This study examined two strategies to improve pregnancy rate (PR) following Ovsynch timed artificial insemination (TAI) for lactating dairy cows (n = 225) and nulliparous heifers (n = 87). Animals were assigned randomly to receive one of three treatments: Ovsynch protocol (GnRH 7 d before and 48 h after one PGF2α treatment), Presynch + Ovsynch (two treatments of PGF2α 14 d apart followed by Ovsynch 14 d later), or Ovsynch + Post-AI GnRH (GnRH 6 d after Ovsynch TAI) for first service breeding. Pregnancy rates among treatments were not different in lactating cows (42.5, 48.0, and 44.9%) or heifers (65.5, 58.6, and 58.6%) for Ovsynch, Presynch + Ovsynch, and Ovsynch + Post-AI, respectively. Cows treated with Ovsynch had lower PR when bred < 76 d in milk (DIM) compared with Presynch + Ovsynch or Ovsynch + Post-AI treatments. In addition, cows and heifers that received Post-AI GnRH had greater progesterone (P4) concentrations on day 21 and day 28 post-TAI than the Ovsynch group. Animals with higher P4 concentrations at initiation of Ovsynch had better PR than those with low P4 concentrations. Presynch animals had a greater proportion of animals with P4 values above 1 ng mL-1 at the initiation of Ovsynch than those animals in the Ovsynch group (74.5 vs. 59.4%). Heifers had lower PR if they were <14.6 mo of age (48.9 vs. 75%) or weighed <380 kg (47.4 vs. 70.8%). Although no significance differences in PR were observed between treatments in cows or heifers, DIM in cows and age and weight in heifers affected PR.Key words: Ovsynch, presynchronization, gonadotropin-releasing hormone, dairy heifer, dairy cow, timed artificial insemination
Effects of Presynchronization and Bovine Somatotropin on Pregnancy Rates to a Timed Artificial Insemination Protocol in Lactating Dairy Cows
