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Journal of Dairy Science Vol. 76 No. 9 2536-2547
© 1993 by American Dairy Science Association ®
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Postinsemination Administration of Receptal: Follicular Dynamics, Duration of Cycle, Hormonal Responses, and Pregnancy Rates

Jeffrey S. Stevenson 1, Arun P. Phatak 1, Immo Rettmer 1, and Robert E. Stewart 1

1 Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201

In Experiment 1, concentrations of LH, FSH, and progesterone, but not estradiol-17ß, in blood serum were increased during 6 to 12 h after injection of 8 µg of receptal (GnRH agonist) administered on d 11 to 14 after estrus (d 0) and at first AI compared with saline treatment in lactating Holstein cows. Beginning 2 to 3 d after injection of receptal, concentrations of progesterone were increased for 3 d in nonpregnant cows and for 12 d in pregnant cows compared with controls of similar pregnancy status. Number of ovarian follicles determined by ultrasonography during 10 d after receptal was reduced, specifically those with antral diameters of ge10 mm. The dominant follicle in both groups began to decrease in diameter on the day following treatment, but the next dominant follicle began to increase in diameter 2.3 ± .7 d later in receptal-treated cows than in controls, accounting for an increase in cycle duration of 2.5 ± .8 d. In Experiment 2, a double-blinded study was conducted in eight herds in which cows (n = 1013) were AI at first detected estrus after 50 d postpartum and assigned randomly to receive either saline or 4, 8, or 12 µg of receptal on d 11 to 14 after first AI. Pregnancy rates were improved in one herd at all doses of receptal, but dose responses were inconsistent in remaining herds. A greater proportion of cows given receptal than controls returned to estrus after 24 d. Administration of a potent GnRH agonist altered number and distribution of ovarian follicles, increased cycle duration, and increased concentrations of progesterone without a consistent increase in pregnancy rates.

Key Words: cattle • gonadotropin-releasing hormone agonist • pregnancy rates • hormones

Submitted on August 10, 1992
Accepted on November 9, 1992




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