Livestock grazing of cereal grain crops during the early portion of their growing cycle is practiced to some degree in many parts of the world. While a substantial body of literature relating grain yields to defoliation practices has been accumulated, most of this work lacks detail concerning the effects of grazing on individual compo· nents of plant yield. Reports of grazing effects on grain yield have been quite variable, ranging from substantial decreases to modest increases in grain yield. Understandably, many grain producers are confused about the exact impact that livestock grazing has on their grain crop. The effects of a single intense March-grazing by sheep, on the components of biomass yield in 'Yamhill' soft white winter wheat (Triticum aestivum L. em Theil.) were evaluated near Corvallis, OR, during the 1979-to-1981 harvest years. Grazing increased total plant biomass yield each year. The greatest increase was in the grain component, which averaged 16% greater on grazed than on ungrazed plots ·during the 3 yr of the study. Contrary to widely held opinion, grazing had no effect on the number of fertile flower heads produced by the crop. Likewise, grazing did not affect either the number of seeds produced per spikelet or the average seed weight. Increased grain yield on grazed plots largely resulted from an increased number of fertile spike lets produced per wheat head on those plots. These data suggest that components of wheat grain yield that are determined prior to tiller elongation may respond to grazing management.

Both quantity and quality of pasture forage produced generally varies with frequency and intensity of plant defoliation. However, intensity and frequency of defoliation have rarely been evaluated simultaneously. The objective of this study was to quantify forage quality response to simultaneous changes in defoliation treatments over a range of values likely to occur in short-duration graxing systems. Effects of defoliation treatments on forage digestibility (DMD), crude protein content (CPC), crude protein yield, and digestible dry matter yield were evaluated on a perennial ryegrass (Lolium perenne, L.)-subclover (Trgolium subterraneum L.) hill land pasture growing on a Ultic Hapioxeroll soil near Corvallis, Oregon. Treatments consisted of all possfble combinations of 4 defoliation intervals (clipped every 7, 21, 35, or 49 days) and 3 stubble heights (High-70, Medium-55, or Low-40 mm of stubble remaining after defoliation) applied during the 1980, 1981, and 1982 growing seasons. Digestible dry matter yield increased with increasing defoliation interval. With the exception of DMD in 1980, both digestibility and CPC of the forage produced decreased linearly as the period between defoliation events Increased. Crude protein content increased linearly as stubble height Increased, while forage digestibility was comparatively insensitive to changes in stubble height. Forage quality was generally adequate on all treatments to meet the needs of most classes of livestock.

While many authors have reported the effects of either varying defoliation intensity or of varying defoliation interval on pasture drymatter production, both intensity and frequency of defoliation have rarely been simultaneously applied over a range of values in such a way that their interaction can be evaluated. The objective of this study was to quantify the response of pasture dry-matter yield to simultaneous changes in defoliation intensity and frequency over a range of values. Effects of defoliation treatments on dry matter production were evaluated on a perennial ryegrass (Lolium perenne L.)subclover (Trifolium subterraneum L.) hill land pasture growing on a Ultic liaploxeroll soil near Corvallis, OR. A combination of four defoliation intervals (clipped every 7, 21, 35, or 49 days) and three stubble heights (70, 55, and 40 mm of stubble remaining after defoliation) were applied during the 1980, 1981, and 1982 growing seasons, respectively. Total annual dry matter production in all3 yr increased as defoliation interval increased from 7 to 49 days between defoliation events. Effect of stubble height on dry matter yield was apparently influenced by forage plant type and sward species composition. When the pasture was dominated by erect-growing ryegrass in 1980, more dry matter was produced when 70 mm of stubble remained after defoliation; but when the more prostrate subclover dominated the sward in 1982, it yielded more with 40 mm of stubble. The results of this experiment suggest that short-duration grazing systems, which provide long periods of nonuse for pasture regrowth, shOuld result in more dry matter production than does continuous grazing.

Three methods of calculating forage disappearance from forage standing crop present on mowed versus protected plots were compared to the actual amount of forage harvested from mowed plots. The method most widely used by range scientists, the difference method, displayed a marked tendency to overestimate forage disappearance during periods of rapid plant growth or when plots were protected for more than 3 weeks. More accurate estimates of forage disappearance were generally obtained using formtdae suggested by Linehan et al. (1947) and Bosch (1956) than could be obtained by the difference method.

Effects of defoliation frequency and intensity on persistence of a perennial ryegrass (Loliumperenne)-subclover (TrYohm 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 j-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.