Comparison of estradiol benzoate doses at beginning of the synchronization and early resynchronization of ovulation for timed- insemination in Bos indicus beef cows

Based on the wide use of estradiol benzoate (EB) in timed-AI (TAI) protocols, with this research we aimed to compare the effect of different doses of EB in synchronization of ovulation and resynchronization at 14 days after TAI in suckled Nelore beef cows. Therefore, this dissertation consists of two studies. In the first study we compared three EB doses (1 mg, n= 20; 1.5 mg, n= 15; and 2 mg, n= 21) associated with a progesterone device (P4) at the beginning of a synchronization protocol for TAI in primiparous (n= 20) and multiparous (n= 36) beef cows. Follicular wave emergence (FWE) did not differ (P > 0.05) among treatment groups, as well as the other features. The FWE occurred later (P < 0.05) in primiparous cows than in multiparous cows. In addition, the follicular diameter of primiparous was smaller (P < 0.05) than multiparous cows at D 2 and TAI. We conclude with this study that doses from 1 to 2 mg EB were efficient in promoting the emergence of the new follicular wave. Furthermore, we also concluded that the delay in follicular emergence in primiparous cows contributed to the reduced follicular diameter at TAI compared to multiparous cows. In the second study, we compared the use of 1 or 2 mg EB associated with a P4 device at the beginning of the resynchronization protocol at 14 days post-TAI in suckled Bos indicus beef cows (1 mg; EB-1 group, n= 516; and 2mg, EB-2 group, n= 510). On D14, along with the beginning of resynchronization, it was possible to assess the presence of an active corpus luteum in 933 cows. In addition, a subgroup of these cows (n= 18-19/group) was submitted to daily ultrasound assessments from day 14 to 22 for localization and measurement of antral follicles and corpus luteum area, and assessment of corpus luteum perfusion. On D22, all cows were submitted to ultrasound evaluations in color Doppler mode to detect luteolysis. Cows with luteolysis detected on D22 were submitted to TAI2 on D24. The conventional diagnosis of pregnancy was performed 30-35 days after TAI1 or TAI2. The pregnancy per AI (P/AI) from first TAI was 21,4% greater (P<0.05) in the EB-1 group than in the EB-2 group. The potential embryonic loss between D22 and D30- 35 was doubled (P<0.05) in the EB-2 group. The P/AI of the second TAI did not differ (P> 0.1) between groups. Thus, with satisfactory pregnancy rates in TAI 1 and TAI 2, the EB-1 group had a greater cumulative P/AI after two TAIs with a 24-day interval than the EB-2 group. Therefore, we can conclude that 1 mg EB is the safe and recommended dose to be used in association with the P4 device in resynchronization protocols started at 14 days after TAI. (AU)