title: Effect of a gonadotropin-releasing hormone agonist on follicle recruitment and pregnancy rate in cattle.
abstract: 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.
authors.affiliation: Department of Dairy and Poultry Sciences, University of Florida, Gainesville 32611-0920, USA.
authors.forename: E J
authors.initials: EJ
authors.lastname: Schmitt
authors.forename: M
authors.initials: M
authors.lastname: Drost
authors.forename: T
authors.initials: T
authors.lastname: Diaz
authors.forename: C
authors.initials: C
authors.lastname: Roomes
authors.forename: W W
authors.initials: WW
authors.lastname: Thatcher
mesh_terms: D000818:Animals; D002417:Cattle / Q000097:blood / Q000502:physiology*; D015237:Dinoprost / Q000008:administration & dosage / Q000494:pharmacology*; D004343:Drug Implants; D005260:Female; D005298:Fertility / Q000187:drug effects / Q000502:physiology; D007987:Gonadotropin-Releasing Hormone / Q000819:agonists*; D006080:Ovarian Follicle / Q000187:drug effects / Q000502:physiology*; D010060:Ovulation / Q000187:drug effects / Q000502:physiology; D011247:Pregnancy; D018873:Pregnancy Rate; D011282:Pregnenediones / Q000008:administration & dosage / Q000494:pharmacology*; D011374:Progesterone / Q000097:blood
chemical_list: D004343:Drug Implants; D011282:Pregnenediones; D007987:Gonadotropin-Releasing Hormone; C100284:norgestomet; D011374:Progesterone; D015237:Dinoprost