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Factors Affecting Function of Induced Corpora Lutea in Postpartum Anestrous Ewes¹

West Virginia University, Morgantown 26506

Abstract

Two experiments were conducted with multiparous, anestrous ewes of mixed breeding that lambed in the autumn. Ewes in each experiment were assigned randomly to treatments in a 2 x 2 factorial arrangement. Ewes in groups 1 and 2 received flurogestone acetate from an intravaginal pessary (containing 20 mg) during days 17 to 21 postpartum, while ewes in groups 3 and 4 received no progestogen. Lambs of ewes in groups 2 and 4 were weaned on day 20 postpartum. All ewes received 150 µg of gonadotropin releasing hormone (GnRH) in a single IM injection on the morning of day 22 postpartum. In Exp. 1, jugular blood was collected just before GnRH and at 30-min intervals over the next 3 hr to monitor release of luteinizing hormone (LH). Jugular blood was collected from each ewe in Exp. 2 just before GnRH and daily for the next 16 days for determination of progesterone. Seven days after treatment with GnRH, all ewes were laparotomized, ovaries were observed and uterine blood samples were collected for assay of prostaglandin F₂ (PGF₂). Ewes in Exp. 2 were checked for estrus at 12-hr intervals beginning the morning after GnRH and were bred when found in standing estrus. Mean concentrations of LH (nanograms/milliliter) during the 3-hr test period were higher (P<.05) in progestogen-treated (59.2) than in control (39.5) ewes. Pattern of release of LH after GnRH was altered by progestogen treatment (P<.01; progestogen x time interaction) but not by the number of lambs suckling before or after day 20 postpartum. Profiles of progesterone over time after GnRH varied with progestogen treatment (P<.01), but not with number of lambs suckling before day 20 or weaning on day 20. Mean concentrations of progesterone (nanograms/milliliter) varied (P<.05) with the number of lambs suckling before day 20 (2.3, 1.6 and .6 for ewes suckling one, two and three lambs, respectively). All ewes had corpora lutea (mean = 2.6) at laparotomy. The life of induced corpora lutea in Exp. 2 averaged 11.8 days and did not vary significantly with treatment. Ewes showed estrus and were mated an average of 17.7 days after GnRH, and 82% lambed to this mating. Concentrations of PGF₂ were lower (P<.05) in ewes receiving progestogen than in those not receiving progestogen (1.7 vs 4.2 ng/ml), but were not affected by number of lambs suckling before or after day 20 postpartum. Weaning of lambs and treatment for 4 days with progestogen did not improve life span or function of corpora lutea induced by GnRH. Variation in duration of luteal function after GnRH could not be explained by the pattern of release of LH after GnRH or by concentrations of PGF₂ in uterine venous plasma 7 days after GnRH.

¹ Published with the approval of the Director of the West Virginia Agr. Exp. Sta. as Scientific Paper No. 1644 from the Division of Anim. and Vet. Sci. Supported by Hatch Project 224 (NE-72). Synchro-Mate pessaries containing flurogestone acetate were provided by G. D. Searle and Co. and GnRH by Abbott Laboratories.

² Present address: Animal Science Institute, Beltsville Agricultural Research Center, Beltsville, MD 20705.

³ Present address: Dept. of Anim. Sci., Univ. of Natal, Pietermaritzburg, Natal, South Africa.

⁴ Dept. of Obstet. and Gynecol.