Grazed and Ungrazed Comparisons

Utilization can be estimated by comparing biomass from paired grazed and ungrazed sample units, with the difference representing the amount of forage consumed (or otherwise destroyed) during the grazing period. Data is collected at the end of the grazing period, by adopting an appropriate method to determine biomass.

Areas can be protected from grazing by either small or larger exclosures that restricted to key areas of the management unit. Small temporary cages are easily moved at the end of each grazing period to eliminate long term effects of cages. Larger permanent exclosures with special fences are useful to separate the effects of livestock and wildlife, but interpretations suffer from the confounding effects of extended periods of time with protection from grazing.

The advantage of the simple approach of grazed and ungrazed comparisons is that it overcomes a common shortcoming of many other methods to determine utilization, which depend on observer experience to imagine the biomass that has been removed. Several other factors, however, reduce the accuracy and precision of this method. First, different growth rates between grazed and ungrazed plants can compromise the legitimacy of protected plots as indicators of potential forage production. In addition, cages can modify the micro-environment to enhance plant growth within small caged plots. Finally, sample size is usually limited by the expenses associated with constructing and locating cages.

In another approach, utilization can be determined by comparing the biomass of forage before and after grazing, without installing structures to protect areas from livestock. However, this technique is only suited to short grazing periods where plant regrowth after grazing is insignificant. Because of regrowth after grazing, this method of estimating utilization will typically underestimate the actual level of utilization.

References and Further Reading

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Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp. 309-310.

Bureau of Land Management. 1996. Utilization studies and residual measurements. Interagency Technical Reference, BLM/RS/ST-96/004+1730. pp. 70-75.

Cook, C.W., and L.A. Stoddart. 1953. The quandary of utilization and preference. Journal of Range Management 6:329-331. (pdf)

Frischnecht, N.C., P.W. Conrad, and P.E. Hansen. 1970. Improved folding utilization cages. Journal of Range Management 23:215-218.(pdf)

Heady, H.F. 1957. Effect of cages on yield and composition in the California annual type. Journal of Range Management 10:175-177.(pdf)

Hinnant, R.T., and M.M. Kothmann. 1982. Durable livestock exclosure for herbage production and utilization sampling. Journal of Range Management 35:127-128.(pdf)

Owensby, C.E. 1969. Effect of cages on herbage yield in true prairie vegetation. Journal of Range Management 22:131-132.(pdf)

Schulz, T.T., and W.C. Leininger. 1990. Differences in riparian vegetation structure between grazed areas and exclosures. Journal of Range Management 43:295-299.(pdf)