Anatomy and Physiology of a Mare's Reproductive System

Moody. Inscrutable. Temperamental. Cantankerous. Mares seem to get tagged with such stereotypical labels. When your female equine displays inappropriate behavior, you might hear someone snort, "Typical mare!" Your mare's nature and individual behavior are influenced by hormones, which dictate events in her reproductive system. Whether or not you ever intend to breed your mare, she is controlled by her anatomy and physiology.

As a performance animal, a mare's actions are guided by you through training. Her success as a mother is determined by her heredity and your management. You can't change her character, but you can improve her health through the environment you provide.

The Mare's Reproductive Organs

As a mammal, the mare shares the same organic makeup as other mammals. She normally conceives and carries a single foal, gives birth, and nurtures her offspring through milk. The organs of her reproductive tract are situated inside the pelvic and abdominal regions. As in other four-footed female animals, the mare's organs are mostly internal.

The mare's udder, the mammary gland, is part of her reproductive system. A bag with two teats, it provides the source of milk to nurture the foal.

In order from the most external to the most internal reproductive anatomy, the vulva is the beginning, as it forms the entrance to the interior structures. Two labia, or lips, surround and protect the vaginal opening. The labia maintain closure and protect the internal organs. Just inside is the mare's clitoris, usually visible when the mare displays sexual interest during estrus.

The internal organs provide the environment for conception, development of the foal, and parturition. The vagina, a muscle-lined tube that receives the stallion's penis, extends from the vulva to the cervix. It measures 15 to 20 cm long and 10 to 12 cm in diameter, although normally its walls are collapsed.

The cervix is the neck of the uterus. It separates the vagina from the uterus and acts as a protective barrier.

In the mare, the uterus is a hollow muscular organ, a Y-shaped body with two cylindrical horns. The inner space of the uterus, the lumen, is lined with endometrium, a mucous membrane. Ligaments suspend the uterus in place. The body is 18 to 20 cm long, and 10 cm wide, with each horn 25 cm long.

Two oviducts, also called fallopian tubes, stretch from the horns of the uterus to the ovaries. Measuring from 20 to 30 cm long, these narrow tubes transport the ova from the ovary to the uterus. One of the oviducts usually acts as the site of fertilization.

The ovaries, the female gonads, are the farthest forward reproductive organs, under the fourth or fifth lumbar vertebrae. Size varies according to the individual and the season, ranging from 2 to 8 cm long and 2 to 4 cm wide. The ovary can become larger in the spring, and smaller in winter.

The ovaries produce both hormones and ova, which become eggs when fertilized by sperm. Follicles in the ovary--present at birth as primordial follicles--develop on an ongoing basis throughout the mare's reproductive life.

Hormones Dictate Functions

The mare is seasonally polyestrous, which means her reproductive tract is active during a certain season of the year. In nature, the mare does not experience heat, or an estrous cycle, during the winter of the year. Her reproductive system goes into a rest period, or anestrus.

Most mares follow a regular cycle, which is the rhythm of the estrous cycle. After a winter of inactivity, the cycle begins just before spring, peaks in the spring and early summer, and halts by the next winter.

Spring is when the mare is most likely to conceive. Michael Riegger, DVM, of Albuquerque, N.M., said, "The highest conception rates occur in March, April, and May and are designed to put the foal on the ground when the weather breaks in the spring."

In the months of the estrous cycle, the mare is either in estrus (heat) or diestrus (not in heat). A chain of interconnected events maintains the alternating pattern of estrus and diestrus. Both the ovaries and the endocrine system control the events of each estrous cycle.

The cycle is measured from one ovulation to the next, with 21 days the average interval between ovulations. Estrus generally lasts seven days, followed by a 14-day period of diestrus.

The pituitary gland and the hypothalamus, a part of the brain, regulate the functions of the gonads. Gonadotropins are released by the pituitary gland, under the influence of the hypothalamus. These include follicle stimulating hormone (FSH) and luteinizing hormone (LH).

Starting at the time of ovulation, the corpus luteum of the ovary develops. This structure appears on the ovary at the site of ovulation and produces progesterone, the hormone that maintains pregnancy and keeps the mare out of heat. The corpus luteum synthesizes progesterone up to the 14th day after ovulation.

The uterus secretes another hormone, prostaglandin F2 alpha, beginning Day 14 or 15 after ovulation.

"The mare's body has to recognize that she's not pregnant," explained Gayle Leith, DVM, from Gilbert, Ariz.

This hormone terminates the life of the corpus luteum, the rate of progesterone falls, and the mare comes into heat to signal the beginning of her next estrous cycle. It also prompts the development of a new follicle.

FSH stimulates the development of follicles in the ovary, and the synthesis of estrogen. In the seasons of the mare's lifetime of estrous cycles, hundreds of follicles will eventually develop into ova. Certain follicles grow in each breeding season, and in each estrous cycle, one will mature into an ovum at the point of ovulation.

LH is released from the pituitary gland to stimulate ovulation. LH triggers the rupture of a mature follicle, and the follicle releases an ovum. Also called an oocyte, the ova moves through an oviduct.

Pregnant or Not?

Like other mammals, a mare becomes pregnant when an ovum becomes fertilized. The fertilized egg, or zygote, occurs when both ovum and sperm meet at the optimum time in one of the mare's oviducts. The zygote develops into an embryo, to grow into a foal ready for birth.

Most horse owners breed a mare prior to ovulation, from 24 to 48 hours prior to the rupture of the follicle. The stallion's sperm arrive in the oviducts four hours after breeding. They attach to the surface and live there until one meets an ova. Sperm survive in the oviducts for approximately 48 hours.

The zygote moves into the uterus about the sixth day after fertilization.

"It's too small for us to see on an ultrasound," said Leith. "Between Days 6 and 15, the embryo moves back and forth, going the length of a football field. This movement is unique to the horse, letting the uterus know that the embryo is there."

The uterus responds to the embryo's free-floating movement by changing the function of the estrous cycle. Dirk Vanderwall, DVM, PhD, of Colorado State University, noted, "The embryo is responsible for blocking uterine secretion of the hormone prostaglandin F2 alpha. This will occur up to seven days after ovulation."

If the uterus does not recognize the embryo's existence, it releases prostaglandin and blocks the ovary's production of progesterone. Without progesterone to maintain the pregnancy, the embryo dies.

Age affects the efficiency of the mare's reproductive system. Citing recent statistics, Terry Blanchard, DVM, of Texas A&M University, reported, "The fertility of mares stays pretty flat till about 10 or 12 years of age. After she starts to get into her teens, there's a decline in her fertility. Those mares that produce desirable offspring will remain in the breeding herd."

A mare reaches puberty at the average age of 18 months, or as early as 10 and as late as 24 months. Although a filly can conceive at an early age, like a teenager she may be too immature to withstand the pressures of motherhood.

The older mare, a veteran of many pregnancies, shows loss of uterine firmness. Vanderwall said, "In the last 10 years, we have begun to understand why fertility starts declining in older broodmares. There's a dramatic loss after 15 to 18 years of age. It's more severe in the mare 18 to 21. Embryo transfer research indicates that the eggs produced by older broodmares may be less viable than the eggs produced by younger mares."

Riegger noted that the mare's condition also improves or inhibits the chances of pregnancy.

"A mare who's lean, in good condition and good health, is obviously going to have a higher conception rate," said Riegger. "You presume a fat, old, out-of-shape horse will probably have a lower fertility rate. But as soon as you say a fat old mare can't get pregnant, then she will."

The technique of flushing can improve a mare's likelihood of conception. In this approach, the horse owner keeps a mare in a lean condition through the winter, and then accelerates her feed ration in the spring. As the mare resumes her estrous cycle, she's more likely to conceive.

The mare's estrous cycle is also subject to the effects of climate and seasons. This presents a problem to the breeder who seeks a foal every year from a valuable broodmare. With an 11-month gestation period, you expect the mare to conceive according to the breeding season, not her natural season.

The length of daylight hours affects the pituitary gland. Vanderwall explained, "We use lights to start the season as early as Feb. 15. Because of that, in order for her to foal each year within that defined breeding season, we only have about 30 days in the post-foaling period to get that mare pregnant so she'll foal approximately the same time next year. That doesn't always occur, and we find that typically the mare may start having her foals later in the year in successive breeding seasons."

The cumulative effect can result in the owner having to skip one breeding season on a mare. She sits out one year as barren, which allows you to start breeding her earlier the following year.

Normal Physiology and Behavior

Your mare might or might not display the events of her estrous cycle. Typically, she will demonstrate that she's in heat by showing interest in the stallion, or even to geldings or other mares. Her sexual interest is often the only indication that her reproductive system is functioning normally, proving the well-being of her ovaries.

If you want to breed your mare, you observe her estrous cycle and record the data. This includes when she starts cycling each year, and the length of her cycles.

"Knowing that she starts coming into heat in March or April is very helpful," said Riegger. "Knowing how often she cycles is very helpful. She may be a 16-17 day cycler, or a 23-25 day cycler."

Some mares undergo a silent heat, where they won't display any signs of estrus. Or, a mare stabled by herself might not show.

Riegger suggested, "Get in the habit of examining the mare. Look for physical evidence that she's cycling, by scratching her a little under the tail, and looking for signs of vaginal milk around the lips of the vulva."

Another challenging mare is the one which appears to be in constant heat. This type might actually follow a usual 21-day cycle, although her behavior doesn't change. The veterinarian will have to check her through palpation to determine when she is actually ready to ovulate.

If you don't plan to breed your mare, an internal checkup of her reproductive system isn't necessary. Riegger said, "She doesn't need a reproductive exam as a routine part of an annual physical. But when you do the annual, the owner might say, 'She's gotten aggressive,' or 'She's mean,' or 'She's cycling all the time.' Such observations may indicate a problem such as cystic ovaries or tumors."

A mare's reproductive system--especially her estrous cycles--can affect her athletic performance. Mares don't have PMS, but some mares earn "witchy" reputations at that time of the month.

"Some mares are very much distracted when in estrus," said John W. Paul, DVM, MS, of Hoechst-Roussel Agri-Vet Com-pany. "Some become rank, and some become totally unmanageable. It's due to that surge of estrogen in estrus, as compared to a stallion that doesn't go through cyclic behavior."

However, mares have proven themselves at the highest levels of equestrian sports. For example, several mares have competed for the U.S. Equestrian Team in three-day eventing. Bally Cor won a total of four gold medals at the 1975 Pan-American Games and 1976 Olympic Games. Gold Chip competed in the 1978 World Championships and 1980 Olympics, and Nirvana II was on the team at Atlanta in 1996.

Two California trainers praised mares as athletes. In 1996, Patrona competed in her fifth season at the Advanced level of combined training. The 11-year-old Thoroughbred was the highest-placed American horse at the 1992 Barcelona Olympics, in the three-day event.

Rider Jil Walton said, "I really prefer mares. We have our differences, but when it comes down to it on cross-country day, she will go the extra mile. She will run when she's hot, like she did in Barcelona. She just gives and gives and gives."

Walton noted that Patrona doesn't act differently when in heat. "She's always the same and is very steady that way. It's always worth it--if you can get a mare on your side, you're there."

Another eventer, Natalie Rooney, currently competes two mares. "They're both totally normal when they're in season," she said about Rafabi, a 12-year-old Trakehner, and Fine Fields, a 6-year-old Irish Thoroughbred. On Rafabi, Rooney won the Gold Medal at the 1994 North American Young Riders Championship. She is aiming toward qualifying one of her mares to compete at the 2000 Olympic Games.

She compared the presence of a mare to that of a stallion. "I love mares. When they're good, they're better than any gelding can be, because they throw extra into it. They have so much personality."

Reproduction and Fertility

Whether an athlete at the highest caliber or a pleasure horse, if an owner wants to breed a mare, the ultimate question revolves around the chance of the mare delivering a live, healthy foal. The mare should first prove her state of health, or freedom from any disease or condition that impacts her fertility, conception, and gestation.

Many factors influence the mare's ability to function reproductively. One important factor is genetic--did she inherit from her ancestors the potential to be a good broodmare? Every mare inherits reproductive conformation, along with other characteristics. In a broodmare, the capacity to nurture the unborn foal is crucial to her motherhood.

Unfortunately, the domestic horse does not have a high rate of fertility. A recent study of Thoroughbreds showed that the average live-foal rate was 65%.

"Traditionally, the group of mares which are the most fertile are the foaling or wet mares," clarified Blanchard. "Right behind them, running a close second, are maiden mares. Finally, the barren mares have a markedly reduced pregnancy and foaling rate. They may have an acceptable pregnancy rate, but they're much more likely to lose the pregnancy."

Prior to breeding, a mare should be in good body condition. If she has been in strenuous training, she needs to be given time to adjust to changes in activity and handling. A fit, active horse has to be let down from the stress of performance before conception.

To carry the foal to term, the mare's uterus must be viable. Infections are one cause of spontaneous abortions. Also, the position and tone of the vulva can predispose a mare to infection. A vulva that's tipped forward signals a potential problem. The Caslick's operation, or suturing the lips together, is geared toward preventing the introduction of air and bacteria into the reproductive tract. Some horse owners routinely use this method to prevent or restrict infection.

Veterinary Care

In a clinical examination, the practitioner looks at the mare's overall condition. He then checks the external genitalia, to see if the vulva is healthy. The breeding soundness examination involves checking the status of internal organs through rectal palpation. Wearing a surgical sleeve, the equine practitioner inserts a hand into the rectum, past the anal sphincter, to examine the mare.

"The first thing you feel for is the mare's attitude," explained Riegger. "When you go in, patience is the number one rule. You carefully and gently move ahead, and you do have to take time."

Some practitioners prefer to restrain a mare in stocks or a chute, or use a twitch. Riegger said, "I'm in the minority. I sneak up alongside a mare and sweet talk her into the procedure, if at all possible." Even when he administers a tranquilizer to relax a tense mare, he still eases into the examination. "The more uptight or tense the horse is, the more likely you are to have a catastrophic accident, which is a rupture of the rectum. You don't try to force your hand up the mare's rectum."

In an examination, the veterinarian feels the internal organs. Knowing the mare's reproductive history helps him or her to relate the size, shape, and texture of the uterus and ovaries. The experienced practitioner, having felt hundreds of organs, can recognize the unusual. He also notices the soundness of the cervix, which can become damaged in foaling. During estrus, the cervix relaxes and softens, to open a path for the sperm.

The practitioner detects the well-being of the ovaries and the state of the mare's estrus through palpation. In the rectal examination, the veterinarian can feel the development of the follicle. Leith described the follicle as "a bubble-like structure on the ovary." She explained, "You can have the veterinarian palpate a mare every other day to feel what her ovaries are doing. I can take the ovary in my hand and gently feel it with my fingertips. I feel how big the egg is getting as it grows in the follicle.

"You breed the mare when the egg is 30 to 35 mm in size. Your vet might say, 'Your mare has a 25 mm follicle.' That's growing, but it's not big enough yet. You want to breed the mare once, with the goal of efficiency."

Ultrasound is an important tool for the pre-breeding assessment. By using ultrasonic waves to image internal structures, the practitioner can view the interior of the uterus. This diagnostic tool can produce images either through a rectal probe, or a transabdominal view.

"You can look at the female reproductive organs with ultrasound," said Riegger. "You can see if they look normal. Is there something going on in the ovaries, uterus, or cervix that we need to look into?"

In a workup, the practitioner performs a thorough medical examination. The means of further internal examination might involve imaging through ultrasound, or lab tests using a culture or biopsy. Whatever the course of action, the mare's history--her previous breeding record and type of performance--is crucial to the practitioner's evaluation of breeding soundness.

"If she's 15 years old and has never been bred, conception's not too likely," said Riegger. "If she's a 5-year-old and never been bred, she's probably going to conceive without a problem. But if she's been bred 15 times without a foal, she's probably not going to get pregnant."

The veterinarian might decide to examine the mare's reproductive organs with the aid of a vaginal speculum. This tool allows for inspection of the vagina and cervix, and lets the practitioner obtain tissue samples of the endometrium from the uterus.

Health depends on the well-being of the organs, and a major problem in the mare is uterine inflammation, or metritis. Mares often get such infections through breeding and birth. The condition can be a low-grade inflammation, or increase to fulminating. Infections result in a hostile environment for sperm, thus inhibiting fertilization. Infections also can result in scars and possible blockage of the reproductive tract.

Metritis can be acute (a rapid onset followed by a short, severe course), subacute, or chronic (long-lasting). Some mares have contracted a metritis known as Contagious Equine Metritis (CEM), which is a highly contagious venereal disease.

Laboratory tests help identify the state of the mare's reproductive health. The veterinarian might decide to examine tissues from the uterus in order to search for infection. As a standard practice, the practitioner might take a culture by inserting a swab into the uterus to wipe against the endometrium. In the lab, the resulting culture is examined to see if specific microorganisms grow that indicate infection.

Riegger defined diagnosing through a culture as "misleading--if it's negative, it may not be negative. If it's positive, it may not be positive. It should be interpreted very carefully when making a judgment." He noted that a fulminating infection will obviously show up positive, but others might be missed.

A biopsy, which removes a small section of tissue, is another method to investigate the integrity of the endometrium. However, like a culture, it brings only a small area outside for inspection.

The veterinarian "pinches" out a sample from the endometrium of the uterus. "You take a tiny piece of tissue, probably the size of the white of your nail," said Leith. "From that, we can grade the mare's chance of conceiving and carrying a pregnancy." The grades, assigned what is known as the Kenney category, evaluate a mare's breeding potential according to the diagnosis of the endometrium.

Another diagnostic method is endoscopic examination. With the endoscope, the practitioner can visually scan the interior of the uterus. "It's much more precise," said Riegger. "A biopsy is done blindly. With the endoscopic examination, you may see a polyp, or another section where something is going on. That's where you take your specimen."

Veterinary Intervention

Humans can control the mare's estrous cycle through pharmacology. Hormonal treatment affects the ovaries or the pituitary/hypothalamus, and it can induce estrus or ovulation. The pharmaceutical drugs manufactured for this purpose are available through prescription only. Their use is primarily to treat problems that affect the estrous cycle, although some horsemen incorrectly use some drugs to enhance performance. This usage can result in an imbalanced reproductive system.

Riegger noted that mares off the track, and some show horses, have a strong likelihood of having been administered various medications. Such a mare might need a recovery period of nearly 12 months before she resumes a normal estrous cycle. He added, "You know a mare's got trouble if she's mounting everything else in sight. With some mares I've purchased, it's taken about a year for them to come off the anabolic steroids."

Veterinarians prescribe hormonal treatments to adjust the cycle to improve the mare's chance of conception. For example, one fertility problem is persistent corpus luteum, treated with prostaglandins. Such administration must be closely supervised.

"These are not innocent drugs," said Riegger. "They do have effects. I'm not saying they're dangerous, but theoretically they're supposed to be used during very specific times."

Vanderwall noted, "We give the mare a shot of hormones (prostaglandin), and we are doing the same that her uterus does in a non-pregnant animal. When the uterus secretes it, progesterone falls and the mare comes back into heat as the next estrous cycle begins."

The practitioner can combine the examination with hormonal treatment, to time either natural cover or artificial insemination with the mare's ovulation. Leith noted, "Once the follicle is big and ready to ovulate, we give the mare a shot of human chorionic gonadotropin (HCG). That makes the follicle release the egg within 36 hours." HCG, an artificial hormone, ends estrus while stimulating ovulation. However, practitioners note that such treatment can backfire, even ending a mare's cycles for the season.

A synthetic progestin (altrenogest, marketed as Regu-Mate) is used to manipulate the cycles of breeding and non-breeding mares. Progesterone can take the mare out of heat, in any stage of her normal estrous cycle.

Paul explained this drug's strategic use for a performance mare, as prescribed by the treating veterinarian: "The veterinarian should discuss the upcoming show circuit, and recommend treatment. We recommend 15 days use. Then, with a week or 10 days before the next show, take Regu-Mate off and let the mare come into estrus.

"The mare ovulates at the end of the cycle, and she produces the corpus luteum and her own progesterone. You can take advantage of that free progesterone for two weeks. As you approach the end of 14 days, go onto another course of Regu-Mate."

This strategy results in the mare being in heat for seven days of a 40-day period. She is out of heat for 15 days (through the drug) and 14 days of naturally produced progesterone. "This reduces the risk," said Paul. "We believe there's a potential risk, because we've circumvented what Mother Nature intended."

Many equine practitioners agree in tampering as little as necessary with the mare's basic physiology. Like any other horse, the mare thrives on regular nutrition and grooming. She also needs to socialize with companions in an outside setting.

Riegger summarized, "You can't beat Mother Nature. The dog, cat, and horse have been here a lot longer than us, and they get along nicely without us."

About the Author

Charlene Strickland

Award-winning writer Charlene Strickland lives in Bosque Farms, N.M. She has published 8 books and over 600 magazine articles, and is a member of the International Alliance of Equestrian Journalists.

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