Estradiol benzoate delays new follicular wave emergence in a dose-dependent manner after ablation of the dominant ovarian follicle in cattle.

Administration of estradiol benzoate (EB) induces atresia of the dominant follicle (DF) in the ovaries of cattle within 36 h but emergence of a new wave of follicular development is delayed by 3-5 days. The present study investigated the role of EB in determining timing of emergence of a new follicular wave after removing the influence of the DF. At 6.4+/-0.2 days after ovulation in Angus and Angus/Simmental cattle (n=26), aged 4.9+/-0.6 years and weighing 634+/-20 kg, all ovarian follicles > or =5mm in diameter were aspirated with a 17-gauge needle using an ultrasound-guided transvaginal approach (Day 0 or Hour 0) and animals immediately received 0 (0EB), 1 (1EB), 2 (2EB) or 4 (4EB) mg EB i.m./500 kg body weight (n=6 or 7 per treatment). Ovarian structures were monitored by ultrasonography on a daily basis until emergence of a new wave of follicular development. Concentrations of estradiol (E2) were different among all treatments between Hours 24 and 72, increasing (P<0.01) with greater doses of EB administered. Hour of peak follicle-stimulating hormone (FSH) was 29.3+/-4.0, 53.3+/-4.5, 81.1+/-15.5, and 91.4+/-8.2 for the 0EB, 1EB, 2EB, and 4EB treatments, respectively, and emergence of a new wave of follicular development occurred on Days 1.5+/-0.2, 3.3+/-0.3, 4.0+/-0.6 and 4.4+/-0.4, respectively. Timing of peak FSH and emergence of a new wave of follicular development was earliest (P<0.05) in the 0EB treatment, similar (P>0.1) among the 1EB and 2EB treatments, and most delayed (P<0.05) in the 4EB treatment when compared to the 0EB or 1EB treatments. The overall mean interval from peak FSH to emergence of a new wave of follicular development was 15.7+/-3.3 h and was not affected by treatment. Concentrations of E2 at 24 h before new emergence were not different among EB-treated animals (20.2+/-5.5 pg/ml), but lower (P<0.01) in the 0EB treatment (1.6+/-0.2 pg/ml). In a dose-dependent manner, EB delayed the pre-emergence surge in FSH that stimulates new follicular development after the DF has ceased to be functional. The importance of using an 'optimal' dose of EB in hormonal regimens using this agent to strategically regulate follicular development is emphasized by the outcomes of this study.