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Effect of Melengestrol Acetate on Plasma Progesterone, Luteinizing Hormone and Total Corticoids in Dairy Heifers^{1, 2,}

Purdue University, Lafayette, Indiana 47907

Abstract

Two groups of 10 yearling Holstein heifers respectively were fed 0.5 or 1.0 mg of melengesterol acetate (MGA) daily for 14 days, and 62 hr. later five heifers in each group were injected intramuscularly with 200 µg of estradiol-17ß. Blood was collected on days 0, 7 and 14 of MGA feeding, at 8-hr, intervals from 24 hr. after withdrawal to ovulation and on days 5 and 9 after ovulation. A similar schedule was used for nine controls except collection at 8-hr. intervals started 2 days before predicted estrus. The objective was to evaluate effects of MGA and estradiol-17ß on ovarian function, synchronization of estrus and fertility and levels of luteinizing hormone (LH), progesterone and total corticoids in jugular venous plasma. Estrus was inhibited during the 14 days MGA was fed. Ovulation without estrus was increased by treatment with estradiol-17ß. No treated heifer conceived from mating during the estrus subsequent to withdrawal of MGA as compared to three of four controls mated while on experiment.

Average levels of LH, progesterone and total corticoids, nonsignificantly different between groups fed 0.5 and 1.0 mg of MGA on days 7 and 14 and after withdrawal, were comparable to controls at 7 and 14 days. Plasma LH increased 10 hr. after injection of estradiol-17ß (P<.025). Irrespective of treatment with estradiol-17ß, plasma LH and progesterone increased after withdrawal of MGA. Plasma LH averaged 2.5 times (1.92±11 ng NIH-LH B6/ml) more than controls 5 and 9 days after ovulation. Progesterone increased from 3.8±.7 ng/ml 24 hr. after last feeding MGA to 8.6±.6 ng/ml at 80 hr. and remained high from 88 to 120 hours. These elevated levels persisted after synchronized ovulation, were observed when no CL were palpable following treatment with MGA and averaged 5.4±1.2 ng/ml during the LH surge subsequent to treatment only with 0.5 and 1.0 mg of MGA as compared to 2.2.±6 ng/ml for controls. Persistently high levels of LH and progesterone in heifers subsequent to feeding MGA indicate that the feedback mechanism(s) regulating plasma LH and progesterone were altered by feeding MGA.

¹ Journal Paper No. 4548, Purdue Agricultural Experiment Station. A contribution from Western Regional Project W-95 by the Departments of Animal Sciences and Veterinary Science.

² We are indebted to Dr. L. E. Reichert, Jr., Emory University, Atlanta, Georgia for providing purified bovine LH, to Dr. G. D. Niswender, University of Michigan, Ann Arbor for providing anti-bovine LH antisera and to the Upjohn Company, Kalamazoo, Michigan for supplying melengestrol acetate.

³ Present address: U.S.D.A. Range Livestock Experiment Station, Miles City, Montana 53901.

⁴ Present address: Department of Dairy Husbandry, University of Missouri, Columbia 65201.