Journal of Animal Science, Vol 74, Issue 1 154-161, Copyright © 1996 by American Society of Animal Science

CLINICAL TRIAL

Effect of a gonadotropin-releasing hormone agonist on follicle recruitment and pregnancy rate in cattle

E. J. Schmitt, M. Drost, T. Diaz, C. Roomes and W. W. Thatcher
Department of Dairy and Poultry Sciences, University of Florida, Gainesville 32611-0920, USA.

Two experiments were conducted to determine whether a GnRH agonist eliminated a potentially persistent first-wave dominant follicle (PDF) and recruited a new dominant follicle with improved fertility upon ovulation. In Exp. 1, five nonlactating Holstein cows were treated on d 7 (d 0 = estrus) with a norgestomet implant and PGF2 alpha (25 mg); a GnRH agonist was injected on d 9. On d 16, the norgestomet implant was removed and PGF2 alpha was injected. The corpus luteum (CL) regressed (5/5 cows), and plasma progesterone (P4) decreased (P < .01) from d 7 (P4 = 10.4 +/- .3) to 9 (P4 = 1.0 +/- .3 ng/mL). The GnRH agonist induced ovulation of the first-wave dominant follicle. New dominant follicles emerged by d 12 +/- 1. In all cows, removal of norgestomet implants and injection of PGF2 alpha on d 16 caused regression (P < .01) of the CL induced by the GnRH agonist. The GnRH agonist-recruited dominant follicles were highly estrogenic on d 17 (estradiol = 19.6 +/- .8 pg/mL) and ovulated on d 19.8 +/- .2 (5/5). In Exp. 2, 147 heifers at a synchronized estrus were assigned disproportionally but randomly to two treatments (GnRHa, n = 94; PDF, n = 53). On d 7, a used controlled internal drug releasing (CIDR-B) device was inserted into the vagina and PGF2 alpha was injected. On d 9, heifers in GnRHa were injected with GnRH agonist. The CIDR-B devices were removed and PGF2 alpha was injected into all heifers on d 16. Within 4 d after removal of CIDR-B devices, 96.8 and 94.3% of heifers in GnRHa and PDF were detected in estrus and inseminated. Pregnancy rates were GnRHa = 60.6% > PDF = 43.4% (P < .05). In summary, fertility after ovulation of a persistent first-wave dominant follicle is reduced, whereas induction of a new dominant follicle following injection of a GnRH agonist results in greater fertility.

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