Understanding the Mare’s Transitional Period

Find out how light can impact a mares’s ovulatory cycles.

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Understanding the Mare
Mares' ovulatory cycles occur in response to increasing day length. | Photo: iStock
Nature has timed the mare’s breeding season to maximize her foal’s chances of survival during winter. This evolution has led the mare to be a long-day, seasonally polyestrous (meaning her estrous cycles depend on the season) animal whose regular ovulatory cycles occur in response to increasing day length. During winter, or increased periods of darkness, the small pineal gland in the brain releases the hormone melatonin, which plays a pivotal role in seasonality of reproduction. The exact mechanism by which melatonin acts, however, is unclear.

At the same time, the mare’s hypothalamic-pituitary axis begins changing. It secretes less gonadotropin-releasing hormone (GnRH, which stimulates ovarian follicle development) and releases minimal levels of luteinizing hormone (LH, a gonadotropin that facilitates follicle maturation and ovulation, or egg release), and waves of follicular development cease in most mares. We call this period anestrus, because the mare’s estrous cycle has paused. As day length and photoperiod (amount of sunlight) increase, the transitional period—the phase between winter anestrus and the ovulatory season—ensues. This can last 60 to 80 days. 

As days continue to lengthen, melatonin production decreases and GnRH secretion increases. This causes the anterior pituitary gland to begin releasing more gonadotropins, particularly LH. As ovarian activity increases, follicles grow and regress, causing the erratic behavior characteristic of mares in estrus. 

Factors such as latitude, climate, age, body condition, and nutrition can cause the onset of fall transition, duration of winter anestrus, and onset of spring transition to vary greatly between and within mare groups. Lower latitudes, warmer ambient temperatures, green grass, and an upward plane of nutrition seem to hasten the natural transition out of the winter anestrus period, and young mares tend to transition earlier than older mares. The early transition phase is characterized by the development of several small follicles (>20 mm) that become atretic (regress and do not ovulate). Then, during late transition, one to three more anovulatory follicular waves (in which follicles develop but then do not ovulate) occur, each with a larger dominant follicle (>35mm) that regresses. Normal cyclicity returns with the first ovulation

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Written by:

Karen Wolsdorf, DVM, Dipl. ACT, is a specialist at Hagyard Equine Medical Institute’s McGee Fertility Center, in Lexington, Ky.

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