Abstract #M193

# M193
A resynchronization of ovulation strategy based on the ovarian structures present at non-pregnancy diagnosis reduced time to pregnancy in lactating dairy cows.
R. Wijma*1, M. Masello1, M. L. Stangaferro1, M. M. Pérez1, J. O. Giordano1, 1Cornell University, Ithaca, NY.

Our objectives were to evaluate time to pregnancy after first AI and pregnancy/AI (P/AI) in dairy cows managed with 2 resynchronization of ovulation strategies. After 1st service, Holstein cows from 2 farms were blocked by parity (1 vs. > 1) and randomly assigned to the Day 32 Resynch (R32; n = 614) or Short Resynch (SR; n = 609) group. Non pregnancy diagnosis (NPD) was conducted 32 ± 3 d after AI by transrectal ultrasonography. Nonpregnant cows in the R32 group received the Ovsynch protocol [(GnRH-7d-PGF2α-56h-GnRH-16h-timed AI (TAI)]. Nonpregnant cows in the SR group with a corpus luteum ≥ 15 mm and a follicle ≥ 10 mm (CL cows) received PGF2α-24h-PGF2α-32h-GnRH-16h-TAI. Cows not meeting the criteria (no CL cows) for ovarian structures received CIDR-Ovsynch (GnRH+CIDR-7d-CIDR removal+PGF2α-24h-PGF2α-32h-GnRH-16h-TAI). Binomial outcomes were analyzed with logistic regression and time to pregnancy [only nonpregnant cows to first AI that reached 210 d after 1st AI (R32 = 376; SR = 367)] with Cox’s proportional regression in SAS. For P/AI analysis, TAI service was the experimental unit (R32 = 561; SR = 667). Models included treatment and parity as fixed effects and farm as random effect. The hazard of pregnancy was greater (P = 0.04) for cows in the SR group (HR = 1.21, 95%CI: 1.01–1.44) and tended (P = 0.06) to be greater for primiparous cows (HR = 1.18, 95%CI: 0.99–1.41). Median days to pregnancy were 105 and 89 for the R32 and SR group, respectively. Overall P/AI did not differ by group [P = 0.18; R32 = 29.8% vs. SR = 33.3%]. At NPD, 71% and 72% of cows had a CL in the R32 and SR group. Treatment did not affect (P = 0.97) P/AI for CL cows (32.0 vs. 31.8% for R32 and SR). For no CL cows, P/AI were greater (P = 0.01) for the SR than the R32 group (37.1 vs. 24.5% for SR and R32). Pregnancy loss from 32 to 63 d after AI was similar (P > 0.10) for all AI services combined (R32 = 9.6% vs. SR = 11.3%), for CL cows (R32 = 8.3% vs. SR = 12.2%), and no CL cows (R32 = 14.7% vs. SR = 9.1%). We conclude that the SR protocol reduced time to pregnancy because of a reduction in interbreeding interval for cows with a CL at NPD and improved P/AI in cows with no CL at NPD. Supported by USDA-NIFA, Hatch under 1007421.

Key Words: resynchronization, dairy cow, timed AI