Follicular dynamics, ovarian vascularity and luteal development in mares with early or late postpartum ovulation

Follicular development and deviation processes during the postpartum period are not completely known in horses. Thus, we aimed to study the characteristics of follicular dynamics and ovarian vascular perfusion during the postpartum period in mares that demonstrated estrous behavior and had early (<10 days) or late (≥10 days) postpartum ovulation. Ten mares were scanned daily by transrectal ultrasonography from the first day postpartum (d1) to the sixteenth day after the first postpartum ovulation (D0 = ovulation). The animals were split in the early (n = 3) and late (n = 7) ovulation groups (average interval between parturition and ovulation: 8.0 ± 0.0 and 14.7 ± 1.2 days, respectively). For the follicular growth, no difference (P > 0.1) was detected between the groups when the data were normalized for the days preceding the first postpartum ovulation (from D-7 to D-1). However, when the data were normalized to days postpartum, the dominant follicle was larger (P < 0.05) in the early-ovulated group in all days during this period (d1 to d7). The number of follicles >25 mm diameter was greater (P < 0.05) in the early-ovulated group during the first 3 days postpartum, and the late-ovulated mares showed greater number of follicles with 20-25 mm during d4 to d7. For blood flow characteristics, no difference (P > 0.1) was detected between groups in vascular perfusion of the dominant follicle or in the ovarian pedicle ipsilateral to the largest follicle. Similarly, no difference (P > 0.1) was detected in progesterone concentrations, corpus luteum (CL) area and vascular perfusion of the CL. Pregnancy rate did not differ (P > 0.1) between the early (3/3; 100%) and late (5/7; 71.4%) groups. Therefore, the characteristics of the follicle growth on the preceding days of ovulation were similar between the early- and late-ovulated mares and were consistent with the follicular dynamics expected in non-pregnant and non-lactating mares. However, when the data were analyzed for the days relative to parturition, greater follicle development was present in mares that ovulate earlier during the postpartum period (<10 days). The results suggest that important events may occur before parturition, resulting in early follicle development, mainly in those mares that show estrus and ovulate within 10 days postpartum.