LAUSR

The objective of this experiment was to compare time to pregnancy and proportion of cows not pregnant 210 d after first service for cows managed for second and subsequent artificial insemination (AI) services with a reproductive management program that promoted reinsemination at detected estrus (AIE) or a program that promoted timed AI (TAI). After first service, lactating Holstein cows were blocked by parity and randomly assigned to d 32 Resynch (D32R; n = 464) or AIE Resynch (AIER; n = 512). To determine the effect of management strategies on time to pregnancy and cows not pregnant by the end of a 210 d at-risk period after first AI service, cows remained in AIER and D32R until pregnancy or herd exit. Cows in D32R received a GnRH treatment 32 ± 3 d after AI (first treatment intervention; FTI). Nonpregnancy diagnosis was conducted 7 d later by transrectal ultrasonography when nonpregnant cows with a corpus luteum (CL) ≥15 mm completed the Resynch protocol (PGF2α, 56 h later GnRH, and 16 to 18 h later TAI) and cows without a CL (NoCL cows) were enrolled in a PreG-Ovsynch protocol (GnRH, 7 d later GnRH, 7 d later PGF2α, 56 h later GnRH, and 16 to 18 h TAI) to receive TAI. For the AIER treatment, nonpregnant cows with a CL ≥15 mm observed by transrectal ultrasonography 32 ± 3 d after AI (i.e., FTI) received PGF2α to induce estrus. Cows not AIE within 7 d were enrolled in Resynch (GnRH, 7 d later PGF2α, 56 h later GnRH, and 16 to 18 h TAI). Cows in the NoCL group in AIER were enrolled in PreG-Ovsynch. Detection of estrus was performed based on visual observation of behavioral signs of estrus and tail-paint removal. Binomial data were analyzed with logistic regression and time to event data with Cox's proportional regression. After the FTI, a greater proportion of cows were AIE in AIER than D32R (36.0 vs. 11.9%) and more cows were AIE within 7 d of the FTI for AIER (25.0%) than D32R (4.8%). Overall pregnancy per AI at 68 ± 3 d after AI did not differ (AIER = 35.5% vs. D32R = 34.7%). The hazard of pregnancy up to 210 d after first AI for all cows enrolled (hazard ratio = 1.04, 95% CI 0.90 to 1.19) and for cows that received treatments only (D32R = 308, AIER = 349; hazard ratio = 1.00, 95% CI 0.85 to 1.19) did not differ. We conclude that a program aimed at increasing the proportion of cows reinseminated at detected estrus by treatment with PGF2α at 32 ± 3 d after AI may be an alternative strategy for dairy farms that prefer or need to inseminate more cows at detected estrus rather than by TAI.