The Height-Weight Method is used to determine average utilization. This method is based on the assumption that there is a relationship between biomass and the height of a plant. Weight based on plant height is estimated by measuring the heights of ungrazed and grazed grasses or grasslike plants. Measurements of plant heights are recorded along transects (line or pace transects). These measurements are then converted to percent of weight utilized through the use of a “utilization gauge” (Lommasson and Jensen 1943). Utilization gauges are developed from height-weight relationships curves from previous studies (a utilization gauge developed by the U.S Forest Service, Rocky Mountain Forest and Range Experiment Station can be obtained from Colorado State University). A utilization gauge can also be developed by clipping 10 or more plants as described on pg 92-93 in the Interagency Technical Reference 1734-3. Ultimately heights are converted to percent utilized with adjustments made for the average ungrazed height of plants.
In addition to determining percent utilization, this method provides a tool that can be used for training, checking personal judgment, and promoting uniformity of results between observers.
This method can be used to obtain utilization data where the key species are bunchgrasses, rhizomatous or sod-forming grasses, or grass-like species. It cannot be used for determining utilization of forbs and shrubs. In bunch grasses, for example, the height of the grazed plant can be highly related to the percent utilization, enabling the development of a relationship between height and weight. Therefore, heights of plants can be measured to estimate percent utilization.
The height-weight method provides uniform, accurate, and reliable utilization determinations for perennial grasses and grass-like species. However, it cannot be used for determining utilization of forbs and shrubs.
This method is objective, but some estimation is required. Because every grass plant is not exactly the same, some error is involved as well. It also requires numerous ungrazed plants, which may be hard to locate.
Another disadvantage is that accurate utilization scales may not be available for the key species. Height-weight relationship curves can be developed in this case; however, preparation of utilization gauges scales can be time-consuming.
This method requires adequate training for field application. It assumes the observers know the relationship between ungrazed height and weight. Observers have to be able to identify the plant species and must know how to measure and record the height of grazed and ungrazed plants, determine the utilization of individual plants from the gauge, and calculate the average utilization by key species.
Related utilization and residue measurement methods include:
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