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Determination of live and dead herbage fractions from mixed herbage samples requires hand separation or specialized laboratory procedures. The constituent differential method is designed to determine the relative proportion of live and dead components in a mixture based on the difference in dry matter concentration between the components. Our objective was to evaluate several characteristics of the constituent differential method under field conditions in tallgrass and mixed grass vegetation. Estimation of live standing crop by this method is most sensitive to the dry matter content of the total mixture and the dead component but becomes less sensitive as the difference between these variables increases. Time-of-day was not usually associated with dry matter content of the herbage components if sampling began after the herbage was thoroughly dry to the touch. Suggested sample sizes in large experimental units for estimating dry matter content are 40-50 samples for herbage mixtures, 10 samples for live herbage, and 5 samples for dead herbage. In 4 field trials the average value for percent live herbage determined by hand separation and the constituent differential method differed by 1.6 percentage units, which was nonsignificant P>0.10). The constituent differential method is a relatively rapid and accurate method for determining live and dead herbage fractions. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020