A comparison of two different esters of estradiol for the induction of ovulation in an estradiol plus progestin-based timed artificial insemination protocol for suckled Bos indicus beef cows

The abilities of two different estradiol esters to induce ovulation in a timed AI (TAI) synchronization protocol in suckled Bos indicus cows were evaluated. In Experiment 1 (synchrony of ovulation), 31 cows were submitted to an estradiol/progestin-based synchronization protocol (Day 0) and randomly assigned to one of three treatments at the time of progestin removal on Day 8: 0.5 or 1.0 mg of estradiol cypionate (EC) at that time or 1.0 mg of estradiol benzoate (EB) 24 h later (Day 9). To determine the timing of ovulation, ultrasound examinations were performed every 12 h from ear implant removal to 96 h after the removal. Orthogonal comparisons were performed to determine the effects of estradiol ester and the effects of the dose of EC on reproductive parameters. Although neither the E2 ester (P = 0.83) nor the dose of EC (P = 0.55) affected the ovulation rate, the interval from progestin removal to ovulation was longer (P = 0.04) in EC-treated cows (1.0 mg EC = 71.1 ± 3.6 and 0.5 mg EC = 78.0 ± 3.5) than EB-treated cows (EB = 66.0 ± 2.3) was detected. Ovulation in 0.5-mg-EC-treated cows was less synchronous than that in 1.0-mg-EC-treated cows (distribution curves compared using kurtosis). In Experiment 2 (pregnancy per AI; P/AI), 660 cows at two different locations received the same synchronization protocol (n = 361 at Farm A and n = 299 at Farm B) and were treated with estradiol esters as in Experiment 1 [0.5 mg EC (n = 220) or 1.0 mg EC (n = 219) at the time of progestin removal or 1.0 mg EB (n = 221) 24 h later]. The cows were inseminated 54 to 56 h after progestin removal. As applied in the Experiment 1, orthogonal comparisons were performed to evaluate the effect of estradiol ester and the dose of EC on P/AI. Although the type of estradiol ester used did not affect the P/AI (P = 0.57; EB - 43.0% vs. EC - 44.6%), the P/AI was higher (P = 0.03) in cows treated with 1.0 mg EC (55.7%) than in those treated with 0.5 mg EC (38.6%). In summary, the administration of 0.5 mg EC at the time of progestin removal altered the distribution of ovulation and resulted in a lower P/AI when compared with the use of 1.0 mg EC in suckled B. indicus cows. However, the P/AI following the administration of 1.0 mg EC at the time of progestin removal did not differ from that after the administration of 1.0 mg EB 24 h later.