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Our objective was to determine whether delaying the PGF2α injection by 24 or 48 hr after the first GnRH injection in an Ovsynch protocol (from a standard 7 days) altered ovarian characteristics in lactating dairy cows. Estrous cycles were synchronized in 36 Holsteins after removal of a progesteronereleasing controlled internal drug release (CIDR) insert and injection of PGF2α. On day 6 of the estrous cycle, cows were administered 100 μg of GnRH (81 ± 2 days postpartum) and assigned randomly to receive a treatment injection of PGF2α 7, 8, or 9 days later. Timed artificial insemination (TAI) was performed at 48 hr after PGF2α, at which time a second injection of GnRH was administered. Ovarian structures were mapped by ultrasonography on day 0 (first GnRH injection), on day 2 to determine responses to the first GnRH injection, at PGF2α injection, and daily thereafter through 72 hr after PGF2α to monitor ovulation of preovulatory follicles. Blood was collected on day 0, day 2, at PGF2α injection, and at 24 and 48 hr after PGF2α, to monitor serum changes estradiol-17s and progesterone. On the basis of serum progesterone and ovarian exams, 2 cows were eliminated because of anestrus and their failure 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 days (n = 13), 8 days (n = 9), and 9 days (n = 10). Twenty-nine of 32 cows ovulated (90.6%) in response to the first GnRH injection. Despite a 24or 48-hr delay between first GnRH and PGF2α injections, the diameter (mm) and volume (mm) of the ovulatory follicle did not differ among treatments. In all 32 cows, at least 1 follicle ovulated after treatment, but ovulation rates did not differ. Serum concentrations of estradiol-17s did not differ among treatments. Two cows in the 7-day treatment and 2 cows in the 8-day treatment were inseminated 24 hr late and were excluded before assessing conception rates: 5/9 (55.6%), 5/9 (55.6%), and 1/10 (10%), respectively. We concluded that delaying PGF2α injection by 24 hr had little effect on outcomes.

Delaying Injection of prostaglandin F2α in an ovsynch protocol