Abstract
Background:
Summer heat stress affects the regularity of the estrous cycle in different animal species.

Aim:
This study aimed to investigate seasonal effects on follicular dynamics, hemodynamics, and associated biomarkers across the estrous cycle in native mares.

Methods:
Doppler ultrasound examinations and blood samples were collected along three sequential estrous cycles during the cold (November to March) and the hot months of the year (May to August). The diameter, antrum diameter, and color area of the dominant follicle (DF) were determined. The diameters, uterine horns, and corpus luteum (CL) diameters, areas, and color areas were estimated. In blood samples, estradiol (E2), progesterone (P4), cholesterol, lactate dehydrogenase (LDH), and nitric oxide (NO) concentrations were determined.

Results:
Season (Cold vs. Hot) significantly (p < 0.01) reduced the total number of follicles in the hot months compared with the cold months. Season × Days of estrous (20 days, -5 to 14) impacted (p < 0.05) total number of follicles. Estrous cycle days (p < 0.001) and Season × Days of estrous influenced (p < 0.001) DF diameter/cm, color area/pixel, and P4. Season (p < 0.001), Days of the estrous cycle (p < 0.0001), and Season × Days of estrous influenced (p < 0.001) the area/pixel, the antrum area/pixel, the circumference/pixel, the color area %, the granulosa color area, the granulosa color area %, the circulatory % of the DFs, and also CL diameter, uterine horn diameter/cm, uterine horn area/pixel, total cholesterol, NO, LDH, and E2. Either season (p < 0.001) or days of the estrous cycle (p < 0.001) influenced the DF antrum diameter/cm and uterine horn color area/pixel. Estrous days of the hot months increased the area, the antrum area, the circumference of the DFs, the area of the uterine horns, total cholesterol, NO, and LDH.

Conclusion:
In mares, the season of the year did not disturb the ovarian macro-environmental dynamics, DF growth, and recruitment but disrupted blood flow mediators, enzymes, and biochemicals that may associate disturbances in the intrafollicular mediators and influence the oocyte quality. It is recommended that mares be bred under suitable environmental conditions during the cold months when good quality green food is available.

Key words: Mares; Season; Ovarian hormones; Dominant follicle; Luteal dynamics.