Hypothalamic-pituitary function in normal ewes and ewes which grazed oestrogenic subterranean clover for several years

Plasma luteinizing hormone (LH) concentrations were measure in normal Merino ewes and in Merino ewes with lowered fertility which had resulted from prolonged grazing of Trifolium subterraneum L. cv. Dinninup. During the anoestrous season, LH was measured at frequent intervals before and following administration of oestradiol-17 beta or gonadotropin releasing hormone (GnRH), and around the time of spontaneous oestrus. All ewes responded to GnRH treatment and there were no differences between the two groups in terms of the amount of LH released or the time to reach maximal plasma hormone concentration. Comparable proportions of ewes from both groups responded to oestradiol-17 beta treatment with LH levels showing initially a negative feedback response (i.e. suppression to approximately 80% of pretreatment levels) followed by positive feedback response. During the positive feedback phase, the quantity of LH which was released was comparable in both groups; however, the time interval over which this hormone was secreted was significantly shorter for clover-infertile ewes (P less than 0.05). During the anoestrous season the mean LH level was lower in normal ewes than in clover-infertile ewes [0.45 +/- 0.41 (n = 32) versus 0.82 +/- 0.73 (n = 48 ng/ml, P less than 0.01]. In the oestrous season the mean LH level for normal ewes rose to 1.23 +/- 0.65 ng/ml (n = 76) but there was no change for the clover-infertile ewes (0.77 +/- 0.59 ng/ml, n = 73). These experiments indicate that in the clover-infertile ewe LH concentration remains static throughout the breeding and non-breeding seasons but the results suggest that this altered endocrine status in not due to any differing level of sensitivity to negative feedback effects of oestradiol-17 beta as compared with normal ewes.