Postinsemination administration of receptal: follicular dynamics, duration of cycle, hormonal responses, and pregnancy rates
1993
Stevenson, J.S. | Phatak, A.P. | Rettmer, I. | Stewart, R.E.
In Experiment 1, concentrations of LH, FSH, and progesterone, but not estradiol-17 beta, in blood serum were increased during 6 to 12 h after injection of 8 microgram 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 greater than or equal to 10 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 microgram of receptal on d 11 to 14 after first AI. Pregnancy rates were improved in one herd at all doses of receptal, but dose response 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 consitent increase in pregnancy rates.
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