Effect of sheep grazing on subterranean clover (Trifolium subterraneum L.) seed production

Graduation date: 1982Subterranean clover (Trifolium subterraneum L.) seed production\ud practices in Oregon are different from those in Australia.\ud There, seed is produced on fields which are managed primarily for\ud forage. In Oregon, seed fields are managed for seed as the primary\ud product and forage is of secondary concern.\ud Some sheep grazing is necessary to prevent low seed yields\ud due to excessive forage growth. However, no information is\ud available describing how grazing can be used to optimize seed yields.\ud The purpose of this study was to determine the effects of\ud defoliation on the seed yield of subclover and to suggest alternative\ud management practices, if necessary, to improve seed yields.\ud Moderate grazing applied during flowering and continued until\ud early bur fill, increased seed yields 51 and 27% in 1979 and 1980.\ud Grazing treatments shorter in duration increased seed yields, but\ud not as much. A short-duration grazing treatment prior to flowering\ud reduced seed yields 13% in 1980. Seed yield increases were due to\ud an increase in the number of seeds produced (r = 0.99 in both\ud years).\ud The effect of grazing was to modify the canopy to allow for an\ud increase in plant growth during the later stages of reproduction.\ud However, the plants needed to be allowed to recover fully from the\ud grazing treatments to allow for adequate seed fill. Plants which\ud were mechanically defoliated did not fully recover and showed seed\ud size reductions.\ud Defoliation, position of leaves within the canopy, phenological\ud stage of development, and the conditions under which the plants are\ud grown all affect the specific leaf weight of the leaves. This introduces\ud variation which reduces the predictability of leaf area from\ud leaf dry weight. Care should be given to selecting the samples of\ud leaves which are used to make leaf area predictions