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Effects of defoliation frequency and intensity on persistence of a perennial ryegrass (Lolium perenne)-subclover (Trifolium subterraneum) pasture were studied near Corvallis, Ore., during 1980 to 1983. While canopy cover of individual plant species did not differ between defoliation frequency or intensity treatments (P6.05) within years, all defoliated plots differed from the undefoliated control plots. Canopy cover of perennial ryegrass declined from its initial value of 43% in 1980 to an average of 30% and 1% in defoliated and undefoliated plots in 1983, respectively. Perennial ryegrass in defoliated plots was apparently replaced by subclover, whose canopy cover increased by 33% during the 3-year period. On undefoliated plots, however, both perennial ryegrass and subclover were replaced by annual grasses, whose canopy increased from approximately 2% in 1980 to 48% in 1983. Density of perennial ryegrass plants at the end of the trial in 1983 was highest when plots were defoliated once every 21 or 35 days compared to those defoliated every 7 days, 49 days, or the undefoliated control plots. Root biomass per plant, however, increased linearly as defoliation interval increased from 7 to 49 days between defoliation events. Viewed together, these data suggest that underutilization of grass-clover pastures may be potentially as damaging to pasture persistance as overutilization. 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