Sheep Grazing Effects on Subterranean Clover Development and Seed Production in Western Oregon

This study was conducted to determine which sheep (Ovis aries) grazing practices could be used to increase seed yields of subterranean clover (Trifolium subterraneum L.) and to study the effects of sheep grazing on plant growth. A further objective was to describe the effects of natural moisture stress in the field on seed production. Four sheep grazing treatments were applied in 1979 and 1980 at a constant stocking rate of 5 and 7.5 ewes/ha, respectively. The location was a hill-land site mapped as a loamy mixed non-acid mesic shallow Typic Xerorthents, loamy mixed mesic shallow Dystic Xerochrept, fine mixed mesic Ultic Haploxeralfs, and fine-loamy over clayey mixed mesic Ultic Haploxeralf. The duration of the grazing treatments was controlled with wire-mesh exclosure cages. Extended grazing applied from before flowering until early bur formation increased seed yields over the ungrazed control by 51 and 27% (234 and 223 kg/ha) in 1979 and 1980, respectively. Two grazing treatments of a shorter duration during this period were not as effective in increasing seed yields. While grazing pressure was being applied, leaf area index, canopy height, and the amount of light absorbed by the canopy was reduced in the extended grazing treatment. Natural variations in soil depth within the experimental area resulted in varying amounts of moisture stress to the crop when rainfall was limited in the 1979 season. Seed yield, seed number, and seed size (g/100) showed correlations of −0.84, −0.80, and −0.84 (P = 0.001), respectively, with increasing moisture stress. Leaf area index, leaf dry weight, and total stem dry weight were reduced equally over the moisture stress levels for both the extended grazing and ungrazed treatments. However, the total amount of dry matter regenerated in the extended grazing treatment was less as moisture stress increased. This left more moisture available for reproductive development. Sheep grazing is essential if maximum seed yields are to be obtained from subterranean clover fields in western Oregon. The optimum grazing period is from just prior to the start of flowering until the time of early bur fill.