Early foal losses in mares


The extrinsic factors in mare reproduction are those that can be manipulated.

The extrinsic factors in mare reproduction are those that can be manipulated.

They can be influenced from a management perspective. "Sorting out how important each of those factors is in an individual mare and her case of embryonic loss is not that easy," Barry Ball, DVM, Ph.D, University of California, Davis, suggests.

A number of studies indicate a significantly reduced fertility, increased embryonic and early fetal loss of foaling mares bred on the first postpartum estrus (foal heat) compared with foaling mares bred at later estrous periods.

Foal heat and lactation

Feces in vagina

Higher incidence of embryonic and early fetal loss has also been seen in lactating versus non-lactating mares.

"There have been suggestions for a long time that mares bred in foal heat were a lot less fertile and also had a higher incidence of embryo loss," Ball explains. "That's not clear from every study. Some say that it is, and others suggest that there is no effect on embryo loss. To me, there is consensus that there is higher embryo loss associated with foal-heat breeding. It might be related to the health of the uterine environment in the immediate post-partum mare. The possibility is that things are not clear in the uterus prior to breeding."

Maternal stress has been proposed as a factor in embryonic or early fetal loss in mares.


The role of maternal stress and its possible relationship to progesterone concentrations and embryonic loss in mares requires further investigation.

Stress is thought to decrease progesterone secretion, which terminates pregnancy. Stressors include excessive heat, van transport, pain, social situations and violent weather. In regard to van transport, work done at Colorado State University has not shown any detrimental effect of routine transportation of mares during early pregnancy.

A poor plane of nutrition, especially lower protein and energy levels, has been shown to be linked to embryonic loss in mares. It was shown that mares on poor quality pasture had more embryonic losses between days 25 and 31 than did mares that received a supplement or better quality grazing.


There are several nutrients, including vitamins A and E that are lower under such circumstances. Mares with poor body condition due to poor nutrition had lower pregnancy rates at 30 days, and had more pregnancy losses noted between days 30 and 90.

In another study, 90-day pregnancy rates were lower in mares that were in thin body condition at the start of the breeding season than in mares in better body condition. Mares that were not in good body condition at the time of foaling had reduced fertility, and suggested a higher embryo loss rate.

After Caslick's procedure

Prevention, management of early pregnancy loss

Some management practices can help prevent early pregnancy loss.

Ensuring good nutrition, reducing stress, selection of good sires and testing and careful breeding practices can limit or alleviate extrinsic factors.

Good quality pasture, hay and supplemental nutrition will help keep breeding mares in good body condition.

Moderation of heat stress and reduction of physical stress during mare transport may assist to reduce embryonic loss from stress factors. Cleanliness of the mare during and after breeding may help to reduce post-breeding endometritis.

Other steps can be used to alleviate embryonic loss.

Other steps

Oocyte transfer to bypass the oviduct may assist mares when primary embryonic defects are not the cause.

Artificial insemination may reduce the problems of uterine contamination of natural breeding and resultant endometritis. Antibiotics may also assist to reduce possible uterine infection. Exogenous progesterone, though commonly used, may not be that effective in preventing pregnancy loss in every case, but may be beneficial in certain situations. Careful monitoring of early pregnancy may be especially helpful, so that if early pregnancy loss occurs, it is possible to rebreed the mare.

Fertility will be better if the mare population is managed carefully.

Mare management

"I think if you take a random group of mares and don't carefully manage them, that's when you come up with these 70 percent conception rates, and 55 percent foaling rates," suggests Bud Hallowell, DVM, of Auburn, Wash.

Good breeding practices, good husbandry, and good management can lend some control of the mares. It is good to keep recently bred mares together in smaller groups, so they are more compatible; manage the mares, so that they are happy and well nourished. With good management there is not this 'war' going on so that the mares are not stressed socially. "When management begins to take care of the situation, you generally see the conception rate and foaling rate improve," Hallowell explains. "I would see 90-95 percent conception rates and hopefully 80-85 percent foaling rates."

In addition to mare socialization and stress reduction, "We did begin to look askance a little more at mares that had poor breeding histories, but basically what we did was just change the environment that a given group of mares were living under. They became happier, more content, less stressed, and had better nutrition - things we know are important things in regard to better management practices."

On a big farm where there are a lot of mares coming and going, where they are mixing the horses together, taking some out, putting new ones in, those situations can produce mare stress. If that situation is eliminated, the fertility of the mares may change. "That's basically why I say that normal fertility of mares isn't that bad, it's just that sometimes we just manage them poorly," Hallowell concludes. "Mares are pretty fertile as long as they've got plenty of feed, the pressure is not too great on them, or they are not a very old group of mares."

From the practitioner's perspective, much of the occurrence of early embryonic loss prior to 40 days depends on the mare.

Mare veterinary care procedures

The primary candidates are older mares and mares that have had problems, whether foaling problems, or those with endometritis early in their season. This also is probable cause for increased EEL losses seen in lactating mares and those bred during foal heat.

"The older mares have several things going against them," says Pete Sheerin, DVM, Rood & Riddle, of Lexington, Ky.

"I don't think there's a lot of EEL," says Sheerin, "though there is a certain percentage of it in the field, probably 5-10 pecent of mares bred, which is obviously more than we want to see."

With mares bred in the natural setting, with actual stallion cover, the contamination of the mare's caudal reproductive tract from natural service is potentially greater than the controlled environment of an artificial insemination program with use of sterile sleeves, sterile pipettes, etc. With this potential for increased contamination, "We use a lot of post-breeding infusions," says Stuart Brown, DVM, of Hagyard-Davidson-McGee in Lexington Ky., "and then maximize our efforts with the Caslick's procedure."

Though every mare does not have to be treated in this way, there is a certain population of mares, approximately 15 to 20 percent Brown estimates, that experiences delayed uterine clearance, being unable to clear their uterus of normal levels of contamination post-foaling and post-breeding. It's hard to predict which mares are likely to be susceptible. Of this group there are a certain number of mares bred that previously had a difficult foaling or had some type of contamination post-foaling, leading to a less than ideal uterine environment. There is then inflammation in the reproductive tract that detracts from their ability to maintain normal uterine defense mechanisms.

They may easily pick up infections due to streptococcus or coliform bacteria.

Post-breeding infusions help to keep that contamination to a lower level, preventing the bacteria from establishing an infection. Post-breeding infusion is often done with antibiotics, such as Timentin® (ticarcillin disodium/clavulanate potassium) or a pH-balanced solution of penicillin and genticin.

This combination is broad spectrum, economical and effective to combat the two most common organisms of concern. Timentin® and some of the other stronger antibiotics are used for more persistent problems and for mares that have known histories related to endometritis. It may be debatable, according to academicians, as to the necessity of using post-breeding infusions, but in practice they do appear to be beneficial in improving fertility rates.

In veterinary practice with Thoroughbred horses, this is especially essential with the understanding of the economics of breeding and the importance to maximize live foal births.

To help remove any contaminated material, it is also simultaneously helpful to flush the mare.


Even semen may be irritating to the lining of the uterus, which can be significant in the sub-population of mares that have delayed uterine clearance.

Uterine lavages are used this way. After they are bred, mares likely to experience problems related to uterine clearance are often scanned to see if they retain fluids. Post-breeding lavages of sterile fluids are used to reduce the amount of contamination a mare might have post-breeding.

In addition to post-breeding infusions, the Caslick's procedure is also used to ensure the mare's reproductive tract is kept clean.

Caslick's procedure

It is done to help prevent the mare's reproductive tract from aspirating air into the vagina, allowing for ascending bacteria to enter through the cervix and reach the caudal reproductive tract. The vulvar lips are sutured together at the level of the pelvic brim to eliminate this problem, as well as episodes of urine pooling.

Veterinarians sometimes treat mares with supplemental progesterone for various reasons.

Progesterone up until a mare is 110 to 120 days pregnant is paramount to maintaining her pregnancy. Luteal insufficiency or lysis of the CL interrupts natural progesterone supplementation. If a mare's foal is ill and she is confined in the stall with it, this stress can lead to falling progesterone levels.

Mares might experience an episode of colic, due to excessive ingestion of spring grass. A colicky mare may release endotoxin causing release of prostaglandin and subsequent CL lysis and progesterone decline, therefore stall confinement stress and colic might necessitate veterinary intervention of progesterone supplementation.

When mares are ultrasounded for pregnancy at about 15 days they are often checked for uterine tone.

If their tone is noted as really poor, a progesterone assay is done to check progesterone levels. If her progesterone is less than 4ng/ml, then it may be decided to give her supplemental progesterone.

"It is one of the things we see and identify in some mares," Brown explains.

A mare may not be pregnant at day 15 to 16 post-ovulation due to whatever reason. Commonly in central Kentucky, most veterinarians are ultrasounding mares 14 to 15 days post breeding, as opposed to doing it at 17 to 18 days.

At this earlier time a lot more twins, or even triplets are discovered. Done as a precaution, these excess embryos in the mare's uterus are manipulated to reduce the number to a single embryo, so that the mare carries only one fetus to term. If not, she might naturally lose them all to early embryonic death or late term abortion.

The mare seems to have her own 'embryo reduction mechanism,' eliminating a twin on her own, which may be a part of her recognition of pregnancy phenomena. Even so, the probability is better improved with veterinary manipulation. During the process of 'squeezing' out the excess embryos, inflammation may be caused inadvertently, which may result in the loss of the pregnancy. Therefore, once again, as a precaution, supplemental progesterone is provided to sustain the single pregnancy's success.

"We do all these veterinary procedures in an attempt to create the best uterine environment," says Brown. "With such procedures the mare is less likely to suffer early embryonic death. All the precautionary things that you do can only improve the mare's conception rate and fertility. In the background of all these losses, mares, compared to other animals, even women, are a pretty strongly scrutinized group."

Early embryonic loss occurs in the population of mares each breeding season.


Whether due to intrinsic or extrinsic factors, some of these factors have a role to play in particular mares. For each mare embryonic loss may be multi-factorial.

"I think it's one of those things that's just accepted," Sheerin says. "There's a certain percentage of pregnancy loss and it is just going to happen. We try to do the best job that we can do, so that farm managers are breeding a reproductively sound mare with a relatively fertile stallion and end up with a positive outcome. Unfortunately, there are just some mares that are going to lose their pregnancies. We can't always figure out why. People concede that there is a certain percentage of mares that will come up empty and we will try to get them bred again this year or next."

Mare management, which farm managers have in their control, is extremely intense, especially in Thoroughbred breeding. Their ability to properly care for the mare during foaling and breeding, maintain excellent nutrition and health and eliminate stresses in her environment, are each important in reproductive success.

With careful management and veterinary monitoring using ultrasound, hopefully a healthy full-term foal will stand and suckle her dam. Veterinary interventions and good management practices will help to ensure that outcome.

Dr. Kane is a freelance writer for equine topics and senior nutritionist with Stuart Products, Inc., Bedford, Texas. He holds a Ph.D in equine physiology and nutrition from the University of Kentucky.

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