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

Overview

• This study investigated how different factors affect the return of ovarian activity in postpartum mares by monitoring hormone levels and ovarian changes under normal and pathological postpartum conditions.
• The research compared hormone concentrations and ovulation patterns between mares experiencing physiological postpartum, pathological postpartum, and non-lactating cyclic mares.

Study Objectives and Design

• The main aim was to evaluate the factors influencing when postpartum mares resume ovarian activity.
• The study also aimed to compare serum hormone levels—estradiol, progesterone, leptin, and cortisol—between postpartum mares (both physiological and pathological conditions) and cyclic non-lactating mares.
• Ovarian follicular dynamics were tracked using ultrasonography every other day from foaling or admission (Tpp/Ta) until a dominant follicle appeared (Td), then daily until ovulation (To).
• Blood samples to measure hormone concentrations were collected at Tpp/Ta, Td, and To for postpartum mares and at Td and To for cyclic mares.

Key Findings on Ovulation and Influencing Factors

• Ovulation occurred in 34 out of 38 mares (90%), showing a high rate of resumption of ovarian activity postpartum.
• Statistical analysis revealed ovulation was significantly more frequent in:
  • Mares experiencing physiological (normal) postpartum conditions (p = 0.005).
  • Multiparous mares, i.e., those that had foaled multiple times (p = 0.03).
  • Lactating mares (p = 0.00011).
• Age of the mare and the month of foaling did not significantly affect the occurrence of ovulation.

Hormonal Dynamics During Postpartum and Cyclic Conditions

• In mares with physiological postpartum:
  • Estradiol and progesterone concentrations significantly decreased over time from foaling to ovulation (p = 0.002 and p = 0.004 respectively).
  • Leptin and cortisol levels remained stable and did not show significant changes over the monitoring period.
• In mares with pathological postpartum conditions as well as in cyclic non-lactating mares:
  • No significant temporal changes were found for estradiol, progesterone, leptin, or cortisol across the examined time points.
• At individual time points, hormone concentrations did not significantly differ among physiological postpartum, pathological postpartum, or cyclic groups.
• Progesterone concentrations were lower at initial measurement (Tpp/Ta) in primiparous mares (those foaling for the first time) compared to multiparous mares (p = 0.048).

Interpretations and Implications

• Parity (number of times the mare has foaled), type of postpartum condition (physiological vs pathological), and lactation status significantly influenced the likelihood of postpartum ovulation.
• Although overall hormone concentrations were similar among groups, mares with pathological postpartum conditions showed delayed or absent ovulation.
• This suggests that postpartum disorders can negatively impact the timely resumption of ovarian activity and fertility in mares.
• Stable leptin and cortisol levels indicate these hormones may not play a major role in ovulation resumption during the postpartum period in mares, or their roles are more complex and require further study.

Summary

• This research provides valuable insights into the reproductive physiology of postpartum mares, highlighting important factors such as parity and lactation in ovulatory recovery.
• Management of postpartum health and lactation could be critical for improving the reproductive performance of mares.
• Monitoring hormonal changes and ovarian follicle development via ultrasonography offers a practical approach to assess reproductive readiness in postpartum mares.
• Further research may help clarify mechanisms by which postpartum pathologies delay ovarian function and identify interventions to support reproductive health in affected mares.