The control of fertility in sheep: endocrine and ovarian responses to progestagen-PMSG treatment in the breeding season and in anoestrus

Summary Seventy-five maiden Border Leicester × Merino ewes were divided into three treatment groups of 25: 1N, injected with PMSG on day 13 of a natural oestrous cycle in autumn (May); 1S, injected with PMSG on withdrawal of a progestagen sponge inserted for 13 days in autumn; 2S, treated as 1S but in spring (November). Five ewes in each treatment received each of 0, 200, 400, 800 and 1600 i.u. PMSG (dose, D). From 16 h after PMSG injection until oestrus, detected by entire rams, all ewes were bled from the jugular vein at 4 h intervals and plasma oestradiol-17β (E2) concentrations measured by radioimmunoassay. Time to onset of oestrus was recorded. Samples for estimation of plasma progesterone were taken on days 6 and 13 of the subsequent oestrous cycle. The ewes were laparotomized on day 5 or 6 of the cycle and all active corpora lutea, cystic follicles > 8 mm (most of which were luteinized) and follicles > 5 mm in diameter were recorded; the sum of these was taken as the total follicular stimulation at oestrus. There were significant (P < 0·001) linear dose-responses to PMSG in terms of corpora lutea, total follicular development, progesterone, and E2. The covariance of E2 on total follicles was significant (P < 0·001). Progesterone concentrations were more highly correlated with the sum of corpora lutea and cystic follicles than with corporalutea alone (τ = 0·80 ν 0·73, both with 73 D.F.; P < 0·001). The time to onset of oestrus for treatment 1N ewes was highly variable due to the relative asynchrony of the time of PMSG injection but, when treatments 1S and 2S were analysed together, there were significant linear (P < 0·001) and quadratic (P < 0·01) effects of dose of PMSG, resulting in a decrease of 14 h in time to onset between the 0 and 1600 i.u. treatments. The covariance with E2 was significant (P < 0·05). There was no difference between treatments in total follicular development but the number of corpora lutea was less (P < 0·05) in treatment 1N ewes than in the two sponge treatments (1S and 2S), again reflecting the asynchrony of treatment. There was no difference in progesterone concentration between treatments, but peak E2 was significantly less (P < 0·05) in treatment 2S (spring) than in 1N and 1S (autumn). Since there was no difference in the numbers of follicles which developed it appears that follicles in spring-treated ewes secreted oestrogen less intensively than in autumn. The lower oestrogen concentration was associated with a longer interval to onset of oestrus; linear regressions of time on dose PMSG (reciprocal) for treatments 1S and 2S separately had a common slope but significantly different intercepts (P < 0·01), so that oestrus was delayed by a mean of 6·4 h at all PMSG doses in spring compared with autumn. Of the 73 ewes alive at lambing, 48 (66%) lambed producing 69 lambs. There was no significant effect of treatment on percentage of ewes which lambed but the lambing rate of those ewes with large numbers of ovulations (7–14) tended to be lower than that of the remainder (P ≃ 0·1). Of the nine ewes in this category, only four lambed and failure to lamb was significantly associated with excessively high plasma E2 (P < 0·05).