Ovarian Traits After Gonadotropin-Releasing Hormone-Induced Ovulation and Subsequent Delay of Induced Luteolysis in an Ovsynch Protocol1

Our objective was to determine whether delaying the PGF2α injection by 24 or 48 h 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 ± 12 kg and body condition score = 2.3 ± 0.1) were administered 100 μg 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 48 h 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 72 h after PGF2α to monitor ovulation of preovulatory follicles. Blood was collected on d 0, 2, at PGF2α injection, and at 24 and 48 h 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 24 h 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 24 h had no effect on outcomes.