Grazed Plant Method

The grazed plant method was developed by Roach in southern Arizona during the early 1950's to determine utilization. This method involves initially developing a regression relationship between the proportion of plants grazed (irrespective of the degree of grazing) and utilization levels, either by visiting sites with different grazing intensities or by repeatedly sampling the same site as grazing progresses through the year. The relationship can concentrate on key species or include all forage species, depending on the objectives of the inventory or monitoring program. Another method to determine utilization must be adopted to provide the data used to describe the relationship between the proportion of plants grazed and utilization levels.

Field sampling is a rapid procedure following the establishment of the regression relationship. Data is collected from pace transects, by classifying the plant closest to the toe at each sampling point as grazed or ungrazed. Once the proportion of grazed plants is calculated, utilization levels are estimated by referring to the regression relationship.

The effectiveness of the grazed plant method depends on grazing patterns of livestock within the management unit. In situations where plants are grazed only once, a value of R2>0.90 for the regression relationship indicates that the proportion of grazed plants is a strong prediction of utilization. However, further increases in utilization cannot be detected once all plants have been grazed, even if only lightly. The method is also insensitive when livestock tend to regraze the same plant, rather than uniformly grazing all vegetation. The regression relationship is also specific to the population where the data is collected, and caution must be taken when attempting to apply it to other years, sites, or different types of livestock.

References and Further Reading

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp. 298-299.

Cook, C.W., and J. Stubbendieck. (eds.). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp. 127-129.

Gierisch, R.K. 1967. An adaption of the grazed plant method for estimating utilization of thurber fescue. Journal of Range Management 20:108-111.

Hurd, R.M., and N.A. Kissinger. 1953. Estimating utilization of Idaho fescue (Festuca idahoensis) on cattle range by percent of plants grazed. Rocky Mountain Forest and Range Experiment Station. Paper No. 12. pp. 1-5.

Roach, M.E. 1950. Estimating perennial grass utilization on semi-desert cattle range by percentage of ungrazed plants. Journal of Range Management 3:182-185.

Springfield, H.W. 1961. The grazed plant method for judging the utilization of crested wheatgrass. Journal of Forestry 59:666-670.