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

Abstract 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 1.0mg of estradiol cypionate (EC) at that time or 1.0mg of estradiol benzoate (EB) 24h later (Day 9). To determine the timing of ovulation, ultrasound examinations were performed every 12h from ear implant removal to 96h 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 (1.0mg EC=71.1±3.6 and 0.5mg 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 [0.5mg EC ( n =220) or 1.0mg EC ( n =219) at the time of progestin removal or 1.0mg EB ( n =221) 24h later]. The cows were inseminated 54 to 56h 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 1.0mg EC (55.7%) than in those treated with 0.5mg EC (38.6%). In summary, the administration of 0.5mg 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 1.0mg EC in suckled B. indicus cows. However, the P/AI following the administration of 1.0mg EC at the time of progestin removal did not differ from that after the administration of 1.0mg EB 24h later.