Reproductive efficiency of Nelore (Bos indicus) cows submitted to conventional resynchronization or ReBreed21...

The aims were to evaluate the reproductive efficiency of Nelore (Bos indicus) cows submitted to resynchronization programs for timed artificial insemination (TAI) and to evaluate the effect of estradiol cypionate dose (EC; 0.5 vs. 1.0 mg) on reproductive outcomes in a ReBreed21 program. A total of 253 primiparous and 645 multiparous cows (3.0 ± 0.4 of body condition score and 51 ± 13.7 days postpartum) were assigned to one of three resynchronization strategies: conventional (Resynch33; n = 299), ReBreed21 with 0.5 mg of EC (REB0.5; n = 299), or ReBreed21 with 1.0 mg of EC (REB1.0; n = 300). All cows received the same synchronization protocol for the first AI (Day 0). On Day 12 after the first TAI, cows in ReBreed21 treatments received a P4 device (0.5 g), which remained until Day 19, when 300 IU of equine chorionic gonadotropin (eCG) and either 0.5 or 1.0 of mg EC were administered. On Day 21, early pregnancy diagnosis was performed in all cows by corpus luteum perfusion evaluation using color Doppler ultrasonography; nonpregnant cows in ReBreed21 received 8.4 μg of GnRH and underwent the second AI. On Day 33, pregnancy was confirmed, and cows in ReBreed21 inseminated on Day 21 were resubmitted to the same ReBreed21 protocol to receive a third AI on Day 42. Cows diagnosed as pregnant on Day 21 but nonpregnant on Day 33 were considered false positives (FP) and, like those in Resynch33, were resubmitted on Day 33 for the second AI on Day 42 to the same TAI protocol that was used for first AI. Ultrasound examinations were performed on Day 54 and Day 75 to assess pregnancy per AI (P/AI), FP, and pregnancy loss (PL). Statistical analyses were done using SAS 9.4. Significance was declared at P ≤ 0.05, and tendencies when 0.05 < P ≤ 0.10. The P/AI, FP, and PL did not differ between ReBreed21 treatments. After 21 days of the breeding season (BS), the pregnancy rate (PR) was greater in REB1.0 (77.7 %) and REB0.5 (75.4 %) than in Resynch33 (52.8 %; P < 0.0001), although final PR was similar among REB1.0 (84.7 %), REB0.5 (84.9 %), and Resynch33 (79.9 %). Time to pregnancy was shorter (P < 0.0001) in REB1.0 (8.5 days) and REB0.5 (10.8 days) than Resynch33 (15.8 days). Regardless of EC dose, ReBreed21 increased PR after Day 0 (59.9 vs. 52.8 %; P = 0.03) and Day 21 (76.6 vs. 52.8 %; P < 0.0001), and improved PR on Day 42 (84.8 vs. 79.9 %; P = 0.06). The incidence of FP was greater in Resynch33 than ReBreed21 (13.7 vs. 7.5 %; P = 0.02), but final PL did not differ (3.2 vs. 3.1 %). In conclusion, the ReBreed21 program anticipated pregnancies, improved PR, and reduced FP, highlighting its potential to enhance reproductive efficiency in Nelore cows. The dose of 1.0 mg of EC had no effect on fertility parameters.