Effects of extending interval between presynchronization and breeding protocols and prostaglandin F2α administration...

Small size of follicle, which may be due to summer, and high progesterone (P4) could hamper the ovulatory response to gonadotropin-releasing hormone (GnRH) at the beginning of GnRH-based protocols for estrus synchronization (GnRH-1), thus decreasing fertility. Accordingly, the objective of this study was to evaluate the effects of two-day extension in interval between presynchronization (Ovsynch) and breeding (Heatsynch) protocols and administration of prostaglandin F2α (PGF_2α) two days before breeding protocol on size of the largest follicle (LF) and P4 concentration (PC) at GnRH-1, respectively, and fertility of cows during summer. Cows were randomly assigned to four experimental groups, including: 1) O7DH (cows with 7-day interval between Ovsynch and Heatsynch; n = 153), 2) O7DH+PG (cows with 7-day interval between Ovsynch and Heatsynch plus administration of PGF_2α two days before Heatsynch; n = 156), 3) O9DH (cows with 9-day interval between Ovsynch and Heatsynch; n = 147), and 4) O9DH+PG (cows with 9-day interval between Ovsynch and Heatsynch plus administration of PGF_2α two days before Heatsynch; n = 154). Further, cows were subjected to ultrasonographic examination and blood sampling at GnRH-1. Extension of the interval between Ovsynch and Heatsynch did not affect size of LF at GnRH-1 (P > 0.05); however, it decreased pregnancy risk (P < 0.05). Administration of PGF_2α two days before Heatsynch decreased PC at GnRH-1 (P < 0.05), but it did not affect pregnancy risk (P > 0.05). In conclusion, the present study revealed that additional two days before breeding protocol did not increase follicle size at GnRH-1 and reduced fertility. Furthermore, PGF_2α administration before breeding protocol could not improve fertility of cows despite reducing PC at GnRH-1.