During her heat cycle, a mare typically and obviously demonstrates estrus when in the presence of a stallion by flirtatious behavior and physical changes of her genitalia. However, many natural factors — as well as disorders — can affect the estrous cycle and the mare’s ability to show heat. Pregnancy, lactating anestrus, winter anestrus, behavioral anestrus, age, and possibly nutrition are the most frequent natural reasons a mare might not come into heat or might fail to show heat.

Seasonal Anestrus

The most common natural reason for a mare to fail to come into heat is seasonal or winter anestrus, says Patrick McCue, DVM, PhD, (Comparative Pathology), Dipl. ACT, Assistant Professor, Department of Clinical Sciences, Colorado State University. "A majority of mares stop cycling in approximately October and do not resume cycling until the spring, sometimes between March and June," says McCue. "During this winter or seasonal anestrous period, mares have inactive ovaries and typically do not express behavioral heat. In the spring, the ovaries begin to develop follicles. Growing follicles produce estrogens which cause the mare to show heat."

The seasonal cycle is related to the period of short day lengths that occur in winter. Explains Terry L. Blanchard, DVM, Dipl. ACT, Professor (teaching and research on mare and stallion clinical reproduction), Dept. of Large Animal Medicine and Surgery, Texas A&M University, "The horse is a seasonal breeder with an inactive hypothalamic-pituitary-ovarian axis during the winter in North America. As day length increases, the hypothalamus begins secreting GnRH (gonadotropin-releasing hormone), eventually stimulating the pituitary gland to secrete gonadotropins. Follicle stimulating hormone (FSH) secretion generally increases first, promoting follicular growth. As longer days persist, eventually luteinizing hormone (LH) increases to a level that ovulation occurs. Then mares tend to have regular estrous cycles throughout the breeding season until day length begins to shorten again. Once competent follicles begin secreting sufficient estrogen, estral behavior generally begins. Since the transitional period-the phase between winter anestrus and regular cyclic ovarian activity-is gradual and progressive, the first estrus of the breeding season tends to be long and is often also somewhat irregular in intensity