Evaluation of presynchronized resynchronization protocols for lactating dairy cows.

The objectives of this experiment were to determine the speed at which cows that had their estrous cycle presynchronized with a GnRH or PGF(2α) injection are reinseminated and become pregnant. Furthermore, this experiment aimed to determine whether treatment with a controlled internal drug-releasing (CIDR) insert during the timed artificial insemination (AI) protocol improves pregnancy per AI (P/AI) of cows that had their estrous cycle presynchronized with GnRH or PGF(2α). Lactating cows from 2 herds were assigned to 1 of 2 presynchronization treatments at 32 ± 4 d after AI: GGPG (n=452)--GnRH injection at enrollment (d 0), 7d before the start of the timed AI protocol, and P11GPG (n=466)--PGF(2α) injection on d 3, 11 d before the start of the timed AI protocol. Cows observed in estrus at any interval after enrollment were reinseminated on the same day. Cows not observed in estrus by d 7 were paired by presynchronization treatment and assigned to receive or not receive a CIDR insert during the timed AI protocol (CIDR = 240, no CIDR = 317). Timed AI protocols were the Ovsynch56 at site A and the Cosynch48 at site B. A subsample of cows from site A had their ovaries scanned by ultrasound at enrollment and on the day of the first GnRH and PGF(2α) injections of the timed AI protocol and had blood sampled at each injection of the timed AI protocol for determination of progesterone concentration. Cows were examined for pregnancy 32 ± 4 and 67 ± 4 d after reinsemination. Cows in the P11GPG treatment had a faster reinsemination rate [adjusted hazard ratio = 1.24 (95% CI = 1.07, 1.45)] and were less likely to be submitted to the timed AI protocol (40.3 vs. 89.8%) and to be reinseminated at a fixed time (38.6 vs. 83.9%). The interval from enrollment to reinsemination was shorter for cows in the P11GPG group (13.0 ± 0.4 vs. 15.0 ± 0.2d). Presynchronization treatment did not affect P/AI 32 ± 4 d (GGPG = 42.3%, P11GPG = 39.3%) and 67 ± 4 d (GGPG = 37.0%, P11GPG = 35.4%) after reinsemination. Pregnancy rate from d 0 to 7 (GGPG = 3.6%, P11GPG = 17.7%) and from d 8 to 14 (GGPG = 1.6%, P11GPG = 5.7%) were greater for cows in the P11GPG treatment. Treatment with the CIDR insert during the timed AI protocol did not affect P/AI 32 ± 4 d (CIDR = 41.7%, no CIDR = 41.4%) and 67 ± 4 d (CIDR = 36.5%, no CIDR = 35.3%) after reinsemination. A greater percentage of cows in the GGPG treatment had progesterone concentration ≥ 1 ng/mL on the day of the first GnRH injection of the timed AI protocol (83.8 vs. 51.5%), but a greater percentage of cows in the P11GPG treatment ovulated in response to the first GnRH injection of the timed AI protocol (66.1 vs. 46.8%). We conclude that the P/AI of cows that had their estrous cycle presynchronized with GnRH or PGF(2α) was not different, but in herds with adequate estrous detection efficiency and accuracy, presynchronization with PGF(2α) may reduce the interval to the establishment of pregnancy.