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Alteration of the GnRH-Induced LH Release by Steroids in Postpartum Dairy Cattle^1,2,3,

University of Missouri, Columbia 65211

Abstract

The effect of elevated plasma concentrations of estradiol-17ß (E₂ß), estrone (E₁) and progesterone (P), in concentrations similar to those observed at the end of pregnancy, on the gonadotropin releasing hormone (GnRH)-induced luteinizing hormone (LH) release in postpartum dairy cows was studied. Twenty-five dairy cows in late gestation were assigned to five groups of five each to receive daily steroid treatments as follows: 1 and 2) no exogenous steroids; 3) 20 mg E₂ß and 30 mg E₁; 4) 150 mg P and 5) 20 mg E₂ß + 30 mg E₁ + 150 mg P. Steroids were dissolved in alcohol (vehicle) and injected sc twice daily. Cows receiving no steroids were given vehicle. Administration of steroids or vehicle began immediately after parturition (d 0) and continued for 7 d to maintain concentrations of steroids in plasma similar to prepartum concentrations. Cows in groups 2 through 5 received an injection of 100 µg GnRH on d 2, 8, 16, 24 and 32 postpartum, while those in group 1 received water (vehicle for GnRH) on the same days. Plasma for hormonal determinations was collected on alternate days beginning 10 d before the expected day of parturition, daily through the period of steroid treatments (d 0 to 6, postpartum) and on alternate days thereafter until d 40 postpartum. In addition, plasma was collected immediately before GnRH or water administration and at .5 h intervals thereafter for 4 h. Trends in response to treatment over days postpartum were studied by partitioning sums of squares due to linear, quadratic and cubic polynomial responses. Injection of exogenous steroids increased plasma E₁, E₂ß and P to concentrations similar to those observed prepartum. Mean steroid concentrations remained elevated for 7 to 10 d after cessation of treatment. Administration of GnRH resulted in a greater release (P<.05) of LH in all groups (2 through 5) at each injection time compared with cows given water (group 1). The LH release in response to GnRH increased (P<.01) linearly with time postpartum in groups 2, 4 and 5, but the amount of LH released and the rate of increase was less (P<.05) for groups 4 and 5. In contrast, the GnRH-induced LH release for group 3 was characterized by a quadratic response (P<.05). In this group, the LH release was elevated on d 2 postpartum and decreased on d 8 and 16 postpartum before increasing rapidly on d 24 and 32. Release of LH after GnRH in progesterone-treated cows was characterized by a decreasing linear response. The LH response to GnRH in cows given progesterone in combination with estrogen was similar to that observed in cows given only progesterone; progesterone abolished the quadratic response to estrogen.

¹ The authors wish to acknowledge Mr. L. Croy; M. Nienhueser, herdsmen and Mr. Ken Bolte, Farm Manager, for assistance in care of experimental animals.

² Approved by the Director. Journal Paper No. 8601. Missouri Agr. Exp. Sta., Univ. of Missouri.

³ This research was conducted as part of regional research project NC-113, Methods for Improvement of Fertility in Cows Postpartum.

⁴ Dept. of Dairy Sci.