Constancy of interval between luteinizing hormone release and ovulation in the ewe

To investigate the time relationships between the preovulatory LH peak and ovulation in the ewe, a "short" radioimmunoassay for measuring plasma LH concentration was developed. This short assay permitted the determination of LH concentration within 6 h of sampling. The interval from LH peak to ovulation was then defined by a single laparotomy on each ewe, at a known interval after the start of the LH surge to determine for each interval the proportion of ewes which had ovulated. Using this method, the time between LH peak and ovulation was investigated in 47 ewes experiencing normal estrous cycles, in 11 young and 16 old ewes experiencing their second estrus after withdrawal of Cronolone-impregnated sponges, and in 19 ewes experiencing their first estrus after withdrawal of such sponges. The results indicated that normally cycling ewes ovulated between 21-26 h after the LII peak whilst the young and old ewes and the Cronolone-treated ewes ovulated 22-26 h after the LH surge. At 23 h, 16.6% of ewes from all four groups had ovulated whereas by 24 h, 66.6% of ewes had ovulated, indicating that a high proportion of ewes ovulated between 23-24 h after the start of the LH peak. It is concluded that the constancy of interval between LH release and ovulation in these experimental situations confirms the importance of the role of LH in the ovulatory mechanism.