Grazed Class Method
The grazed class method has been widely used by land management agencies to determine utilization of key species. This method uses data that is collected along pace transects where the plant closest to the toe at each sampling point is assigned to one of about 6 classes of utilization (e.g., 0%, 10%, 30%, 50%, 70%, 90%). Once the data is collected, utilization is estimated by multiplying the class value by the number of times it was recorded, adding the results for each class, and calculating an average by dividing by the total number of observations in the transect.
The grazed class method recognizes the difficulties in assigning an exact utilization level to each plant. Ranking data into broad classes is also a relatively rapid procedure because observers are not required to spend as much time contemplating utilization levels to the closest percent! In fact, rapid evaluation is the key to success for this approach because a large sample is less sensitive to the occasional incorrect ranking. However, the method is still subject to personal bias, and sampling accuracy and precision will benefit if time is devoted to training observers. Photographic manuals that illustrate the typical appearance of plants at various levels of utilization have been developed to guide utilization rankings for key species of southern Arizona.
References and Further Reading
(Note: pdf files require Adobe Acrobat (free) to view)
Bureau of Land Management. 1996. Utilization studies and residual measurements. Interagency Technical Reference, BLM/RS/ST-96/004+1730. pp. 81-85.
Schmutz, E.M. 1978. Estimating range use with grazed-class photo guides. University of Arizona Cooperative Extension Service and Agricultural Station. Bulletin, A-73. pp. 1-16.
Schmutz, E.M. 1983. Browsed-class method of estimating shrub utilization. Journal of Range Management 36:632-637. (pdf)
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. (pdf)
Sundt, P.L., and M.P. McClaran. 1993. The binomial use monitoring (BUM) method for desert shrubs. Rangelands 15:224-227.