A transitional ovary
Keywords: stud, transition, ovary, equine, mareThis appearance (of both ovaries) is typical of mares in the springtime. Multiple medium-sized follicles predominate on the ultrasound image; a dominant, pre-ovulatory follicle has yet to be "selected". This is well illustrated in the ultrasound image.
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Transition is a normal condition; an intermediate state between anestrus and normal estrous cycles. Although transitional mares are in estrus for prolonged periods of time, they are not cystic and do not require treatment. Therefore this condition is not analogous to cystic ovarian disease in cattle.
Mares in transition are often bred repeatedly because they usually stand for the stallion. However, they will obviously not become pregnant until estrous cycles begin. Therefore transition is a common cause of "infertility".
When mares in transition arrive at a stud, this author normally suggests that the owner takes the mare home and returns in about two to three weeks to see if the mare has started to ovulate. The presence of a large, dominant follicle with uterine edema on ultrasound suggests that ovulation is imminent and the mare should be left at the stud. The presence of a CL visible on ultrasound confirms that a mare has begun to have estrous cycles and should stay at the stud farm.
Dopamine agonists such as sulpiride and domperidone can hasten the onset of estrous cycles once mares are in the transitional state (but not deep anestrus). However, treatment produces variable and unpredictable results. Therefore, from a practical standpoint there is little one can do to hasten the onset of ovulation in these mares. When owners become impatient at the lack of progress of such mares, the author usually suggests that a lighting program be used the following year, at least 8 to 10 week before breeding is anticipated.
