Pregnancy per artificial insemination in lactating dairy cows subjected to 2 different intervals from presynchronization to initiation of Ovsynch protocol

A protocol for presynchronization of ovarian status with 2 injections of PGF2α given 14d apart, with the last PGF2α injection given 12 or 14d before Ovsynch increases pregnancy per artificial insemination (P/AI) in dairy cows. We determined the efficacy of reducing the interval from the last PGF2α injection (500μg of cloprostenol) of presynchronization to initiation of Ovsynch on response to treatment and P/AI. Lactating dairy cows were assigned to an Ovsynch protocol, with the initial injection of GnRH given either 9 (PRE-9; n=135) or 12d (PRE-12; n=135) after the second PGF2α injection of presynchronization. The Ovsynch protocol consisted of 2 injections of 100μg of GnRH given 9d apart and 1 injection of PGF2α given 7d after the initial GnRH injection, and cows were subjected to timed artificial insemination (TAI; 70±3.5 DIM) approximately 16h after the second GnRH injection. Body condition score (1–5 scale) was recorded at TAI. Blood samples were taken for progesterone determination at the PGF2α injection of Ovsynch, at TAI, and at 11d after TAI. Ultrasonographic examinations were done in all cows at the second PGF2α injection of presynchronization, initial GnRH injection, PGF2α injection of Ovsynch, at TAI, and 24h after TAI for cyclicity status and ovarian responses to treatments, and at 32 and 60d after TAI for confirmation of pregnancy. Overall, 29 cows (10.7%) were determined acyclic or cystic and excluded from the study. The percentage of cows responding to initial GnRH injection (62.2 vs. 61.5%) did not differ between PRE-9 and PRE-12 but more cows in the PRE-9 group failed to respond to PGF2α treatment of Ovsynch compared with PRE-12 (22.7 vs. 10.7%). Body condition score at TAI (2.9±0.02) and mean ovulatory follicle diameter (16.4±0.2mm) were not different between treatments. Overall P/AI at 32d was reduced in PRE-9 (33.6%) compared with PRE-12 (44.3%) but pregnancy losses (5.0 vs. 3.7%) did not differ between treatments. Primiparous cows in the PRE-12 group had higher mean progesterone concentration 11d after TAI and greater P/AI 32 after TAI than primiparous cows in the PRE-9 group (6.4±0.5 vs. 4.6±0.5ng/mL and 55.8 vs. 30.0%, respectively). In conclusion, reducing the interval from the last PGF2α injection of the presynchronization treatment to initiation of Ovsynch (from 12 to 9d) did not affect ovulatory response to initial GnRH injection but reduced response to PGF2α injection of Ovsynch and P/AI at 32 and 60d after TAI. The reduction in