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RHD Helps Predict Forage Supply Relative to Livestock Demand

Posted Jul. 1, 2002

How do you predict your forage supply into the future? One could solicit help from a fortuneteller, a crystal ball, an Ouija board or perhaps a weatherman. Most people just wait for the inevitable, take whatever Mother Nature provides and deal with problems when they arrive. It's mid-summer - early July. Do you know how your pastures are holding out?

If you are using rotational grazing and spending time in your pastures with the cattle, there is a more reliable way of predicting future forage supply relative to livestock demand. All that is needed is to understand the relationship between the cowherd and pasture utilization, to develop a good working knowledge of the expected production from forage types and a targeted date to reach using the current grazing scheme. The method is called the Reserve Herd Day (RHD) concept, and it is credited to retired Foundation forage specialist R.L. Dalrymple.

In recent years, Dalrymple has written often about the RHD concept and the three methods of estimating RHDs - visual, calculation and measured estimate. The calculation method is primarily used in research and demonstration studies in which more precise estimates are needed. It requires taking clippings in each pasture for each forage and soil type. The measured estimate method requires taking height measurements and sample clippings for each forage type to calibrate an estimated pounds-per-acre-inch measure. Average pasture height measurements then are used to calculate production per pasture. It is not as accurate as the calculation method, but is quicker and more easily applied.

The visual method (the method to be discussed in this article) is the simplest and quickest method, and it is sufficiently accurate for eyes trained to rotational grazing. This two-step method is something most producers could and should be willing to do if they are serious about grazing management. The first step is simple - assess the duration that the existing cowherd could graze in each pasture before attaining the desired residual height. Tables 1 and 2 are examples of RHDs for a bermudagrass property and a native range property using the visual method.

Knowing RHDs is all well and good, but what do you do with them and what do they mean? Not much unless you have some RHD parameters to gauge them by, which brings us to step two - developing RHD ranges, or "ebb and flow" tables, for the growing season of predominant forage types. Table 3 indicates the ebb and flow of bermudagrass pastureland and Table 4 shows the ebb and flow of native grass pastureland. The essential elements needed to begin are the percentage of production expected by month relative to the growing season (sometimes found in county soil survey books) and the days until target date. The RHD range is the final and most crucial element. The front end of the range is determined by multiplying the percentage of annual production expected at a particular date by the number of days until target date. The backside of the RHD range is determined either by a factor derived from experience relative to the front end of the target date or by the number of days until target date.

Comparing the examples in Table 1 and Table 2 to their respective ebb and flow tables (Tables 3 and 4) demonstrates the utility of the RHD concept. In the bermudagrass example, we assessed 49 RHDs. If the assessment was performed on July 1 and the target date is Nov. 1, one can easily determine that the forage supply is running short - at least 56 percent short (75 RHD required - 49 RHD assessed / 75 RHD required). Drought management strategies should be implemented. Options might be to sell, move or purchase substitute feed immediately for 35 percent of the herd. If the target date is Sept. 1, then the 49 RHDs assessed falls within the RHD range (40 to 60) and no action is needed at the moment. If we had assessed 100 RHDs on July 1 and we wanted to capture some excess at a decent quality in the form of hay (target date of Nov. 1), we could consider baling between 11 percent (100 RHD assessed - 90 RHD required / 90 RHD required) and 33 percent (100 RHD assessed - 75 RHD required / 75 RHD required) of the grazeable acres.

In the native grass example (Table 2), 185 RHDs were assessed. Referring to Table 4, the July 1 assessment date with the April 1 target date and an RHD range of 190 to 250 indicates a slight shortage. Continued close monitoring is needed throughout the remaining growing season (at least every two weeks) to determine if reserve measures need to be undertaken. However, if the target date is Feb. 1 and the RHD range is 140 to 210 days, all is well. If we had assessed 250 RHDs on July 1 with 30 percent of the annual production yet to occur (target date April 1), we might have considered temporarily increasing the stocking rate by 15 percent ([250 - 190 of the RHD range / 2] / 190 RHDs required) to 31 percent (250 - 190 of the RHD range / 190 RHDs required).

The examples in this article only address the visual method of assessing RHDs. It works well in a relatively intensive rotational grazing program. It does not work as well in a continuous or very simple rotation. The measured estimate method is more applicable and will be communicated in a future article. Remember the more aggressively producers stock their pastures, the more often they should evaluate RHDs throughout the grazing season.

Although intriguing, there is nothing magical about the RHD concept. It is certainly not as romantic as using a fortuneteller or Ouija board, but it's not as risky, either. If you consider yourself to be a decent manager of a rotational grazing unit using the RHD concept, expect to perform at least as accurately as the weatherman. More importantly, you will establish a relative degree of predictability for forecasting the future relationships between livestock demands and forage supplies on your operation.

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