Effect of defoliation treatments on forage quality, quantity, and species composition of a Lolium perenne (L.) - Trifolium subterraneum (L.) pasture

Graduation date: 1984As part of investigations to adapt a suitable grazing system in\ud western Oregon, this research was designed to monitor the effects of\ud defoliation treatments on dry matter production, forage quality,\ud regrowth after defoliation, and species composition of a Lolium\ud perenne Trifolium subterraneum pasture for three consecutive\ud growing seasons from 1980 to 1982. Defoliation treatments were three\ud replications of all possible combinations of four defoliation intervals\ud (clipped every 7, 21, 35 or 49 days) and three stubble heights\ud (70, 55 or 40 mm of stubble remaining). A rear bagging rotary\ud mower was used to defoliate the plots and to collect the phytomass at\ud the assigned dates and heights. Grab samples of the forage produced\ud from different treatments were analyzed to determine their digestibility\ud and crude protein content. Leaf Area Index of the herbage\ud produced and the chlorophyll content of the remaining stubble after\ud defoliation were measured in the first seven weeks of 1981 and 1982\ud growing seasons. Species composition and canopy cover were estimated\ud prior to defoliation treatments each year. Plant density, basal area\ud and root phytomass of perennial ryegrass plants were determined at\ud the completion of the experiment. Collected data were analyzed as a\ud split plot in time. Data within each year were a factorial arrangement\ud of treatments in a randomized complete block design.\ud Defoliation treatments affected dry matter production and the\ud rate of regrowth after defoliation. Total dry matter production in\ud all three years increased as defoliation interval increased from one\ud to seven weeks. Effects of stubble height on dry matter production\ud were probably dictated by forage plant type and pasture species\ud composition. Erect growing ryegrass, which was the major\ud pasture species in 1980, produced more dry matter with lax\ud defoliation; while the more prostrate subclover, which dominated the\ud pasture in 1982, yielded more with close defoliation. Greater daily\ud forage production per unit of land (herbage accumulation rate) on\ud less frequently defoliated plots was associated with increases in\ud leaf area index with time since defoliation. The stubble quality, as\ud evaluated by its chlorophyll content and leaf area index, was\ud lower in plots defoliated less frequently. However, lower initial\ud photosynthetic capacity of the stubble on such plots was less important\ud in determining total forage production than was the higher leaf\ud area accumulated on plots which were defoliated less frequently. Defoliation treatments were also effective in changing forage\ud quality. In vitro digestibility and crude protein content of the\ud forage decreased as the period and intensity of defoliation\ud increased. Forage quality, however, was generally adequate on all\ud treatments to meet the needs of most classes of livestock.\ud Finally, defoliation treatments had some effects on pasture\ud species composition and perennial ryegrass persistence. Perennial\ud ryegrass was replaced by subclover in all of the defoliated plots\ud with time over the trial. Both of these plants, however, disappeared\ud and were replaced by annual grasses in nondefoliated control plots.\ud Plots defoliated every one or seven weeks, as compared to three or\ud five weeks, had fewer ryegrass plants at the end of the experiment.\ud Root phytomass of perennial ryegrass also decreased as defoliation\ud frequency increased. The basal area of this plant, however, was\ud similar for all defoliation treatments