Exploring the postpartum period in mares: hormonal dynamics and ovarian activity under physiological and pathological conditions.

The aims of the study were to evaluate factors influencing the resumption of postpartum ovarian activity and to compare hormone concentrations among mares with physiological/pathological postpartum period and cyclic non-lactating mares. Follicular dynamics were monitored ultrasonographically in 38 postpartum mares from foaling or admission (Tpp/Ta) every other day until the emergence of a dominant follicle (Td), then daily until ovulation (To). Blood samples were collected at Tpp/Ta, Td, and To in postpartum mares and at Td and To in cyclic mares to determine serum estradiol, progesterone, leptin and cortisol concentrations. Ovulation occurred in 34/38 (90%) mares. The χ2 test showed that ovulation was significantly more frequent in mares with physiological postpartum (p = 0.005), in multiparous (p = 0.03) and in lactating mares (p = 0.00011). Age and foaling month were not associated with ovulation. In physiological postpartum mares, estradiol and progesterone concentrations decreased significantly over time (p = 0.002; p = 0.004 respectively), whereas leptin and cortisol remained unchanged. No significant temporal changes were observed for any hormone in mares with pathological postpartum or in cyclic mares. Hormone concentrations did not differ significantly among groups at any time point. Progesterone concentrations at Tpp/Ta were lower in primiparous mares (p = 0.048). Overall, parity, type of postpartum and lactation status influenced the likelihood of postpartum ovulation. Although hormone concentrations were largely comparable among groups, pathological postpartum mares exhibited delayed or absent ovulation, suggesting that postpartum disorders may affect the resumption of ovarian activity.