Ultrasonographic Appearance and the Pattern of Uterine Edema to Time Ovulation in Mares

The mare, on the average, has an estrous period that lasts around 6.5 days. Most mares will ovulate follicles ranging between 40 and 45 mm, 24–48 h before the end of their standing heat. Therefore, mares can show strong signs of heat up to 48 h after ovulation. Although these parameters are well established for the average mare, follicle size at ovulation and the duration of estrus in mares vary widely. Some mares might consistently ovulate 35-mm follicles, whereas others will not ovulate until the follicle has reached more than 50 mm in size. For pregnancy rates to be maximized at breeding farms, normal mares must be bred within 24–48 h prior to ovulation. However, mares bred artificially will have higher conception rates if bred within 24 h before ovulation. Furthermore, mares bred with subfertile stallions or with frozen semen will have higher fertility if bred within 12 h of ovulation. Therefore, determining the time of ovulation becomes a critical factor in achieving maximum pregnancy rates at breeding farms. Current systems to determine the time of ovulation in mares rely mostly on (a) teasing, and (b) the softening of a preovulatory size follicle detected by rectal palpation and in some instances the ultrasonographic appearance of the follicle. In addition, human chorionic gonadotropin (hCG) or deslorelin (Ovuplant) implants are routinely used for induction of ovulation in mares in an effort to minimize the variation in the time of ovulation. Although it has been shown that most mares that are in standing heat and have at least a 35-mm follicle will ovulate between 36 and 48 h after treatment with these products, there is still a wide variation in the response of individual mares. Perhaps this is due to the wide variation in the size of the follicles that individual mares ovulate. The mare under the influence of estrogen will have an increase of edema of the reproductive tract. This includes mild hyperemia of the vulvar lips and vagina with a concomitant relaxation of the vaginal vault and cervix. In addition the uterus under the influence of estrogen will display a characteristic pattern of edema that could be described as a cart wheel pattern. Once the mare has ovulated and a reproductive tract begins to be under the influence of progesterone the endometrial edema is no longer present under normal circumstances. The appearance and disappearance of endometrial edema are REPRODUCTION I