Ovarian and Uterine Dynamics During the Estrous Cycle in Criollo Breed Mares

After having been brought to America 400 years ago, the Criollo horse reproduced freely for centuries in the southern part of the American continent. Roughness, resistance, and endurance are typical characteristics of this breed of horses. Although the natural selection that occurred over the centuries may have positively influenced the fertility and longevity of this breed, information regarding ovarian function and other reproductive aspects of Criollo breed mares under natural or controlled management conditions is scarce. The objectives of the present study were to (1) characterize the follicular dynamics of Criollo breed mares, (2) characterize growth and regression of the corpus luteum, and (3) study the cervical and uterine tone and ultrasonographic echotexture changes during two consecutive estrous cycles. In this study, 26 interovulatory intervals (IOI) were evaluated in 13 mares. Spontaneous estrous cycles were characterized by the following: (1) the IOI length was 21 days; (2) dominant and subordinate follicles emerged together at day 5 (ovulation = day 0), and their growth rates were slower until the day of deviation; (3) the deviation in growth rates between the two largest follicles of the ovulatory wave occurred when the dominant and subordinate follicles reached 22 and 21 mm in diameter, respectively; (4) the mean diameter of the preovulatory follicle at the day before ovulation was 43 mm; (5) different combinations of follicular waves were observed in each interovulatory interval; (6) the corpus luteum reached its maximum diameter about 2 days after ovulation and decreased gradually afterward; (7) after ovulation, the cervical and uterine tones were positively correlated and remained elevated until the onset of the expected luteolysis (days 12-14); and (8) after the expected luteolysis, the endometrial echotexture started to increase and reached maximum values 5-3 days before ovulation, when it started to decrease. Moderate-to-strong positive correlations between IOIs revealed repeatability within animals for the diameter of the preovulatory follicle at maximum and at the day before ovulation, cervical and uterine tones, endometrial echotexture, and corpus luteum diameter. The findings herein presented are of fundamental importance for a better understanding of the reproductive physiological patterns of the estrous cycle in the Criollo breed mare.