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Reproductive Responses, Progesterone Profiles and Ovarian Characteristics of Cycling, Fine-Wool Ewes Treated with an Inhibitor of Progesterone Synthesis (Epostane)¹

New Mexico State University², Las Cruces 88003

Abstract

In Exp. 1, 45 fine-wool ewes received (sc) either 0, 50 or 100 mg epostane (3-β-hydroxysteroid dehydrogenase inhibitor) to examine effects on return to estrus and conception rates. Treatments were imposed on d 10 of an estrous cycle (estrus = d 0) and jugular blood samples were collected once daily on d 8 and 9, twice daily on d 10 through 13 and once daily from d 14 until first post-treatment estrus or d 20 (15 ewes/treatment). Intensive samples were obtained hourly for 6 h after treatment (d 10; five ewes/treatment). Serum progesterone (P₄) before treatment was similar among groups; but by 2 h after treatment, epostane-treated ewes had lower (P<.10) values than controls. By 6 h post-treatment, serum P₄ in ewes receiving 50 (1.0 ng/ml) and 100 (.9 ng/ml) mg epostane was well below control values (3.2 ng/ml). By 7 d after treatment, 93.3% of ewes treated with 50 mg epostane had recycled compared with 66.7 and 33.3% of those receiving 100 and 0 mg, respectively (P<.10). Similarly, 93.3, 53.3 and 26.7% of ewes receiving 50, 100 or 0 mg epostane, respectively, lambed to breeding within 7 d of treatment (P<.05). Overall conception rates during a 34-d breeding season were similar among groups. Preweaning performance of offspring did not differ among maternal treatments. A single injection of epostane lowered serum P₄, but P₄ levels were not maintained low enough and(or) long enough to allow ewes to return to estrus immediately after treatment. Experiment 2 attempted to determine if two 50-mg epostane injections could successfully hasten onset of estrus and(or) influence follicular growth. Ten fine-wool ewes received two injections at 12-h intervals containing either 0 or 50 mg epostane. Treatments were administered on d 10 (1800) and 11 (0600) of an estrous cycle. Blood samples were collected twice daily (0600 and 1800) from d 10 to onset of estrus or d 20. Intensive samples were obtained before and after second treatment (d 11, 0600) at 30-min intervals for 4 h and hourly for 8 h. Mid-ventral laparotomies were performed on all ewes on d 13. Serum P₄ was similar before treatment among groups, but by 12 h after first injection, P₄ was lower (P<.01) in ewes treated with epostane. A second epostane injection did not further reduce P₄ at 2, 6 or 12 h after treatment. Progesterone remained lower (P<.05) in treated ewes for 84 h after first treatment. Epostane did not alter number of corpora lutea or follicles and did not reduce the interval from treatment to onset of estrus compared with that observed in controls. Reduction of serum P₄ induced by epostane administration does not appear to allow early return to estrus or enhance follicular growth in cycling ewes.

¹ Journal article 1190 of the New Mexico Agr. Exp. Sta., Las Cruces.

² Dept. of Anim. and Range Sci. Appreciation is expressed to Sterling Winthrop Research Institute (Rensselear, NY) for the supply of epostane and to Dr. G. D. Niswender (Dept. of Physiol. and Biophys., Colorado State Univ., Fort Collins) for the progesterone-11 BSA antiserum (GDN 337) used in progesterone determinations. We also thank D. Sanson, S. O'Neill, M. Neave, C. Roberts and M. Hallford for their assistance. Direct reprint requests to D. M. Hallford.