Open Mares

If your mare isn't in foal, there could be many causes. It takes the examination of an experienced veterinarian to help solve the myriad of problems that can afflict mares.

Photo: Anne M. Eberhardt/The Horse

There are few occurrences in the equine world more frustrating and disappointing than to breed for that foal of your dreams and have the mare come up empty. Unfortunately, this happens frequently and the reasons are many and varied--ranging all the way from an infertile stallion to a mare with serious reproductive problems.

Modern technology and research have helped solve the problem--also in a variety of ways. Solutions include embryo transfer, oocyte transfer, gamete intrafallopian tube transfer (GIFT), injection of a single cell into an egg, the use of drugs such as Ovuplant and Regumate...the list goes on.

Unfortunately, some of these procedures are still in the developmental stages and others, which are utilized with some frequency, are expensive. Embryo transfer is an example. Its use is just short of being commonplace at major breeding establishments, but it is on the expensive side for the backyard breeder. A foal at weaning time, it has been estimated, should be worth between $10,000-$15,000 to justify utilization of the procedure. That can be a bit high for the person who just wants to breed on a favorite bloodline for a future trail riding horse.

So, one might ask, what is the average breeder to do to ensure a pregnancy and resultant foal? There are a couple of basics that almost all equine practitioners will espouse. Make certain to the best of the veterinarian's ability that the mare is reproductively healthy. Second, make certain that the stallion is fertile and, if shipped cooled semen is to be used, that his semen can withstand the rigors of cooling and shipping.

With those two basics out of the way, it is time to learn about some of the leading causes of infertility in mares. What are the most common causes?

That question was put to Mats Troedsson, DVM, PhD, Dipl. ACT. He was asked to present a hit list of what he considers the most common causes of mare infertility/subfertility. He listed the following five in order of frequency of occurrence:

  1. Persistent breeding-induced endometritis.
  2. Uterine pathology characterized by fibrotic changes to the endometrium.
  3. Placentitis, a cause of late-term abortions.
  4. Breeding at the wrong time.
  5. Old maiden mare syndrome.

We will take a look at Troedsson's top five culprits and add some others, as well.

Breeding-Induced Endometritis

Troedsson has collaborated on a great deal of research involving breeding-induced endometritis, and he has presented papers on this subject and others involving reproduction at American Association of Equine Practitioners' conventions and Society of Theriogenology annual gatherings.

Just what is breeding-induced endometritis? When mares are bred, either by natural cover or artificial insemination, it is normal for them to develop endometritis (inflammation of the endometrium, the membrane lining the uterus) in the wake of breeding. There are a couple of reasons for this. The mare's body reacts to the deposited sperm as it would to the introduction of any other foreign body. Her defense mechanisms are summoned to rid the body of the foreign substance.

This works to the mare's reproductive advantage. By ridding the uterus of excess semen and inflammatory products, a hospitable environment is established prior to the descent of the fertilized embryo from the oviduct five to six days after ovulation.

In most mares, the inflammation quickly disappears, but in others it does not. It is a persistent condition. This is known as delayed uterine clearance (DUC). Mares with DUC often retain fluid within the uterus that can serve as a source of continued inflammation and also a site where invasive bacteria can set up shop. When this is the case, a hostile environment has been established within the uterus, which can result in early embryonic death.

One of the tools in a veterinarian's arsenal to treat DUC is oxytocin, which causes the uterus to contract and expel the fluid. Uterine lavage is another tool.

Breeding-induced endometritis is not uncommon. One study involving Walter Zent, DVM; Troedsson; and Jin-Lian Xue, DVM, PhD, was reported at the 1998 AAEP convention. Researchers examined 411 mares from seven farms over 746 estrous cycles. Transrectal ultrasonography was used to examine the mares for the presence of intrauterine fluid the day after breeding--14 to 30 hours post-breeding.

The incidence of uterine fluid the day after breeding was 15.6% for all examined cycles, or 13.7% of the 411 mares. A moderate to large amount of fluid was detected and the mares were subsequently treated in 11.4% of all examined cycles, or 9% of the mares involved in the study.

The researchers found that barren mares were more likely to accumulate fluid than were maiden and foaling mares. Also high on the fluid accumulation list were mares with a history of troublesome births (dystocia). It also was found that some stallions had a greater percentage of their mares accumulate fluid than others.

Additional research at the University of Minnesota resulted in another finding, according to Troedsson. The retained fluid did increase sperm mortality and decrease motility, which is considered highly important in sperm being able to fertilize an egg.

One of the solutions in cutting down on the amount of retained uterine fluid, Troedsson says, involves breeding the mare with less frequency. He said it has been determined that in normal mares, the post-breeding inflammation peaks at 12 hours and subsides within 24-36 hours. This means that the semen from breedings performed every second day should not be affected by the inflammatory environment that results from a previous breeding.

However, he says, it is relatively commonplace to breed again within six to 24 hours with cooled or frozen-thawed semen. When that is the case, according to Troedsson, the inflammation can persist and decrease the motility of sperm.

"Breeding mares repeatedly within 24 hours may have a detrimental effect on the inseminated semen," he says.

He added, "Seminal plasma has an important role in modulation of breeding-induced inflammation and sperm elimination from the female reproductive tract. Maintaining some seminal plasma when semen is processed for cryopreservation might reduce the detrimental effect on sperm cells in the uterus when mares are re-inseminated repeatedly."

Uterine Pathology

Troedsson had this to say about fibrotic changes that often afflict older mares which have had numerous foals (multiparous): "Degenerative changes of the endometrium, such as periglandular fibrosis and glandular dilation, are often seen in older, multiparous mares. They are associated with susceptibility to persistent endometritis and may result from repeated uterine inflammation. However, the conditions have also been observed in older mares without any known history of endometritis, suggesting that degenerative fibrosis of the endometrium can be a process of aging rather than inflammation. Sclerotic changes (hardening) in the uterine vascular bed impair blood flow to both the endometrium and myometrium (the smooth muscle coat of the uterus that forms the main mass of the organ), and may be a predisposing factor to uterine fibrosis. The condition may lead to insufficient nutrition to the embryo and is often the cause of pregnancy loss in these mares."


While breeding-induced endometritis is often the cause of early embryonic death, placentitis (infection within the placenta) frequently is implicated in late-term abortions.

"In a retrospective study of more than 3,000 diagnosed abortions," Troedsson reported, "placentitis was found to be the most common cause of later term pregnancy loss. Placentitis in the mare is most often caused by an ascending infection that enters the uterus via the cervix, but the route of infection can also be hematogenous (produced by or derived from the blood)."

Troedsson presented a paper on the subject at the annual conference of the Society of Theriogenology in 2000. Clinical signs of placentitis, he told his listeners, include udder development, premature lactation, cervical softening, and vaginal discharge. However, he added, abortion can also occur when these clinical signs are absent.

One of the prime tools in determining if placentitis is present in the pregnant mare, says Troedsson, is ultrasonography. Its use, he says, enables the practitioner to determine whether there is a thickening of the placenta. In a field study on a commercial Thoroughbred farm, Troedsson told the group, it was found that an abnormal increase in the combined thickness of the uterus and placenta often portended placental failure and pending abortion.

Research on treatment protocol Troedsson and Zent conducted, which concluded in 2003, showed that transrectal ultrasonography on a regular basis of mares at risk, and the treatment of afflicted mares with antibiotics (as indicated by ultrasound), resulted in the majority of the infected, treated mares producing viable foals.

Breeding at the Wrong Time

Ironically, one of the advances in breeding--artificial insemination--can be at the root of this problem. The ever-increasing use of artificial insemination is resulting in situations where mares are inseminated without having been teased, Troedsson says. It is not unusual that these mares are misdiagnosed to be in heat based on a large diestrus (not in heat) follicle.

Underlining Troedsson's belief that a good teasing program is essential is Edward L. Squires, PhD. In an earlier interview, he had this to say about the importance of teasing:

"Inadequate or improper teasing constitutes a major cause of poor reproductive performance in mares. Normally, cycling mares should be teased daily with at least one stallion. Mares that have not achieved normal cycles and mares exhibiting the first day of diestrus should be teased with two stallions. The mare should be teased by the stallion head-to-head, and at the buttocks and external genitalia."

Old Maiden Mare Syndrome

Here's a scenario for this syndrome. A mare has served well during the first half of her life as a valued pleasure horse or show partner. She's passed her 10th birthday and you decide you'd like to have a foal to carry on her bloodline. You breed her, but she doesn't get pregnant. You have her examined by a veterinarian. She appears to be reproductively sound. Yet she doesn't become pregnant, even after repeated breedings.

"The cause of this condition is not well understood," Troedsson says, "and it is more a clinical observation that older maiden mares--10 years and up--can be difficult to breed successfully. Data from H.A. Schoon's laboratory at the University of Leipzig suggests that these mares may have an underlying vascular (blood vessel) pathology that may interfere with endometrial function."

Another problem with older maiden mares is cervical incompetency. In these mares, the cervix typically does not dilate normally, which can result in accumulation of fluid in the uterus. It has also been hypothesized that in a small number of mares bred by live cover, there is actually a problem with sufficient sperm numbers being present in the uterus due to the cervix not dilating properly.

Additional Problems

Unfortunately, there are other problems that can cause at least temporary infertility in the mare. Following are some of them and suggested solutions from a variety of researchers:

Failure to cycle at the appropriate time in the spring--One solution involves providing artificial light 60 days ahead of the intended breeding season. (For an in-depth discussion on the use of lights for mares, see Let There Be Light! on

Hormonal insufficiency--Prior to 80 days gestation, if a mare fails to produce adequate progesterone, this could contribute to fetal loss, but the true incidence of this is unknown. Supplementing progesterone under the advice of a veterinarian could be a solution.

Urine pooling--This more frequently occurs in older mares which lose muscle and ligament tone along the reproductive tract as a result of age and frequent foalings. When this occurs, urine is pooled within the vagina and enters the uterus during estrus, causing uterine inflammation and thus compromising the mare's ability to become pregnant. Several treatment protocols are utilized to solve the problem, ranging from simply removing the urine to lavage and even surgery to help the mare evacuate urine.

Cysts and scar tissue--Again, problems with uterine cysts and scar tissue are more common in the older mare and sometimes are the result of numerous pregnancies. Laser surgery has been successful, particularly in the removal of cysts. There is no treatment for scar tissue.

Fungal endometritis--This occurs when a fungus or yeast enters the uterus and causes an infection. Drug therapy generally is advocated, but treatment of fungal endometritis is difficult. Many mares will clear the fungus with treatment only
to come back with a bacterial uterine infection.

Fescue toxicosis--Mares ingesting fescue with fungal endophytes late in pregnancy often suffer from placental abnormalities that can result in foals born dead or severely weakened. Prevention involves removing mares from the infected fescue. Treatment often consists of administering domperidone 10-15 days before the expected foaling date.

Take-Home Message

If your mare isn't in foal, there could be many causes. It takes the examination of an experienced veterinarian to help solve the myriad of problems that can afflict mares. Even if you have a mare which isn't destined to be a broodmare, she can suffer from some of these problems and might need help to be at her healthiest.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners