Seasonal Changes in LH Secretion in Normal Ewes and Ewes which Grazed Oestrogenic Clover

Abstract

Plasma luteinizing hormone (LH) concentrations were measured in normal (control) Corriedale x Merino (comeback) ewes and in clover-infertile comeback ewes which had grazed oestrogenic Yarloop clover (Trifolium subterraneum L. cv. Yarloop) for more than 4 years. Plasma LH concentrations were measured in samples taken at 20-min intervals for 6 h during the dioestrous stage of the oestrous cycle in the breeding season (BS) and during the anoestrous season (AS). In the control ewes during BS, transitory elevation in plasma LH concentration (pulses) occurred, reflecting secretory episodes, with a frequency of one per 5 ·2 h. This frequency fell to one per 16·5 h during the anoestrous season. In clover-infertile ewes, LH pulses occurred with a frequency of one per 4·5 h during BS and one per 4·9 h during AS (difference not significant).