with a known conversion factor. Table 1 lists some suggested frame sizes with conversion factors.

Samples have to be dried, weighed, averaged and converted to a pounds- per-acre basis. The more samples taken, the greater the accuracy, but for practical purposes in a uniform stand, six to 10 samples are sufficient. Height measurements need to be taken before and after clipping plots to determine the height of available forage. At this point, it becomes simple math. The formula to determine pounds per acre inch is:
Pounds/acre inch = [average dry weight (grams or ounces) X conversion factor]/[Total height residual height (inches)]

For example, wheat pasture is 10 inches tall. Desired residual height is 4 inches. Your average dry-weight sample is 30 grams and your frame size is 16.6 inches square. Therefore, pounds/acre inch = (30 grams x 50)/(10 inches 4 inches) = 250 lbs./ac.in. Using this formula, one can take forage height measurements in each pasture or paddock and develop an estimate of available forage production. Once total production is determined, harvest efficiency is factored in to determine grazeable production. Then RHDs or grazing duration can be determined. Let's use Table 2 as an example.

Harvest efficiency is set at 100 percent since we are in a good grazing rotation. Therefore, total forage production equals grazeable forage production. If each paddock or pasture was continuously grazed, harvest efficiency would have been less 70 percent, for example.

To determine RHDs, divide grazeable forage production by estimated forage demand (estimated average weight per head x total number of head x intake as % of body weight). For example, if you have 320 stockers averaging 580 pounds per head consuming 3 percent per head per day, forage demand per day is 320 head x 580 lbs. x 3% = 5,568 lbs. per day. RHDs equal 140,000 lbs. grazeable forage/5,568 forage demand = 25 days of forage for current stocker herd.

The measured estimate method is more complicated and labor intensive than the calculation method, but is much more accurate. It requires harvesting representative samples of each forage type in each pasture or paddock to determine total production using clipping frames with known conversion factors (similar to the calculation method but without the height measurements). For a brief and simplistic overview of determining RHDs using the measured estimate method, see the following example (see Table 3).

  1. Grazeable production = Total forage x harvest efficiency = 167,950 x 90% = 151,155 lbs.
  2. Forage demand = 320 Steers, estimated 580 lbs. per head x 3% intake (.03) = 5,568 lbs. of forage needed per day.
  3. RHD = 151,155 lbs. of forage per 5,568 lbs. of forage needed per day = 27 RHDs.

The RHD assessment has great utility in determining grazing duration of a group of livestock. It can also be used to determine the number of head of livestock that can be grazed on a particular area if grazing duration is known. Keep in mind, some assumptions have to be made relative to harvest efficiency, growing conditions, and rate of intake. As you begin to use RHDs, visit with a forage specialist to assist with assumption parameters.