Resynchronization of follicular wave using long-acting injectable progesterone or estradiol benzoate at 14 days post-timed AI in Bos taurus x Bos indicus beef heifers.

We compared pregnancy rates in beef heifers resynchronized 14 days after the first timed-artificial insemination (TAI) using a P intravaginal device associated with either long-acting injectable progesterone (iP) or estradiol benzoate (EB). Braford and Brangus heifers were submitted to a TAI (D0). On D14, all animals were given a P intravaginal device and were randomly divided into two groups, EB (1 mg; n = 339); or iP (75 mg; n = 338). On D22, P devices were removed, and non-pregnant (NP) heifers were identified by assessing morphological luteolysis with Doppler ultrasonography. The NP heifers had the dominant follicle diameter measured and were submitted to a second TAI on D24. Dominant follicle diameter (mm) on D22 in NP heifers did not differ (P > 0.05) between EB (9.77 ± 0.25) and iP (9.92 ± 0.22) groups. No difference was observed between EB and iP groups for pregnancy rate on D22 (56.3% vs. 60.1%, respectively), and D40 post-first TAI (49.6% vs. 53.3%, respectively). The rate of potential pregnancy losses from D22 to D40 did not differ between EB (12%, 23/191) and iP (11.3%, 23/203) groups. The resynchronization pregnancy rate in the EB group (45.9%, 68/148) was greater (P<0.05) than the iP group (31.8%, 43/135). In conclusion, treatment with either 1 mg EB or 75 mg iP in combination with P device at 14 days after TAI are equally safe for the ongoing pregnancy. The EB treatment can improve the reproductive efficiency, as it resulted in greater resynchronization pregnancy rates than iP treatment in beef heifers resynchronized 14 days after TAI.