Utilization – Grazed Class Method

written by Karen Colson and Jason Karl

Description and Uses

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.

Advantages and Limitations

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 applied 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.

Manuals

Utilization Studies and Residual Measurements (Technical Reference 1734-3) http://www.blm.gov/nstc/library/pdf/utilstudies.pdf

Technical and Application References

Schmutz, Ervin M. 1971. Estimation of range use with grazed-class photo guides. Coop. Extension Service and Agr. Exp. Sta., Univ. of Ariz., Tucson, AZ, Bull. A-73. 16 p.
Schmutz, E.M., G.A. Holt, and C.C. Michaels. 1963. Grazed class method of estimating forage utilization. Journal of Range Management. 16:54-60.

The following are sources of existing photo guides:

University of Arizona, Cooperative Extension Service and Agricultural Experiment Station, Tucson Arizona 85721. Bulletin A-73.
University of Idaho, College of Forestry, Wildlife and Range Science, Moscow, Idaho 83843. Station bulletin 54.
Montana State University. Forage Use – A Tool for Planning Range Management. Extension Service. July 1988.
Kinney and Clary. 1994. A Photographic Utilization Guide for Key Riparian Graminoids. USFS GTR-308.

Similar Approaches

Related utilization and residue measurement methods include: