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.
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."
Because the length of daylight triggers the mare's estrous cycles, owners can get a jump on the season and induce cycling earlier in the year by putting the mare under artificial lights. Says Dale Paccamonti, DVM, MS, Dipl. ACT, Professor of Theriogenology, Dept. of Veterinary Clinical Sciences, Louisiana State University, "Providing 16 hours of light beginning no later than the first of December will cause mares to start cycling earlier in the year. Starting in November might even be better. But if you start the process too late-late December, early January, for example-you won't get much benefit."
It's important to provide light of sufficient intensity. "A 100-watt bulb for a 12 foot by 12 foot stall is usually enough," says Paccamonti. "To test for sufficient light, use a 35 mm single lens reflex camera set at 400 ASA, 1 D4 second shutter speed, and place a plain white Styrofoam cup over the lens. Hold the camera like you're taking a picture at the level of the mare's eye. Check the aperture reading (f-stop), and if it is at least 4 or greater there is sufficient light."
Some experts advocate providing artificial lighting at the end of the natural daylight. "That is, if natural daylight is 8 a.m. to 4 p.m., turn the lights on from 4 p.m. until midnight, rather than from midnight to 8 a.m.," says Paccamonti. Other experts, says Blanchard, feel it is best to add equal increments of light in both the morning and evening to get the best response to artificial lighting. "There remains some arguments about the best methods," states Blanchard.
Once started, the lighting schedule must be strictly maintained. Warns Blanchard, "If it is not, it is possible that the mare may actually perceive a decreasing day length and stop cycling too early. We have noted this problem in some show mares that are blanketed and lighted in the winter to keep their hair coats short. Sometimes the owner stops the lighting program when spring begins and warmer days arrive, and the mare enters a period of anestrus early even though it is only May or June."
Artificial lighting should be maintained until natural day length alone equals the sum used in natural/artificial lighting programs. The month that occurs will vary according to the latitude.
Note: Although artificial lighting promotes earlier estrus, it will not eliminate the transitional period. "She'll still have a period of up to six to eight weeks of erratic cycles and prolonged estrus without ovulation," Paccamonti states.
Pregnancy is another major cause of anestrus, says McCue. "Pregnant mares will not usually show heat due to the high levels of progesterone present during pregnancy. However, it has been well documented that some pregnant mares will show heat and even stand to be mated by a stallion."
Adds Paccamonti, "Pregnancy is an often overlooked reason for mares not to cycle. Probably every veterinarian has been asked to examine a mare that is not cycling and finds her to be pregnant, much to the surprise of the owner."
Common causes of behavioral anestrus or "silent heat" include a mare's concern about her foal and fright or shyness due to inexperience or being a new addition to the herd.
Mares with young foals by their sides might cycle normally (and ovulate), but not show estrus because of the foals. Says Paccamonti, "These mares are very protective of their foals and although they are actually cycling, will not exhibit normal estrous behavior."
McCue says that, "In many instances, a foaling mare does not show heat if her foal is running free: The mare appears to be very anxious about her foal. These mares may express estrus when teased if their foal is restrained."
Paccamonti adds, "If artificial insemination is allowed, one may breed these mares on the basis of ultrasound exams, regardless of behavior."
Frightened mares likewise might not show signs of heat. "When teased, these mares may walk off to a corner of the pen and show estrus after the stallion is taken from the pen, but not show estrus when they are teased directly by the stallion," says Blanchard.
Examining the mare at regular intervals by palpation per rectum and transrectal ultrasound to ascertain if the mare is having regular estrous cycles is important. "As far as how often to examine the mares that are not yet cycling, that very much depends on what size of follicles are present," Blanchard says. "When follicles are still small early in the transition period, once weekly may be enough. Once they get larger, say 25 mm in diameter or more, examination frequency should increase so the first ovulation is not missed-eventually from every two to three days to daily."
"Alternatively, weekly or bi-weekly blood progesterone assays can be run by an endocrine testing laboratory," he says. "If progesterone rises and falls as expected during normal estrous cycles, yet the mare does not show signs of estrus, she is cycling regularly."
A mare with a foal by her side might not be cycling due to lactational anestrus. "This is seen in some mares which foal and often go through a foal heat, but then cease to cycle," Paccamonti states. "When examined, the ovaries are inactive. Domperidone has been used with success to get mares in lactational anestrus to begin cycling."
Says Paccamonti, "Very young mares may not have reached puberty or be sexually mature." Obviously, this condition is something that time will address.
Very old mares might have abnormal cycles or fail to ovulate. "This seems to be tied in with ovarian senescence or old age," Paccamonti says. "The mare will show estrus and follicle development. Then the follicle develops a characteristic appearance on ultrasound and fails to ovulate."
McCue also notes that in the older mare, "the first cycle of the year may be delayed and they may have a longer period of follicular growth during a given cycle."
As the mare ages, her ovaries atrophy and her estrous cycle ceases. "The age at which this occurs is variable among mares and breeds, as some mares continue having regular estrous cycles (and produce foals if bred) well into their 20s," Blanchard reports. "This 'reproductive senescence' is not well studied in the horse, but probably involves the hypothalamic-pituitary-ovarian axis in many cases.
"Prescribed treatments have included GnRH therapy to stimulate the pituitary to secrete gonadotropins that will stimulate follicular development of the ovaries. This probably is a simplistic approach and is likely to be expensive with low likelihood of success in most cases. However, some 'testimonials' suggest that it may work in some cases."
Paccamonti believes that anestrus usually is not caused by nutrition unless the mare is obviously malnourished. He does add, however, that there seems to be "some positive effect on cyclicity in the spring when the lush pastures becomes available to the mare, so there does appear to be some positive influence of nutrition on cyclicity, but it is unclear exactly what it is or how it works."
Blanchard says that good nutrition does play a role in "at least allowing the mare to begin regular estrous cycles." He says, "Adequate energy levels must be present in the diet and the mare should be in good body condition. Remember that reproduction is a luxury, and if poor body condition exists, estrous cycles will cease."
Gary D. Potter, professor, equine science, Texas A&M, has done considerable research on body condition and fat stores, and he has shown that mares in good to fat (not obese) condition cycle better and are "more fertile" than mares losing weight or in poor body condition. Adding fat to the diet to increase energy density of ingested feed might be helpful in thin mares and in lactating mares losing weight due to heavy milk production.
Beyond that, Blanchard states that he's unaware of any specific feed ingredients that favor estrous behavior or the onset of regular estrous cycles. "Vitamins A and beta-carotene are often suggested to be important," he says, "but if general nutrient requirements are met, I suspect adding more would not make much difference."
Disorders And The Hand Of Man
Certain pathologic abnormalities or human influences can also cause anestrus.
Insufficient or improper teasing -"For our research studies, we pen the mares, then take a stallion on a lead into the pen to walk among the mares to detect those in estrus," says Blanchard. "Then we place the stallion in a box and allow the mares to come to the stallion on their own and express estrus. Finally, we catch the mares and place a halter and lead on them and bring them to the stallion where he can tease them over a protective rail."
Blanchard points out that response to teasing and intensity of estral behavior vary among individual mares. "Some mares require considerable time and effort with all these methods before they show estrus, while other mares show estrus to any method used to tease them. Some mares always show only subtle signs of estrus. In contrast, occasional mares show intense signs of estrus with very little teasing and even occasionally show estrus to other mares, geldings, and even humans."
No stallion-"Here at Texas A&M University, we often have individual mare owners with no access to a stallion that believe their mares are not cycling," says Blanchard. "Yet, institution of teasing the mare with a stallion often demonstrates the mare is having regular estrous cycles. This has become more of a problem with shipped semen breeding, as many of these owners are reluctant to send the mare to a breeding farm where teasing is an integral part of the breeding program."
Ovarian tumors-The most common type of ovarian tumor in horses is a granulosa cell tumor, which occurs in mares of all ages, says Tomas Gimenez, MVZ, Professor (equine reproduction and endocrinology) and Researcher (reproductive physiology and endocrinology in the mare) at Clemson University. The ovary becomes very large (untreated, it can grow to the size of a basketball, Gimenez says) and begins to secrete hormones that cause the other ovary to become nonfunctional.
"Normally, the ovary produces the estrogen that makes the mare behave like she's in heat," Gimenez states. "But with granulosa cell tumors, the ovary can produce many different types of hormones, particularly estrogen (which makes the mare act like she's always in heat) and testosterone (which causes the mare to behave male-like and studdish)."
States Blanchard, "Usually the tumor only affects one ovary, does not spread to other organs, and only occasionally re-occurs in the remaining ovary a year or so later. Surgical removal of the affected ovary most commonly results in the mare returning to regular estrous cycles and a fertile condition, but often not until the next breeding season."
Persistent corpus luteum-The corpus luteum is a structure that develops in the ovary on the site where an ovum is released and that secretes progesterone if fertilization occurs and pregnancy is maintained. Because progesterone blocks estrous behavior, the mare will not show signs of estrus.
"Normally, if the mare is cycling and does not become pregnant, the corpus luteum will live only for about two weeks, then the mare comes back into heat," explains Gimenez. "This is because the uterus produces prostaglandin, a hormone that goes from the uterus to the ovary and results in the death of the corpus luteum." Consequently, progesterone production is halted and estrus might occur.
In approximately 10-20% of estrous cycles, says McCue, the corpus luteum that forms after ovulation is retained longer than normal and the continued production of progesterone can cause the affected mare to fail to return to estrus at the predicted time.
Treatment for this condition is simple. "All you have to do is give the mare one injection of prostaglandin, and she'll come back into heat," Gimenez says. "This works well in most mares."
Persistent anovulatory (noncyclic) follicles-"In this condition," McCue says, "an affected ovary may have a follicle that initially grows normally, but fails to ovulate. After a few days, the amount of estrogen produced by the anovulatory follicle declines and the mare will go out of heat. The mare will not come back into heat until another follicle develops, which may be ten to thirty days or more later. In addition, some anovulatory follicles begin to produce progesterone that takes the mare out of heat. This can be successfully treated with prostaglandins."
Research has shown that persistent anovulatory follicles occur in 5-10% of estrous cycles.
Cushing's disease-"This is a disease caused by a tumor of the pituitary," says Gimenez. "Although Cushing's disease is not a reproductive disease, it does affect the adrenal gland, which indirectly affects reproduction, causing the mares not to cycle."
The disease typically is a problem in horses more than 15 years of age. Besides anestrus, there are varied clinical signs that might be present. Treatment is geared primarily to addressing symptoms. "As far as reproduction goes, it's pretty much over," Gimenez says.
Racing fitness-"Maiden mares that are training or 'racing fit' often have atrophied ovaries and do not have regular estrous cycles," notes Blanchard. "Many mares off the track can arrive at the breeding farm already cycling, but those that have static (inactive) ovaries may require quite a bit of time to start cycling. Occasionally, they won't cycle the first year at the farm. No hard and fast rules here, and many practitioners have their own 'let-down' programs that they have success with. Tincture of time obviously plays a large role!"
Anabolic steroids and drugs-"Young mares in training and competition," Blanchard says, "unfortunately may be given certain drugs such as anabolic steroids, ACTH, certain anti-ulcerogenic drugs, or even estrus-suppressing drugs such as altrenogest, all of which when given in large doses or for prolonged periods may cause a delay in establishment of regular estrous cycles once the mare is retired from a performance career. For this reason, it is wise to provide plenty of adjustment time for maiden mares that have been in training; i.e., move them to the breeding farm in plenty of time to allow them to adjust to their new surroundings and environment, settle down, gain some weight, and begin regular estrous cycles."
Although much is known about the reasons and remedies for anestrus, researchers are actively involved in further examining the condition.
Says Paccamonti, "Researchers are looking at domperidone, gonadotropins, acupuncture, and other treatments for spring transition. Domperidone looks promising. Gonadotropins require repeated administration, which can be expensive and/or labor intensive and results so far haven't been that impressive. Acupuncture has reportedly been effective in other species, but no work to date has been done in this area in horses."
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.