Ovarian assessment for pre-selection of embryo donor ewes

The present study aimed to evaluate whether two-dimensional ovarian measurements and the identification of antral follicle populations by ultrasound at different stages of the estrous cycle are related to the superovulatory response and embryo production in ewes. The estrous cycle of 57 Santa Inês ewes was synchronized by administering two doses of 75 μg of prostaglandin (PGF) 10 days apart. After 2.5 (proestrus/estrus), 4.5 (metestrus), and 9.5 (diestrus) days from the last PGF injection, ultrasound examinations were performed to quantify antral follicles ≥ 2 mm and ovarian measurements. Subsequently, the number of follicles and ovarian measurements were correlated with the superovulatory response and embryo production of each ewe. The mean follicle population (≥ 2 mm) and ovarian dimensions were similar in all phases of the estrous cycle (P > 0.05). Positive correlations were identified between the number of follicles and ovarian dimensions (P < 0.05) in different phases of the estrous cycle. Donor ewes with a high number of follicles (≥ 7) in the metestrus phase showed a significant increase in the number of ovulations, total structures, total embryos, and viable embryos (P < 0.05). In conclusion, ultrasonographic assessment of the number of follicles ≥ 2 mm, performed during metestrus (4.5 days after synchronization pre-treatment), is an easy-to-perform approach and can be used for pre-selection of embryo donor ewes.