Effect of ingestion of oestrogenic clover on luteinizing hormone release in the ovariectomized ewe

Luteinizing hormone (LH) concentration was measured in the jugular plasma of six groups of ovariectomized ewes which were fed oestrogenic clover (Trifolium subterranean L. Gray cv. Yarloop) for 3 days. Ewes in groups B, D and F received an injection of oestradiol-17beta (E2) 24 h after first being offered clover, whilst ewes in groups A, C and E received no oestrogen treatment. Following the initial ingestion of clover, plasma LH concentrations for all groups fell within 3 h and then increased significantly. Each subsequent feeding of clover usually resulted in a temporary decrease and then an elevation of the LH concentrations. When ewes in groups B, D and F were injected with E2, LH concentrations fell and remained low for approximately 9 h, then increased to a peak of 72-3 +/- 10-4 ng/ml within 5 h. The LH concentrations remained elevated for a period of about 12 h. The present paper indicates that the ingestion of oestrogenic clover by ovariectomized ewes results in a depression and then a release of LH, and that this release does not appear to deplete the pituitary of LH or cause refractoriness to an oestrogenic stimulus.