Ovarian Traits After Gonadotropin-Releasing Hormone-Induced Ovulation and Subsequent Delay of Induced Luteolysis in an Ovsynch Protocol

Abstract

Our objective was to determine whether delaying the PGF2α injection by 24 or 48h after the first GnRH injection in an Ovsynch protocol (from a standard 7 d) altered ovarian characteristics in lactating dairy cows. Beginning 9 d after removal of a progesterone-releasing controlled internal drug release (CIDR) insert and injection of PGF2α (d 6.4 of the estrous cycle), 36 Holsteins (average body weight=707±12kg and body condition score=2.3±0.1) were administered 100μg of GnRH (81±2 d in milk) and assigned randomly to receive a treatment injection of PGF2α 7, 8, or 9 d later. Timed artificial insemination was performed at 48h after PGF2α at which time a second injection of GnRH was administered. Ovarian structures were mapped by ultrasonography on d 0 (first GnRH injection); on d 2 to determine responses to the first GnRH injection; at PGF2α injection; and daily thereafter through 72h after PGF2α to monitor ovulation of preovulatory follicles. Blood was collected on d 0, 2, at PGF2α injection, and at 24 and 48h after PGF2α to monitor serum changes in estradiol-17β (E2-17β) and progesterone (P4). Based on serum P4 and ovarian exams, 2 cows were eliminated because of anestrus and their failure to ovulate a follicle in response to the first GnRH injection. Two other cows in which luteolysis failed to occur after PGF2α treatment also were eliminated. Final numbers of cows per treatment were: 7 d (n=13), 8 d (n=9), and 9 d (n=10). Twenty-nine of 32 cows ovulated (90.6%) in response to the first GnRH injection. Of those cows not ovulating in response to the first GnRH injection, 2 had 1 original corpus luteum and 1 had 2 original corpora lutea. Despite a 24- or 48-h delay between first GnRH and PGF2α injections, the diameter (mm) and volume (mm3) of the ovulatory follicle did not differ among treatments: 14.3±0.6 and 1,526±62 at 7 d; 14.1±0.8 and 1,479±97 at 8 d; and 15.3±0.9 and 1,490±69 at 9 d. In all 32 cows, at least 1 follicle ovulated after treatment, but ovulation rates did not differ: 1.2±0.1, 1.1±0.1, and 1.3±0.2, respectively, for the 7-, 8-, and 9-d treatments. Serum concentrations of E2-17β did not differ among treatments. Four cows in the 7-d treatment were inseminated 24h late and were excluded before assessing conception rates, which were 5/9 (55.6%), 5/9 (55.6%), and 1/10 (10%), respectively. We conclude that delaying PGF2α injection by 24h had no effect on outcomes.