written by Karen Colson and Jason Karl
Utilization – Quantitative
Description and Uses
The Twig Length Removal Method is a way of measuring utilization of browse where utilization is defined as the amount of plant material removed by livestock or wildlife. According to Utilization Studies and Residual Measurements reference, “With the Twig Length Measurement Method, utilization is determined by measuring twigs on 25 to 50 browse plants after full annual growth has occurred and again after the period of use.” The difference between the before and after measurements estimates the amount of browse that has been used. Plants to sample are selected at fixed intervals along a transect and tagged so they can be relocated. Different transects are selected for different browse species. This technique is most often used on wildlife winter range.
Two important considerations when implementing methods for quantifying utilization are selection of areas that have been utilized and selection of key species for measurement. Factors to take into account for site selection include whether the objective is to sample a critical area or to sample key areas that are representative of a larger landscape. For selecting key species, it is important to consider the forage value of each species, when during the year it is utilized, and the potential for the key species to reflect changes in the whole plant community.
Advantages and Limitations
From the Utilization Studies and Residual Measurements manual:
- Percent utilization determined by measurement is more accurate than utilization determined by ocular observation. This method is useful in determining the amount of use made on browse plants by livestock and the amount of use made on the same browse plants by wildlife, wild horses, and/or wild burros. The degree of direct forage competition among different kinds of animals can be determined where there are discrete periods of use by different animals. Growth and use indexes can also be determined.
- Good utilization estimates can be obtained with this method even though twig volume is not uniformly distributed along the length of twigs. The results will vary with species due to twig growth characteristics.
- The method is not reliable on species that do not clearly exhibit annual twig growth, such as sagebrush and serviceberry. It is also not reliable in areas of the Southwest where annual twig growth may be masked by almost continuous growth or erratic seasonal growth after rains.
- The time and expense needed for gathering data are doubled because the measurements must be made twice a year.
Technical and Application References
- Jensen, Charles H. and George W. Scotter. 1977. A comparison of twiglength and browsed-twig methods of determining browse utilization. J. Range Manage. 30:64-67.
- Ferguson, Robert B. and Michael A. Marsden. 1977. Estimating overwinter bitterbrush utilization from twig diameter-length-weight relations. J. Range Manage. 30:231-236.
- Smith, Arthur D. and Philip J. Urness. 1962. Analyses of the Twig Length Method of determining utilization of browse. Utah State Dept. of Fish & Game. Publication No. 62-9. 35p.
- U.S. Department of Interior Bureau of Land Management. 1984. Rangeland Monitoring – Utilization Studies, TR4400-3.
Other browse removal methods include:
Related utilization and residue measurement methods include:
- Stubble Height
- Paired Plot Method
- Utilization – Grazed Class Method
- Herbaceous Removal-Height-Weight Method
- Herbaceous Removal-Actual Weight Method
- Herbaceous Removal-Ocular Method
- Printable data forms for the Twig Length Measurement Method can be found on pages 30-33 of the Utilization Studies and Residual Measurement Methods Interagency Technical Reference.