‹ Back

Breeding an older mare is rarely just about producing a foal in the most cost-effective way. Many of these mares have completed their athletic careers without ever having had the opportunity to carry a foal — demanding training and competition schedules simply left no time for it. Along the way, they’ve become cherished companions and beloved members of the family. The process of trying to get an older mare in foal can be an emotional journey for both owner and veterinarian. Just as advances in human fertility medicine have helped older women conceive, veterinary science has made meaningful progress in supporting older mares to conceive and carry foals to term. Still, expectations must remain realistic — and it’s the responsibility of a well-informed veterinarian to clearly communicate both the possibilities and the limitations.

Despite scientific advances, success often depends on patience, careful planning — and no small measure of luck. Whether the journey ends with a healthy foal or the heartbreak of repeated unsuccessful attempts, an informed owner, supported by an experienced and properly equipped veterinarian, can take comfort in knowing that every decision was based on sound science, not wishful thinking.

This article is written for the educated, informed horse owner. My goal is to outline the key reproductive health considerations in older mares, in the hope that deeper understanding will lead to more thoughtful care, more realistic expectations, and better outcomes for our senior broodmares.

Age-related reproductive decline affects both sexes across mammalian species, but it is particularly well-documented — and more predictable — in females. In this article, I’ll explore how advancing age impacts a mare’s oocytes (eggs) and the overall health of her reproductive system, and suggest ways owners and veterinarians can work together to give older mares the best possible chance of reproductive success.

Infertility: the fundamentals

When a breeding attempt fails, the disappointing outcome can usually be traced to one of two points: either the mare did not conceive, or she conceived but subsequently lost the pregnancy. Although not always straightforward, distinguishing between these two scenarios is important as they stem from different causes and require differing management strategies. Conception and the maintenance of pregnancy represent two separate biological challenges. Some mares stumble at the first hurdle, others at the second — and a significant number may struggle with both. Older mares are more susceptible than their younger counterparts to the factors contributing to both conception failure and early pregnancy loss. Generally, failure to conceive points to issues involving the gametes — sperm or eggs — whereas failure to maintain a pregnancy more often indicates problems with the uterus or other reproductive structures.

Differentiating between these two distinct causes of breeding failure — failure to conceive versus early pregnancy loss — can be especially challenging in the early stages of gestation, even for experienced stud veterinarians. This difficulty stems from the unique characteristics of equine pregnancy, the limitations of available diagnostic tools, and the financial constraints that often limit the use of advanced reproductive diagnostics in breeding mares. In human fertility medicine, distinguishing between conception failure and early pregnancy loss is a little easier. A hormone called human chorionic gonadotropin (hCG) becomes detectable in a pregnant woman’s blood as early as 6–7 days after ovulation — around the time the embryo begins forming the earliest stages of a placenta, often before any missed period. Measuring hCG levels allows clinicians to differentiate between the two outcomes: if hCG is never detectable, conception did not occur; if hCG is initially present but fails to rise appropriately, it suggests that conception occurred, but the early pregnancy was lost.

In contrast, equine pregnancy presents a different challenge. The equine embryo remains mobile within the uterus until around day 16 and does not produce any equivalent, easily measurable hormone like hCG. Because the placenta develops much later, and no unique early pregnancy marker exists in the mare's blood or urine, veterinarians must rely on ultrasonography for early pregnancy detection. The equine embryo is enclosed in a capsule and forms a fluid-filled structure that can be visualised on ultrasound from an early stage – as early as day 10-11, which is much earlier than any of our other domestic animal species (Figure 2).

If a mare loses the embryo at any point between conception and her first pregnancy scan, the loss typically goes unnoticed, and we can’t distinguish between a failure of conception and a lost early pregnancy. The only definitive way to confirm fertilisation failure is through in vitro fertilisation (IVF), where the developmental progress of the egg after sperm binding can be directly observed.

However, this approach remains out of reach for most veterinary practitioners due to its cost and the specialised expertise required — though it is likely to become more accessible with time.

Older mares produce less healthy eggs, and have less predictable cycles

As William Harvey stated in 1651, Omne vivum ex ovo — all living things come from eggs. As in all mammals, in horses the egg is the origin of life, and its quality dictate’s a mare’s ability to conceive and the health of the resulting embryo. IVF has been used extensively in research settings, and it is through such studies that we’ve learned an important fact: the eggs of older mares are less likely to be successfully fertilised than those of their younger counterparts.

In the early stages of embryonic development, almost all the sub-cellular machinery needed for normal function is provided by the egg. The stallion’s sperm contributes DNA, but little else. This machinery — the egg’s organelles — must function properly to support fertilisation and the first critical steps of embryo development. Among these organelles, the mitochondria, responsible for producing cellular energy, are especially vulnerable to age-related decline. In older mares, mitochondrial numbers tend to decrease while the proportion of abnormal mitochondria increases. Once fertilisation occurs, the embryo’s energy needs rise quickly, placing significant stress on these already-compromised structures. If the mitochondria can’t keep up with energy demand, embryonic development may slow down or stop altogether.

Because an 11-month pregnancy is such a major investment for the mare, nature is particularly unforgiving of underachieving embryos. In the days before about day 8 of gestation, there's very little tolerance for delay — even minor developmental issues can lead to the embryo being lost. If a weak embryo forms from an aged egg with malfunctioning mitochondria, it faces a much higher risk of being naturally eliminated before the pregnancy even gets started.

We know that egg quality declines with age, and we strongly suspect that mitochondrial dysfunction plays a big role in that. What we don’t yet know is exactly how to prevent it. Right now, most approaches focus on supporting the mare with dietary supplements around the time of breeding. These include ingredients thought to help the immune system or act as antioxidants, in the hope that some will reach the egg and help protect it from damage.

For years, adding antioxidants and certain fatty acids to the diet has been known to improve sperm quality in males, including stallions. More recently, research has shown that similar supplements can also boost the fertilising ability of eggs in older mares. Because we don’t fully understand how each ingredient works — or whether some combinations might work better together — many commercial supplements contain a mix of components believed to support reproductive health. Common examples include D-alpha tocopheryl acetate (a form of Vitamin E), omega-3 fatty acids, and L-carnitine. These are typically chosen for their potential to support cell health, boost energy production, and protect against oxidative stress.
 

While mitochondrial damage is a well-known issue in older mares, it’s not the only concern. As in other species, older mares are also believed to accumulate more DNA damage and to lose some of their ability to repair it. This can reduce the chances of normal development and may contribute to early embryo loss. It’s possible that antioxidants and fatty acids could help reduce the impact of these changes as well — though more research is needed to understand exactly how, and to what extent.

In older mares, the follicles that support and nourish developing eggs often become less responsive to hormonal signals. As a result, oestrous cycles may become more irregular, and follicles are more likely to regress before ovulation. In addition, the cumulus cells — specialised follicular cells that are directly attached to the egg — seem to become less effective at doing their job as mares age.

While the egg is the starting point for new life, it doesn’t mature in isolation. Its quality and potential depend heavily on the environment around it — especially the support of the follicle and the surrounding cumulus cells. These cells form a close partnership with the egg, passing on nutrients, hormones, and protective signals through tiny channels. In younger mares, this system runs like clockwork, helping the egg grow, mature, and prepare for fertilisation. But as mares get older, this support system starts to wear down. Cumulus cells may become less active, less efficient at passing along nutrients, and less able to protect the egg from stress. So even if the follicle looks normal on ultrasound, the egg inside might not be as healthy as it appears.

When we look at all these factors together — from mitochondrial wear and tear, to DNA damage, to weakened hormonal and cellular support — it becomes easier to understand why fertility declines with age. If we know about these potential egg quality issues in our older brood mare, we can do our best to optimise her preparation for breeding.

Older mares are more likely to have uterovaginal issues

As if declining egg quality weren’t enough, the function of the uterus, cervix, and vaginal tissues also tends to deteriorate as a mare ages.

You can think of the mare’s reproductive tract as a layered security system. The outermost barrier is the vulva and lower portion of the vagina, which should remain tightly closed to keep out air and environmental contaminants. If this first line of defence is breached and something enters the upper vagina, the cervix serves as the next gatekeeper, preventing access to the uterus. And if bacteria or debris reach the uterine cavity, the endometrial lining must respond quickly and effectively to neutralise the threat before it can multiply and cause lasting damage.

The way a mare’s reproductive anatomy is structured has a big influence on how vulnerable she is to contamination — and this risk increases with age. Poor perineal conformation (such as a recessed anus and a long, forward-sloping vulva) makes a mare more likely to pool urine in the vagina, as the urethral opening isn’t ideally positioned for rapid drainage. These mares are also more prone to pneumovagina (sucking air into the vagina), which can introduce faecal bacteria, leading to inflammation and reduced fertility.

You’ve very likely heard of the Caslick’s procedure, a simple surgical technique used to close the upper portion of a long vulvar cleft, reducing the risk of vaginal contamination in mares with poor vulvar conformation. To help assess this risk, veterinarians use a measurement called the Caslick’s index. Imagine drawing a straight line between the two pinbones (tuber ischiadica). The length of the vulvar cleft above this line (in cm), multiplied by the angle of its forward tilt (in degrees), gives a numerical value — the Caslick’s index — which helps us objectively compare vulvar conformation between mares.

As a mare ages, muscle tone supporting the vulva may decline, causing the vulva to tilt forward and increasing the Caslick’s index. This change allows air, contaminants, and urine to enter the vagina, setting up inflammation or infection that can ascend into the uterus. Similarly, cervical tone often diminishes with age, increasing the chance of uterine contamination. Mares that have had difficult foalings in the past may also have cervical scarring, further reducing the cervix’s ability to seal properly.

Beyond these structural issues, the uterus itself becomes less efficient with age. Weaker uterine contractions and reduced lymphatic drainage can impair the uterus’s ability to clear contaminants after breeding.

Ideally, a mare’s uterus should expel excess semen and remove any contaminants shortly after insemination or natural breeding. In older mares, it’s common to see uterine fluid pooling after breeding — a sign that the uterus is failing to clear itself properly. If this fluid remains, it can lead to post-breeding endometritis, which may persist and create an unfavourable environment for embryo survival.

To evaluate uterine health, veterinarians can collect and examine endometrial samples — most commonly through biopsy or cytology. These samples provide insight into the health of the uterine lining and the mare’s chances of carrying a pregnancy to term. This is especially valuable for identifying mares that are unlikely to succeed as broodmares and helps avoid time and financial investment in unsuccessful breeding attempts.

So how should one go about breeding an older mare?

After reading all this, you might be wondering if there’s any light at the end of the tunnel. Breeding an older mare is rarely straightforward — it can be an emotional journey filled with hope, uncertainty, and patience. Age-related changes in egg quality, uterine function, hormonal regulation, and anatomical conformation all play a role in reducing fertility, but their onset and severity vary widely between individuals. One fourteen-year-old mare may conceive and carry a foal with ease, while another of the same age may struggle despite every effort. That’s why a thoughtful, case-by-case approach is so important. A recent foaling remains one of the best indicators of ongoing reproductive potential, but no single factor can predict success. Careful assessment, realistic expectations, and good communication are key.

Broadly speaking, a successful approach includes four key pillars:

(1) Assess each mare’s individual risk through careful history-taking, diagnostics, and timely treatment, considering nutritional support as adjunctive treatment.

(2) Breed optimally by closely monitoring her cycle and ensuring insemination is accurately timed to ovulation.

(3) Manage the risk of post-breeding endometritis with prompt follow-up and appropriate uterine care.

(4) Be prepared for more than one attempt — persistence is often part of the process.

Above all, remember: your veterinarian is your greatest ally. With the right partnership and a science-based plan, even the most experienced mare may have one foal left in her.

 

** If the reader would like references, they are welcome to send me an email. I have not included them in this draft to allow for easier readability.