Estrous cycles and certain disease processes can affect mares’ reproductive hormones; fortunately, veterinarians have ways to quell resulting behaviors.

Many owners stand by the statement that there’s nothing quite like a hard-trying, intelligent mare. But if you ask for her full attention and cooperation while she’s showing signs of estrus (heat), good luck! Whether they’re causing unsavory behavior in the show arena or preventing a broodmare from coming into estrus, a mare’s hormones can present challenges for owners. With a veterinarian’s help, however, owners can either prevent their mares from cycling or make mares’ estrous cycles more predictable. Let’s first review what estrus is and then explore conditions that can affect a mare’s cycle and how to treat them.

Signs of Estrus

A mare in heat typically exhibits gait abnormalities, raises her tail, urinates repeatedly, and interacts unpredictably with people and other horses. This estrous behavior normally lasts five to seven days, becoming more intense as the ovarian follicles increase in size and produce more estrogen. As the mare ovulates, she goes out of heat and structures on her ovaries begin producing the hormone progesterone (which prepares the uterus for pregnancy). This quiescent period when the mare is not receptive to a stallion is called diestrus, and her behavior can vary from ear pinning and unwillingness to cooperate to kicking, squealing, and striking at other horses. A mare should normally be out of heat for 14 to 15 days; this is the most consistent period in the estrous cycle, so any deviation should alert an owner to have a veterinarian examine the mare.

That Time of the Year

Horses are seasonally polyestrus, meaning their estrous cycles depend on the ¬season. The length of a mare’s natural breeding season varies, but for the northern hemisphere it typically starts in March/April and extends until October/November. Primarily, the amount of daylight hours regulates a mare’s reproductive activity: As day length increases in the spring so does ovarian activity, and vice versa when day length decreases in the fall. This seasonality results in transition periods in the spring and late fall when a mare can show signs of heat for weeks at a time. Transition heats can frustrate owners and riders, as mares are often unpredictable and unresponsive to ovulatory drugs during this time.

Upon veterinary examination, a transitional mare’s ovaries typically have multiple small- to mid-sized, grapelike follicles. They might also possess one or more large hemorrhagic (ovulating) follicles. By definition, the mare’s transitional period ends after her first ovulation of the year. It’s possible to hasten her body into predictable estrous cycles sooner by tricking it into thinking it is later in spring; this is generally done by exposing mares to increased periods of daylight, up to 14 hours per day. There are two primary reasons to shorten this transition period: 1) to begin breeding early so the mare foals early the following year; and 2) to cause a show mare to begin having a predictable cycle earlier in the spring for behavior purposes.

The simplest way to expose mares to increased daylight is to house them in well-lit (artificially) stalls from dusk until midnight. If mares are placed under lights in early December, most will ovulate eight to 12 weeks later. Another method of shortening the transition period, which can be used in addition to supplemental lighting, is to administer oral or injectable progesterone for 10 days. This method is only effective if a veterinarian determines the mare has adequate-sized follicles on her ovaries, indicating she’s ready to ovulate. After this ovulation a mare should continue having predictable 21-day cycles until the fall. During late fall about 80% (according to published research in Veterinary Clinics of North America Large Animal Practice) of mares transition into a quiescent period as daylight hours dwindle.

Abnormal Cycles

Some disease processes can affect mares’ reproductive hormones, resulting in inconsistent or extended estrus. Etta Bradecamp, DVM, Dipl. ACT, ABVP, of Rood & Riddle Equine Hospital, in Lexington, Ky., says potential causes of abnormal cycles include ovarian tumors, uterine infection, equine Cushing’s disease, and poor ¬nutrition.

Uterine infection "The most common cause I see of a mare having erratic cycles is a uterine infection, either acute and chronic," says Bradecamp.

Veterinarians see acute infections more frequently in mares that have been bred or have foaled recently. They rarely find them in young maiden (never been bred) mares. Older mares or any mares with poor perineal conformation (that of the region encompassing the anus and genitalia), however, could be susceptible to either acute or chronic infections.

In both cases, excessive fluid and inflammatory cells accumulate in the uterus, which can cause a mare to "short cycle." This limits the time she is out of estrus and results in more frequent heats.

If you suspect your mare has a uterine infection, based on clinical signs such as frequent cycling, vaginal discharge, and shortened diestrous period, have your veterinarian evaluate her reproductive tract. He or she can perform an ultrasound exam to determine if there is fluid or inflammation in the uterus, obtain a uterine culture to evaluate the cause of inflammation, and formulate a treatment plan based on diagnosis. The veterinarian likely will lavage (flush) the uterus and infuse an appropriate antibiotic into it for several days.

Granulosa cell tumor Another issue affecting mares’ cycles involves reproductive tract tumors, usually granulosa cell tumors (GCT) of the ovary. Veterinarians have reported these tumors in all ages and breeds, but they detect them most commonly in ¬middle-aged and older mares.

A GCT causes an increase in various hormone levels; this imbalance can affect a mare’s reproductive behavior. Mares with GCTs exhibit three types of behavior, depending on the amount and type of hormones the tumor produces: 1) prolonged anestrous behavior (not showing signs of heat); 2) stallionlike behavior and aggressiveness; or 3) persistent or intermittent periods of estrous behavior.

Initially, performance horses with GCTs might show signs of mild lameness or a change in attitude. Owners might also cite back pain or saddle fit problems, because as the GCT grows in size, it can place pressure on the ligaments suspending the ovary in the abdominal cavity. Riders might notice a sore back, abdominal pain, resistance to pressure on the flank, and/or reluctance to move forward at speed or when jumping. All of these complaints should initiate veterinary investigation.

Veterinarians diagnose a granulosa cell tumor based on rectal palpation, ovarian ultrasound, blood hormone levels, and these behavioral changes. The affected ovary is generally larger than the unaffected one. This size difference is due to tumor growth and production and secretion of high levels of the hormone inhibin, which inhibits the other ovary’s follicular development.

After determining a mare has a GCT, a veterinarian should surgically remove the affected ovary. The approach used depends on the tumor’s size. Brett Woodie, DVM, MS, Dipl. ACVS, a surgeon at Rood & Riddle, says most affected ovaries can be removed via a flank approach during standing sedation. The ventral (toward the belly) approach, on the other hand, requires general anesthesia and is reserved for large ovaries that cannot be removed through a flank incision. Some surgeons now use less-invasive laparoscopic surgery. Most mares cycle and behave reproductively normal after three to 12 months, and they can conceive and have a normal pregnancy with one ovary.

Manipulating Estrus

If a mare has a history of being difficult to handle or unwilling to perform while she is in or out of heat, a veterinarian can offer methods to help prevent or limit the behavior. First have your veterinarian evaluate the mare’s reproductive soundness and estrous cycle stage; it’s important to rule out reproductive abnormalities before moving forward with manipulation methods, and some approaches depend on the stage of the mare’s cycle to be successful. It is possible to prevent a mare from coming into heat, lessen the severity of behavioral changes as she comes into heat, as well as shorten the time she is in estrus.

Mares can also ingest progesterone (pregnancy hormone) supplements that suppress estrous behavior. As a mare goes out of estrus her natural progesterone levels rise, preventing her from showing signs of heat. Traditionally, Regu-Mate (an oral synthetic progestin) has been the most widely used method of estrus suppression. Clinical trials using Regu-Mate have demonstrated that it suppresses estrus in approximately 95% of treated mares within three days of administration. Caretakers administer Regu-Mate daily and can stop giving it at any point, at which time a mare should come into estrus within five to seven days. Wear protective gloves when handling Regu-Mate, as the solution can be absorbed through human skin and adversely affect women’s hormone activity.

Other forms of supplemental progesterone include a daily injectable solution or a long-acting injectable given every 10 days up to a month. The longer-acting formulations can be very effective in keeping mares out of estrus and last for seven to 10 days, but these can cause soreness and fibrous tissue development at injection sites.

Injectable medroxyprogesterone (Depo-Provera), a synthetic progestin, can be effective for 60-90 days. Its advantages include infrequent administration and a relatively low incidence of injection site reactions. However, studies have shown this product to be only sporadically effective.

Bradecamp says placing a sterile marble in the uterus near the time of ovulation can keep a mare out of heat for up to 90 days. She notes that this method does not work in all mares, but it’s an inexpensive approach that does not require administering drugs or injections. A permanent method of preventing estrus is an ovariectomy, or surgical removal of the ovaries.

Take-Home Message

A mare that misbehaves periodically or is unpredictable in her interactions with other horses due to her heat cycles can benefit from estrus suppression. Several methods have been proven effective, and a veterinarian can help determine if your mare is a candidate for hormone therapy to suppress such behavior.