Beyond Routine Ultrasonography: Peri-Insemination Biomarkers in Relation to Reproductive Competence in Warmblood Mares

Efficient reproductive management in mares relies on the accurate assessment of periovulatory status; however, routine ultrasonographic parameters cannot fully reflect underlying reproductive competence. The aim of this study was to evaluate hormonal (estradiol), inflammatory (IL-6), and clinical (follicle size, uterine edema, age, parity) biomarkers in relation to pregnancy outcome in mares and to assess whether these parameters may provide additional information alongside routine transrectal ultrasonographic evaluation. A total of 31 mares were examined. Estradiol concentration, follicle size, uterine edema score, and interleukin-6 (IL-6) concentration were assessed 24 h prior to insemination. Pregnancy outcomes were determined 14 days post ovulation. Statistical analysis included non-parametric tests and correlation analysis. Estradiol concentrations were significantly higher in pregnant compared to non-pregnant mares (p = 0.004). A moderate positive correlation was observed between mare age and estradiol concentration (Spearman’s rho = 0.599, p < 0.01), although age itself was not associated with pregnancy outcome. Stratified analysis confirmed higher estradiol concentrations in pregnant mares within both age groups, with statistical significance observed in younger mares (p = 0.003). No significant associations were found for uterine edema score, follicle size (continuous), or IL-6 concentration. These findings indicate that among the evaluated parameters, estradiol concentration at insemination showed the strongest association with subsequent pregnancy status and may provide additional information about periovulatory reproductive status. Although estradiol concentrations varied with age, its association with fertility was not solely explained by age-related differences. Rather than serving as a stand-alone predictor, peri-insemination estradiol was associated with pregnancy outcome and may provide additional information about periovulatory reproductive status. However, due to the relatively small sample size and observational design, these findings should be interpreted cautiously.