Understanding heat cycles is key to successful breedingThe horse has the lowest conception rate of any domestic animal. This is partly because horses are bred in unnatural conditions; most mares are hand-bred rather than living in harem groups with a stallion.

Horses are seasonal breeders. Mother Nature programmed mares to conceive best in late spring or during summer so that the resulting foal would be born when weather and food (green grass) are optimum for survival and growth. But man has tried to change this, wanting mares to conceive well ahead of the natural breeding season. He has had some success, using artificial lighting to trick the mare's hormones into thinking it is spring, or using additional hormones to manipulate her cycles, but getting mares bred and settled early in the year remains a challenge.



Breeding basics

Let us look at the "normal" scenario-getting a mare bred during the natural breeding season (April-June)-to understand her reproductive cycles.

For successful breeding, timing is very important. Ovulation usually takes place about 24-to-48 hours before the end of the mare's heat period. Because mares vary in length of heat period, they have traditionally been bred on the second or third day and every other day thereafter as long as heat lasts. Breeding a mare too early (before she ovulates) will not conceive a foal. Thus, many farm managers monitor mares closely with daily palpation by a veterinarian, to determine when the developing follicle is mature enough to release its egg (ovulation). Thus, the optimum time for breeding can be pinpointed.

Some mares are obvious about their heat cycles, but others are not. Some must be teased with a stallion every day or checked to determine when they are ready to breed. Some "shy" mares become more obvious about their cycles if there is a stallion nearby and some breeding activity going on. A mare that never sees a stallion may not show obvious heats. Horses are very gregarious animals, and often "social stimulation" will cause a mare to make more outward signs of what is going on in her cycle.

Good teasing programs have long been recognized as one of the keys to efficient breeding and high rates of conception. Experienced farm managers know that some mares show stronger estrus the more they are teased, and some will not accept the stallion unless they first go through a lengthy teasing process. And now scientists have discovered that the act of teasing stimulates the release of oxytocin in the mare, setting into motion a multitude of physiological actions that enhance ovulation and transport of the egg and sperm.

If a mare does not conceive when bred, she should come back into heat about 17-to-21 days later. The estrous cycle is defined as the interval of time from first day of heat until the first day of the next heat, and the average length of cycle for most mares is 21-to-23 days, but this can vary. After being bred, a mare should be teased daily or checked frequently until confirmed pregnant.

The length of estrous cycle can vary during different seasons of the year. For practical purposes in discussing this cycle, it is divided into two parts, estrus and diestrus. During estrus or heat period, the mare shows signs of being ready to accept the stallion and a few follicles (containing the immature eggs) in the ovaries are rapidly growing and secreting estrogen. Estrogens are the hormones which cause the mare to come into heat and show sexual receptiveness. These hormones are also responsible for changes in the reproductive tract that permit passage of sperm from the stallion.

Finally, one of the follicles ruptures and releases a mature egg into the oviduct, where it can be fertilized by sperm. This ovulation usually takes place just before the mare goes out of heat. Because the egg lives only a few hours, breeding must be timed so live sperm are in the tract during this short time span.

The second part of the mare's cycle is the diestrus period, and it follows immediately after ovulation. Diestrus is the time the mare is not in heat. One thing every breeder should bear in mind is that ovulation is usually more closely related to the end of estrus than the beginning. Half of all mares are out of heat within 24 hours of ovulation, and about 80% are out of heat 48 hours past ovulation.



The hormonal regulators

The follicle within the ovary has several functions. It nourishes the developing egg and secretes estrogen to bring the mare into heat and prepare her to accept the stallion. Another function occurs after the follicle has released its egg. The follicle then becomes a corpus luteum (CL), which secretes progesterone. The corpus luteum is the structure that forms in the area where ovulation has just occurred, and the progesterone it produces keeps the mare from coming into heat. Progesterone causes a behavioral response in which the mare is not receptive to the stallion. Progesterone is also responsible for changes in the uterus that seal it off from the outside, and which prepare the uterine tissues for supporting a pregnancy. Progesterone inhibits follicular development and prevents ovulation.

The cyclic rise and fall of progesterone in the mare's blood is one of the main regulators of her heat cycles. When this hormone is above a certain level, she does not come into heat. The corpus luteum usually lasts about 14-to-16 days. Then it must cease functioning so the progesterone level will fall, allowing a new follicular phase to begin so the mare can come into heat and start her next cycle.

The corpus luteum is vital to successful pregnancy. If the mare is bred at the proper time, her egg is fertilized and becomes an embryo. If all is normal, the CL persists, producing progesterone and keeping the mare from coming back into heat (which could destroy the embryo).

After entering the uterus, the embryo signals its existence so the CL persists. Otherwise, progesterone production would begin to diminish about the 12th day and the mare would soon come into heat, flushing out the embryo. Up to 10% of pregnant mares show one heat cycle after being bred and settled. If the mare is not rebred, the embryo in some cases will survive the heat and continue into successful pregnancy. If rebred at this time, the mare will usually lose the embryo, but not always.

Most embryos that are lost die within the first weeks. The mare settles, fails to show her next heat cycle or two (and shows pregnant on ultrasound) and is considered pregnant. Then she suddenly comes back in heat again. Chances are, she was pregnant, but somewhere along the way the embryo died.

If the fetus gets through these early shaky times, another crisis occurs between the fourth and fifth months. The endometrial cups and multiple CL (which have been producing progesterone to safeguard the pregnancy) disappear. The job of producing the necessary progesterone is taken over by the placenta, which has been developing around the fetus.Once this shift is successfully completed, the pregnancy is quite stable and about the only things that can endanger it are uterine infections or twins.



Transition from anestrus

An important aspect of equine reproduction is that a mare is a seasonal breeder. Most mares stop showing heat during winter when days are short. This period of winter rest is called anestrus (without estrus). About 75% of mares show some degree of anestrus during the winter. As early as September, some mares stop cycling. By January, only 25%-to-35% are still having heat periods. Mares in the Southern Hemisphere have a pattern just opposite, since the seasons are reversed. In North America, winter anestrus occurs from November through February.

As spring days lengthen, the mares that were in anestrus start cycling again. During this transitional time, sexual behavior may vary from pronounced rejection of the stallion to prolonged acceptance. Variable degrees of both acceptance and rejection may continue for about two months, and neither the teaser nor the human handler can be sure if the mare is really in heat or not. Ovulation does not occur during the mare's transitional phase and breeding will not result in pregnancy.

By the vernal equinox (March 20), daylight and dark are of equal length. Days continue to lengthen until the summer solstice (June 21), which is the longest day of the year. The natural breeding season for most mares begins soon after the vernal equinox in late March and is at an optimum by the time of the summer solstice in June. Ultrasound images show that follicles grow largest during April, May, and June, peaking in May at 48 mm (1.89 inches). This is the easiest time of year to breed and settle mares.

The first cycles in early spring are usually erratic and the mare is not very fertile. Follicles start activity, finally becoming large enough to bring the mare into heat. But these early follicles may not ovulate. Heat periods during this transitional time are often long because the follicles persist, and fertility is low because often the follicles fail to ovulate; they develop and then recede. Some mares will stand for the stallion for 30 days or longer, yet not become pregnant. The mare has a follicle; it continues to produce the hormone that keeps her in heat, yet it never ovulates. Eventually, it either ovulates or degenerates, and the mare goes out of heat.

Other mares show erratic behavior, coming in heat for a day or two, going back out for a week, coming in for two weeks. These mares, and the ones with long heats, usually come into a relatively normal estrus during the end of this transitional period. A mare in a 45-day heat during her transition is just as infertile as a mare in anestrus, unless she does ovulate at the end of it and is bred at the right time.

Heat cycles become shorter and more normal as spring moves into early summer, and conception rates become better. By late April, nearly 80% of mares will be cycling and in June almost all will be coming regularly into heat, with properly functioning cycles. Heat periods in June are short (3-to-5 days) and getting mares pregnant during May, June, or July is quite easy.



Length of heat, erratic cycles

The length of diestrus is dependent upon the life of the CL, which generally lasts about 14-to-16 days. This can vary from 5-to-33 days, but most mares have a diestrus within a 14-to-19 day range. Length of diestrus is actually more consistent and predictable than length of estrus; you can be fairly sure most mares will be back in heat about 15-to-17 days after they go out.

About 90% of mares ovulate within a five-day span, which includes the two days prior to end of heat and the two days after. About 75% ovulate within the three-day span from the day before end of heat to the day after. About 35% of the time, ovulation occurs on the day the mare goes out. Fertility drops rapidly once she goes out, so it is important to breed close to ovulation but before she actually goes out of heat. The egg survives only about 12 hours after ovulation and must be fertilized within this time span. Sperm will usually live up to about 60 hours in the mare's tract. (Some stallions' sperm live longer than this, some not as long.).

In earlier years (before the advent of pinpointing time of ovulation by palpation or ultrasound), it was general practice to start breeding a mare on her third day of heat and every other day thereafter (as long as heat lasted), to keep live sperm in the oviduct. But it is best to breed a mare only once; breeding too often is a waste of the stallion and also makes a mare more susceptible to infection. Modern techniques have made breeding easier on mares and stallions alike, eliminating unnecessary covers.

Some mares ovulate after they go out of heat. They can still become pregnant if bred on the last day, unless they ovulate too late after heat ceases. Sometimes a mare will go out of heat unexpectedly just because she is upset, such as by abrupt changes in her routine or environment. It is not unusual for a mare that was teased and in strong estrus to later arrive at the breeding shed in a nervous state, no longer showing signs of heat, or becoming actively aggressive to the stallion.



Tricking Mother Nature

Mares in transition between anestrus and normal cycling can create a challenge because it is not known when their first ovulation will occur. The best approach here is to combine practical observations with modern science, checking on the progress of follicular development.

For breeders who do not want to wait until April to start breeding mares, the transitional period can be started early with use of artificial lighting. About 20 years ago, the use of artificial lights was initiated to encourage mares to cycle earlier, putting mares under lights in late November or early December (16 hours of light per day, keeping them under lights until about mid-April, or whenever the "natural" light hours catch up with the 16-hour regimen). Mares will then go through their 60-day transitional period in early winter instead of early spring and may be able to start cycling normally by February rather than waiting until April.

Another shortcut has been made possible with the use of hormones. Once follicular development reaches 25mm-to-30mm in size, there are ways to shorten the duration of a mare's transition chemically; by temporarily suppressing follicular growth and then allowing the rebound development of one or more follicles to full maturity (and ovulation); or by stimulating continued follicular growth. Once ovulation occurs, the mare will then be finished with transition and have predictable and fertile cycles.

For hormone treatment to work, a mare's ovaries must be active-which means she will need to have been exposed to 16 hours of light daily for at least 60 days before the use of hormones is started. A mare should be checked to determine whether her ovaries are active enough for hormones to work. Hormones should be used with caution and under the supervision of a veterinarian experienced in equine reproduction.