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The control of fertility in sheep endocrine and ovarian responses to progestogen pregnant mare serum gonadotropin treatment in the breeding season and in anestrus



The control of fertility in sheep endocrine and ovarian responses to progestogen pregnant mare serum gonadotropin treatment in the breeding season and in anestrus



Journal of Agricultural Science 94(1): 69-88



Maiden Border Leicester .times. Merino ewes (75) were divided into 3 treatment groups of 25: 1N, injected with PMSG [pregnant mare serum gonadotropin] on day 13 of a natural estrous 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 (Nov.). Five ewes in each treatment received each of 0, 200, 400, 800 and 1600 IU PMSG (dose, D). From 16 h after PMSG injection until estrus, detected by entire rams, all ewes were bled from the jugular vein at 4 h intervals and plasma estradiol-17.beta. (E2) concentrations measured by radioimmunoassay. Time to onset of estrus was recorded. Samples for estimation of plasma progesterone were taken on days 6 and 13 of the subsequent estrous 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 estrus. 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 corpora lutea alone (r = 0.80 vs. 0.73, both with 73 D.F. [degree of freedom]; P < 0.001). The time to onset of estrus 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 analyzed together, there were significant linear (P < 0.001) and quadratic (P < 0.01) effects of PMSG dose, resulting in a decrease of 14 h in time to onset between the 0 and 1600 IU 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 2 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 estrogen less intensively than in autumn. The lower estrogen concentration was associated with a longer interval to onset of estrus; linear regressions of time on PMSG dose (reciprocal) for treatments 1S and 2S separately had a common slope but significantly different intercepts (P < 0.01), so that estrus 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 the 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 .simeq. 0.1). Of the 9 ewes in this category, only 4 lambed, and failure to lamb was significantly associated with excessively high plasma E2 (P < 0.05).

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Accession: 006620639

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The control of fertility in sheep: endocrine and ovarian responses to progestogen-PMSG treatment in the breeding season and in anoestrus. Journal of Agricultural Science, UK 94(1): 69-88, 1980

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