A novel approach to synchronize ovulation of dairy heifers for the second and subsequent inseminations

Our objective was to evaluate the effect of a novel resynchronization strategy as well as the effect of a modified Short Resynch protocol on pregnancies per artificial insemination of nonpregnant dairy heifers. Heifers that presented nonpregnant at pregnancy diagnosis 32 ± 3 d after artificial insemination (AI) were blocked by ear tag number (n = 851). Nonpregnant heifers with an odd ear tag number were blindly resynchronized (BL; n = 378) with the 6-d Cosynch protocol with a progesterone-releasing intravaginal device (PRID; GnRH and PRID insertion on d 0; PGF_2α on d 5; PGF_2α and PRID removal on d 6; GnRH and AI on d 8) without previous examination of the ovaries. Nonpregnant heifers with an even ear tag number were subjected to ultrasonographic examination of the ovaries (US; n = 473). Heifers with a corpus luteum ≥10 mm and a follicle ≥9 mm concurrently present at the ovaries were resynchronized with a modified Short Resynch protocol (US-Short; n = 241; PGF_2α on d 0; PGF_2α on d 1; GnRH and AI on d 3), whereas heifers that did not meet the criteria established for the group US-short were resynchronized with a 6-d Cosynch + PRID protocol (US-PRID; n = 232). To increase sample size in group US-Short and group US-PRID, nonpregnant heifers were assigned to groups BL and US with a 1:2 ratio during the last 2 years of the trial. At every farm visit, the first eligible heifer was enrolled in group BL, and the next 2 heifers were assigned in group US and resynchronized based on the existing ovarian structures. Blood samples were collected from a subset of heifers (group BL = 233, group US-PRID = 114, and group US-Short = 115) to measure progesterone concentrations. Pregnancy per artificial insemination at 32 ± 3 d after AI did not differ (P = 0.5) between group BL and group US (50.2% vs. 52.6%, respectively) nor between (P = 0.2) group BL, group US-PRID, and group US-short (50.2%, 49% and 55.6%, respectively). The hazard of pregnancy did not differ (P = 0.3) between group BL and group US (hazard ratio = 1.2; 95% CI: 0.9-1.4) and median time to pregnancy was 8 d for both group BL (95% CI: 8-37) and group US (95% CI: 8-19). Group US resulted in 51% reduction in PRID usage and 12.75% reduction in hormonal treatments (GnRH and PGF_2α), compared with group BL, leading to reduced synchronization costs.