Comparison of three doses of estradiol benzoate for synchronization of follicular wave emergence in suckled Bos indicus beef cows

We aimed to compare the effect of three estradiol benzoate (EB) doses on follicular wave emergence (FWE) and dominant follicle growth of suckled Nelore cows submitted to TAI (D0). On a random day of estrous cycle (D-10), multiparous (MULT; n=36) and primiparous (PRIM; n=20) suckled Nelore cows received an intravaginal progesterone (P4) device and were assigned in three groups. Cows in the EB-1 (n=20), EB-1.5 (n=15) or EB-2 (n=21) groups received, respectively, an im treatment with 1, 1.5 or 2 mg EB. A subgroup (n=10-13 cows/group) were subject to daily ovarian evaluations from D-10 to D0. On D-2, P4 devices were removed, and all cows received the same treatment: 1 mg estradiol cypionate, 0.53 mg sodium cloprostenol, and 300 IU eCG. Statistical analyses were performed considering only the main effects of treatment group and parity order. The proportion of cows with a synchronized FWE and the moment of the FWE did not differ (p>0.05) among the treatment groups (overall: 80% {[}28/35] and 4.1 +/- 0.4 days); however, the FWE occurred earlier (p=0.007) in MULT (3.8 +/- 0.2 days) than PRIM (5.1 +/- 0.4) cows. The proportion of animals detected in estrus was greater (86% {[}31/36] vs. 70% {[}14/20]; p=0.02) and the dominant follicle was larger on D-2 (9.7 +/- 0.3 mm vs. 7.8 +/- 0.7 mm; p=0.006) and D0 (11.9 +/- 0.4 mm vs. 10 +/- 0.5 mm; p=0.008) in MULT than PRIM cows. In conclusion, the three EB doses presented similar efficiency to synchronize the FWE in suckled Nelore cows. Moreover, a delayed FWE and smaller dominant follicle is observed in PRIM cows, contributing to the reduced reproductive performance in this parity category when using similar TAI protocols of MULT cows. (AU)