By the time a broodmare enters her late teens and early 20s, her reproductive ability begins to decline, and getting and keeping her pregnant become more problematic. The causes of reproductive difficulties in the older mare are numerous, but in many situations, good management, accurate diagnostics, and appropriate medical or surgical therapies can optimize the chances of producing healthy foals right up until the time there are essentially no more eggs in the basket.

Pregnancy Prohibitors

Contamination and infection, both of which can cause an inflammatory response, are primary reasons for reproductive failure in the older mare. The presence of inflammation in the uterus can destroy the semen or embryo.

In the aging mare, the leading cause for contamination and infection is poor anatomical conformation and, consequently, the reduced efficiency of the anatomical barriers that help keep infection and urinary, fecal, and air-born contaminants out of the reproductive tract. John Dascanio, VMD, Dipl. ACT, Dipl. ABVP, Associate Professor, Equine Field Service/Theriogenology, Virginia-Maryland Regional College of Veterinary Medicine, identifies the three most common conformational disorders. The first barrier to infection is the vulva. A number of older mares ‘will have a tilted vulva where their anus is sunken inward,’ he says. ‘This causes the vulva to tilt, allowing the mare to defecate on top of her vulva and contaminate her reproductive tract. In addition, the lips of the vulva may not seal as tightly, often due to multiple Caslick’s operations or past trauma.’

The second barrier is the vestibulovaginal fold, a fold of tissue about a third of the way between the lips of the vulva and the cervix, says Dascanio. This can become nonfunctional due to anatomical changes. The third barrier is the cervix. Mares which have had trauma to their cervix through abnormal foalings do not form a tight seal to prevent ascending bacterial infections. These changes all add up to a decreased ability to prevent contamination of the mare’s reproductive tract.

Another common cause of contamination is fluid accumulation in the uterus or failure of uterine clearance. Says Dale Paccamonti, DVM, MS, Dipl. ACT, Professor of Theriogenology, Dept. of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, ‘Basically, all mares develop an endometritis (inflammation of the endometrium or the membrane lining of the uterus) after mating. This is a natural physiologic response to the deposition of sperm into the uterus. The majority of mares are able to clear this up without problems and provide a hospitable environment for the embryo when it enters the uterus a few days later.’

Mares with problems in uterine defense mechanisms, especially those with delayed uterine clearance, retain some of that fluid within the uterus, which then serves as a source of inflammation and potentially a source for bacterial colonization that can prevent the establishment of pregnancy.

Dascanio says the exact reasons for delayed uterine clearance are not known. ‘For some reason, the muscular layer (myometrium) of the uterus does not contract as strongly or as long in older mares as in younger mares. In addition, poor anatomical position and decreased lymphatic drainage may contribute to delayed uterine clearance.’

Aside from delayed uterine clearance, the very act of breeding—particularly with natural breeding, which is not as clean as artificial insemination—can be a source of contamination carried by an unclean mare or stallion.

Besides contamination, infection, and inflammation, other factors play a role in infertility.

Aged oocytes or eggs, says Paccamonti, ‘may partially explain relative infertility of the older mare compared to the younger mare, even when biopsy, uterine clearance, etc., look good.’ That might be because old oocytes are unable to undergo fertilization or might develop abnormally, Dascanio says. ‘Older oocytes may tend to undergo early embryonic death because the chromosomes may have defects in them, and the embryo does not form naturally, so you get a reduction in fertility even if the mare does conceive.’

Hypothyroidism or a low thyroid level is a defect that results in the low production of thyroid hormones. The disorder sometimes is thought to hinder conception and other body functions. Paccamonti points out that although the relationship between hypothyroidism and reduced fertility is a popular notion, ‘there is little scientific evidence to support hypothyroidism as a cause of infertility in mares.’

Overzealous use of antibiotics ‘sometimes can result in secondary infections,’ Dascanio explains. ‘You can eliminate relatively innocuous bacteria and accidentally select for more pathogenic organisms, such as yeast, which can be very, very difficult to get rid of.’

Toxins, including fescue toxicosis, often are raised as a possibility for affecting conception or gestation. In truth, says Dascanio, toxins are not a widespread problem for either older or younger mares, with the exception of fescue toxicosis that can lead to failure to cycle, early embryonic death, or prolonged gestation in some regions.

Genetics, while not a condition of aging, is a possibility to consider for the mare which never becomes pregnant. Explains Dascanio, ‘Some mares that have never gotten pregnant may actually have reproductive tract abnormalities that the veterinarian can’t detect during a reproductive examination. Blood tests may reveal that these mares have a chromosomal abnormality such as gonadal dysgenesis. Instead of the normal mare XX chromosome complement, these mares may be an XO. Thus, she’s completely infertile and will never be able to conceive because her reproductive tract never developed normally.’

Uterine cysts are more common in older mares. ‘A large cyst may interfere with embryonic movement and thus maternal recognition of pregnancy or may interfere with early placental formation causing placental insufficiency,’ notes Dascanio.

General health, fitness, and nutrition are important. ‘The natural barriers to infection are usually decreased in a really thin mare,’ Dascanio says. ‘Ovarian function is considered a secondary system to the body. If the mare is really malnourished, ovarian function can stop or oocyte quality may diminish.’ In addition, thinness robs the pelvic canal of the fat that helps support some of the reproductive structures, Dascanio notes, and that contributes to the reproductive tract falling forward, which, as mentioned earlier, increases risk of contamination and can inhibit uterine clearance. A mare which is fit and of good body condition will have better muscle function and will be better able to handle reproductive problems.

Repeated foaling can cause problems. Says Paccamonti, ‘There is some thought that repeated foaling may result in stretching of support ligaments for the uterus and repeated damage to innervation of the uterus. That then contributes to poorer clearance because innervation, or neural stimulus, is what causes muscles to contract, which is what promotes clearance.’

Not foaling enough, the converse, also can be a problem. Many believe that not being in foal every year can affect fertility. Explains Dascanio, ‘We generally tell people that if you keep your mare in foal every year you have a better chance of getting her pregnant versus letting her have a foal, then a year off, then letting her foal again, then letting her have a year off again. We tend to have more difficulty in those mares that are bred occasionally: If the mares go through multiple heat cycles, sometimes the wear and tear to their uterus become more severe versus leaving them pregnant. Every time a mare comes into heat and the cervix relaxes, a little bit of contaminant goes into the uterus from the vagina. A pregnant mare doesn’t have these multiple cycles, so you don’t have that little bit of wear and tear occurring during each one of those reproductive cycles.’

Old maiden mares can be tough. ‘There is a group of mares,’ says Paccamonti, ‘that we refer to as ‘old maiden mares.’ They are not really that old, but have never been bred before (for example, a nine-year-old maiden). This mare may have poorer fertility, which in some cases can be attributed to a very tight cervix inhibiting uterine clearance after mating.’

Stallion choice is important. ‘Certain stallions have a longer life span for their spermatozoon,’ says Dascanio, ‘so if insemination is not coordinated accurately with ovulation, having a stallion with a longer longevity to his spermatozoa—lasting for a couple of days versus a half-day—would allow a better chance of conception.

‘Then, too, certain stallions may have a low number of motile spermatozoa in their ejaculate; a large volume of semen will need to be used to breed the mare to achieve an adequate number of live motile spermatozoa. Adding this extra volume does not help those mares with delayed uterine clearance.

‘Finally, each time a stallion breeds a mare naturally, a significant contamination occurs. Mares that have delayed uterine clearance or an impairment in immunologic function have difficulty in clearing this contamination. By breeding mares using a minimal contamination technique (artificial insemination), the contamination is limited.’

Other less common disorders and causes of infertility include endometrial atrophy resulting in placental insufficiency, blocked oviducts, anti-sperm antibodies, uterine and/or ovarian tumors, and cervical, vaginal, or vulvular trauma.

Failure To Carry To Full Term

After getting the older mare pregnant, you must make sure she stays that way.

The first 30-35 days in the older mare’s pregnancy are critical. ‘Early on in pregnancy,’ Dascanio says, ‘the baby is nourished via two mechanisms. One is via a yolk sac, which is contained within the embryo. The second means is via histotroph or uterine milk. Histotroph is a secretion by uterine glands that provides nutrients for absorption across the embryonic surface.’

Once the embryo leaves the oviduct and enters the uterus at about Day 6 or Day 7 post-ovulation, it needs a healthy environment to survive. Says Dascanio, ‘That’s where poor uterine defense mechanisms and poor uterine environment come into play with the older mare. Additionally, if older mares have had multiple insults to their uterus over time, they are going to have wear and tear to their uterine lining, so it’s not going to be as optimum an environment for embryonic nourishment and good placental attachment.

‘The next period of concern is around Days 35 to 40 post-ovulation, when the placental formation and attachment begin. If the uterine lining (endometrium) has significant pathology, the placenta may not form enough of an attachment to allow the fetus to survive. Once the mare makes it about halfway through pregnancy, most mares will carry to term because the placenta/endometrial unit has a capability for compensating for slight abnormalities.’

Beyond that, mares of all ages are at some risk of abortion due to a number of infectious and non-infectious causes such as twins, herpes virus, fescue toxicosis, etc. The older mare, in particular, is more susceptible to ascending uterine infections due to potential cervical incompetency and/or poor external conformation.


Obviously, an accurate diagnosis of the cause of infertility is needed in order to administer a therapy most likely to produce optimum results.

‘A reproductive examination starts with evaluation of conformation and natural barriers. The next step is to perform a palpation per rectum and an ultrasound examination of the reproductive tract. Following that, a vaginal speculum exam and manual exam of the mare’s vagina and cervix are done,’ says Dascanio. ‘This may be followed by a uterine culture, cytology, and biopsy.’ A uterine biopsy can help predict the mare’s chances of getting pregnant.

Paccamonti warns that a uterine culture always should be accompanied by cytology. ‘This will confirm that a pathogen isolated by culture is indeed a pathogen and not a contaminant,’ he says. ‘Culture results without cytology results provide no information.’

Should preliminary results turn up normal, Dascanio says other, more specialized tests can be performed. ‘We have the ability to test for oviductal patency, perform uterine endoscopy, test for anti-sperm antibodies, assess hormonal status, and check for chromosomal abnormalities among some of our tests. Some of these procedures require special equipment or facilities to perform and may not be done on the farm. Diagnostic information can even be gained by frequent ultrasound examinations, such as looking for fluid accumulation with the first day post breeding to help diagnose uterine clearance problems.’

Hypothyroidism can be difficult to diagnose, as testing usually does not produce a definitive diagnosis. ‘Certain drugs like phenylbutazone can make a mare appear to be hypothyroid when she isn’t,’ says Dascanio, ‘so you need to eliminate all other causes potentially contributing to hypothyroidism before you start thyroid medication.’

Additional tests that might be offered, says Paccamonti, include ‘a ‘breeding challenge’ or infusion of charcoal into the uterus in late estrus and examination of uterine clearance in the ensuing 48-hour period or embryo collection seven or eight days post-ovulation, although this is not routinely done.’


Anatomical defects of the vulva, vaginal fold, and cervix can be corrected surgically. ‘Cervical reconstruction, urethral extension, and Caslick’s procedure (in which the upper section of the vulva is sutured closed, allowing an opening large enough for urine to be eliminated but small enough to help prevent contaminants from entering) may address anatomical problems,’ says Paccamonti. ‘But before doing surgery, do a breeding soundness exam (including uterine biopsy) to see that everything else is in order. There is no use in correcting a urine pooling problem if the mare has an endometrial biopsy score of Grade III (less than 10% chance of getting in foal) due to fibrosis.’

To improve uterine clearance, mares can be treated within hours after breeding with lavage. ‘Dilution is the solution to the pollution,’ says Dascanio. ‘There’s a window of opportunity between the time of ovulation and the embryo actually coming down into the uterus,’ he explains. ‘After breeding, the semen is deposited in the uterus and the spermatozoa enter the oviduct between the uterus and the ovary, where they wait until ovulation occurs. At this time, they are not actually in the uterus per se. Conception occurs in the oviduct, and the embryo that results stays in the oviduct for about six or seven days before it comes into the uterus. You can start to treat the uterus as soon as roughly four hours after breeding and not decrease conception rates significantly. You have between the day of ovulation and about three days post-ovulation to treat the mare’s uterus.’

A simple lavage instillation and removal of warm saline solution can help uterine clearance in a number of ways. Says Dascanio, ‘The saline dilutes contaminants present in uterus. Warm water of about 45 degrees Celsius stimulates uterine tone. Plus, saline itself produces a slight inflammatory response, which brings in more immune cells that, it’s been theorized, help bolster the mare’s immune function.’

After the lavage procedure, oxytocin (a naturally occurring hormone that causes uterine contractions during labor) or a prostaglandin product (a drug commonly used to lyse a corpus luteum to bring a mare into heat, but which also has the ability to contract smooth muscles) might be used to help increase muscle contractions within the uterus in order to dispel more fluid.

Antibiotics might be introduced early in the heat period if an infection is confirmed via a culture and cytology. ‘If a mare is treated pre-breeding with saline lavages, with or without antibiotics, I will stop my treatment at least one day prior to when I breed her,’ Dascanio states. ‘This will allow her uterus time to recover from therapy and provide a better environment for semen deposition. Whether you use antibiotics depends on your specific situation, but for delayed uterine clearance in the mare, saline lavage and oxytocin are the primary treatments.’

Paccamonti notes that exercise and possibly being housed near a stallion also can improve uterine clearance. ‘I’ve seen some mares with clearance problems improve their condition somewhat by being housed in proximity to a stallion,’ he says. ‘The apparent auditory and possibly other sensory stimuli seem to help them to improve tone in the reproductive tract. This is possibly due to endogenous oxytocin release, but this is just supposition based on clinical observations.’

Assisted reproductive techniques are available for those willing and able to invest in more advanced technologies. Says Paccamonti, ‘Embryo transfer, GIFT (oocyte or gamete intrafallopian transfer), in vitro or ISI (intracytoplasmic sperm injection), all these techniques utilize the oocyte from a valuable mare but use a recipient mare as a surrogate to carry the pregnancy. This is especially helpful if the mare has a uterine or oviductal problem, but not much help if the problem is due to the oocyte.’

Optimizing Reproductive Conditions

Good management and sensible, careful breeding techniques also can go a long way in improving the older mare’s chances of pregnancy.

Be clean with breeding techniques. ‘Rather than natural mating, use a minimal contamination breeding technique that involves artificial insemination,’ suggests Dascanio. ‘Wash the mare, paying special attention to any contamination that may be within the few centimeters of the lips of the vulva. You may want to perform a vaginal speculum examination on the mare to see if there is any contamination in the cranial part of the vagina. If not, put on a sterile sleeve, use semen that is filtered and possibly has a semen extender with antibiotics, and breed her in as sterile manner as possible.’

Some registries require a natural cover, which is more contaminating than artificial inse mination. If so, make sure both the mare and stallion are clean.

Time the breeding so the mare only has to be bred once instead of invading the reproductive tract multiple times during a single heat. ‘Don’t breed during transition and don’t breed repeatedly, i.e., every other day, during the heat cycle,’ Paccamonti recommends. ‘During spring transition, mares can be in heat for three or four weeks, and repeated breeding causes repeated contamination of the uterus, but without ovulation. It seems some mares have their best fertility during their first few cycles of the year, then have more problems with uterine clearance and uterine defenses as the season progresses.’

Make sure the stallion is a good candidate by having his sperm tested. The ideal candidate has spermatozoa with good longevity, a majority of which are live sperm, and which doesn’t carry harmful organisms.

Maintain a quality health maintenance program for the mare. Says Paccamonti, ‘Make sure the mare is on a good vaccination program—i.e. rhinopneumonitis at five, seven, and nine months of gestation; annual influenza, tetanus, and encephalitis boosters, preferably at 10 months gestation. Provide her with regular deworming, and dental and foot care. Keep her physically fit and keep an eye on her body condition. If the mare has a history of problems, observe her for indication of problems via ultrasound for placental pathology, and monitor her for impending parturition using milk calcium.’

Good nutrition is important, too. Feed hay that’s leafy, green, free of dust, and rich in nutrients, says Dascanio. ‘Also, give a trace mineralized salt block and appropriate nutritional additives, if needed. Don’t overcondition a mare. Mares that are extremely fat may have foaling difficulties and not produce as much milk.’

New Horizons

While many techniques already are available to enhance the chances of pregnancy, work continues to increase the odds even more. ‘Assisted reproductive techniques are continuing to be developed and refined,’ says Dascanio. ‘Techniques like in vitro fertilization, oocyte and embryo freezing to retain genetic material, intracytoplasmic sperm injection, and gamete intrafallopian transfer may offer means of providing offspring to infertile mares.’

Additionally, researchers are examining the underlying causes of infertility problems in order to deal with them in a preventive, rather than reactive, approach. Says Dascanio, ‘A number of investigators are looking at reasons why the older mare’s uterine muscle function is not functioning adequately. Is it a defect in the muscle itself or a defect in some pathway that leads to muscle contraction that is causing these mares not to want to clear from their uterus? Is it a defect in the immune function in older mares that is leading to these problems? We’re trying to figure out why the problems are occurring and trying to attack them that way. So far, all we’ve been doing is responding to the problem that is already there.’

Whether knowing the causes will lead to better solutions is anyone’s guess, and certainly is in the future. For the here and now, many tools and techniques are available to help the old gray mare be the horse she used to be.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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