Effect of timing of estradiol benzoate administration upon synchronization of ovulation in suckling Nelore cows (Bos indicus) treated with a progesterone-releasing intravaginal device

Abstract The present study investigated how the timing of the administration of estradiol benzoate (EB) impacted the synchronization of ovulation in fixed-time artificial insemination protocols of cattle. To accomplish this, two experiments were conducted, with EB injection occurring at different times: at withdrawal of the progesterone-releasing (P4) intravaginal device or 24 h later. The effectiveness of these times was compared by examining ovarian follicular dynamics (Experiment 1, n  = 30) and conception rates (Experiment 2, n  = 504). In Experiment 1, follicular dynamics was performed in 30 Nelore cows ( Bos indicus ) allocated into two groups. On a random day of the estrous cycle (Day 0), both groups received 2 mg of EB i.m. and a P4-releasing intravaginal device, which was removed on Day 8, when 400 IU of eCG and 150 μg of PGF were administered. The control group (G-EB9; n  = 15) received 1 mg of EB on Day 9, while Group EB8 (G-EB8; n  = 15) received the same dose a day earlier. Ovarian ultrasonographic evaluations were performed every 8 h after device removal until ovulation. The timing of EB administration (Day 8 compared with Day 9) did affect the interval between P4 device removal to ovulation (59.4 ± 2.0 h compared with 69.3 ± 1.7 h) and maximum diameter of dominant (1.54 ± 0.06 a cm compared with 1.71 ± 0.05 b cm, P  = 0.03) and ovulatory (1.46 ± 0.05 a cm compared with 1.58 ± 0.04 b cm, P n  = 119) and Group EB8-AI54h (G-EB8-AI54h; n  = 134) received 1 mg of EB on Day 8 and FTAI was performed, respectively, 48 or 54 h after P4 device removal. Group EB9-AI48h (G-EB9-AI48h; n  = 126) and Group EB9-AI54h (G-EB9-AI54h; n  = 125) received the same treatments and underwent the same FTAI protocols as G-EB8-AI48h and G-EB8-AI54h, respectively; however, EB was administered on Day 9. Conception rates were greater ( P