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The Grazed Class Method uses photo guides of key species to make utilization estimates. These estimates reflect herbage removed but also show biomass remaining. Using this technique the observer compares the sample plant(s) with the photo guides for that species and classifies the species according to one of six grazed-classes (0, 10, 30, 50, 70, or 90 percent use). The utilization estimates are based on growth form of the plant. Because variations in height are automatically adjusted for by the eye, variations in height growth due to site characteristics and seasonal precipitation are disregarded.
This method is best adapted for use on perennial grass, perennial grass-forb and grass-shrub rangelands where the key species are either bunch or rhizomatous/sod-forming grass or grass-like species. It is designed for use after the plants have achieved full seasonal growth.
Errors can be encountered because of variations in plant growth between sites and ecoregions, therefore photo guides should be compared with growth curves of local plants. New guides need to be prepared if the existing photo guides do not correspond to the growth curves. In addition, it may be necessary to develop several guides for each key species in order to capture year-to-year or site-to-site variations in growth form.
This method is rapid and easy to learn and use with proper training. The guides serve as standards of comparison that promote consistency in estimates. This method also tends to reduce errors caused by variability in height growth, one of the major sources of error in height-weight methods.
Another advantage is that, while the estimate of utilization is based on forage removed, each grazed-class shows both the degree of use and the amount of herbage remaining.
A disadvantage however is that in poor growth years when plants do not mature, the guides will not distinguish between use and no-growth.
It can also be difficult to develop photo guides if they are not available and it is hard to take good photos of “average” plants. However, a guide that is properly developed for a given species and a typical site, can usually be appled to all sites over a fairly broad area in good and bad production years without serious error.
Minimal training of observers is needed to use this method. It is important that observers are able to identify the plant species. Error and bias will decrease dramatically with the more on-the-ground experience the observer has. The biggest difference between experienced and inexperienced observers is that inexperienced observers tend to underestimate the use on more heavily grazed plants.
The following are sources of existing photo guides:
Experiment Station, Tucson Arizona 85721. Bulletin A-73.
Moscow, Idaho 83843. Station bulletin 54.
Management. Extension Service. July 1988.
Graminoids. USFS GTR-308.