Fertility of lactating dairy cows treated with Ovsynch after presynchronization injections of PGF2α and GnRH

Synchronization of ovulation (Ovsynch) using GnRH and PGF(2alpha) allows control of follicle growth, corpus luteum regression, and ovulation, but resulting pregnancy rates vary. This study examined whether presynchronization to allow initiation of Ovsynch during diestrus would improve pregnancy rates at timed artificial insemination (AI). Lactating dairy cows (n = 427), 69 to 75 d postpartum, were randomly assigned to two groups by parity. Control cows received Ovsynch (GnRH, d 0; PGF(2alpha), d 7; GnRH, d 9; timed AI 16 h after second GnRH). Treated cows received presynchronization injections of PGF(2alpha), and GnRH, 10 and 7 d, respectively, before starting Ovsynch. Pregnancy diagnoses were performed 36 d after AI Progesterone (P-4) concentrations from a subset of cows (n = 84) were determined in serum samples collected on d 0, 3, and 7 of Ovsynch. Presynchronization increased the percentages of cows with greater than or equal to1 ng/ml serum P-4 compared with control cows at first injection of GnRH (d 0; 93 vs. 56%) and on d 3 (90.7 vs. 51.2%) during Ovsynch. On day of PGF2., d 7 during Ovsynch, percentages of cows with greater than or equal to1 ng/ml serum P-4 were similar (95.3%, treated vs. 82.9%, control) but more treated cows had greater than or equal to2 ng/ml serum P-4 (95.3 vs. 63.4%). However, pregnancy to timed AI was similar between treated (41.5%) and control cows (38.3%). Cows with above-average milk production had greater pregnancy rate (45.8 vs. 33.8%) compared with lower producing cows. Although presynchrony increased the proportion of cows with luteal function at onset of Ovsynch, pregnancy rate to timed AI was not improved. Cows with above-average milk production had greater fertility at timed AI than herdmates with lower milk production.