There is no single cutoff age when a mare becomes “too old” to breed, but fertility declines gradually starting surprisingly early, and the risks to both mare and foal increase significantly after age 15. Most mares are bred between ages 3 and 20, with peak reproductive performance around age 8. Beyond 20, the odds of producing a live, healthy foal drop sharply, and the physical dangers to the mare become serious enough that many veterinarians advise against it.
How Fertility Changes With Age
A large study of Thoroughbred broodmares found that live foal birth rates decrease gradually with every year of age, starting as young as 3 years old. That doesn’t mean a 5-year-old mare is already struggling. It means fertility peaks early and then follows a long, slow downward curve rather than falling off a cliff at a specific birthday.
Roughly 99% of all matings in the study occurred with mares between ages 3 and 20. After 20, the numbers thin out dramatically because breeders recognize the declining returns. Two factors compound each other: the mare’s chronological age and the number of years she’s been breeding. Both independently reduce the likelihood of a live foal, so a mare who has been producing foals for 15 consecutive years faces steeper odds than her age alone would predict.
What Happens Inside an Aging Mare
The decline isn’t just about the uterus. Egg quality deteriorates with age in horses much the same way it does in humans. Older mares produce eggs with altered structure, reduced energy output from their mitochondria, and a higher likelihood of chromosomal problems during cell division. These changes make it harder for an embryo to form correctly and survive the early weeks of development.
The uterus itself also accumulates damage over time. Years of breeding cycles, pregnancies, and exposure to bacteria leave behind scar tissue and inflammation in the uterine lining. A veterinarian can assess this damage through a uterine biopsy, which grades the tissue on a scale. Mares with minimal scarring (Grade I) have about a 78% chance of carrying a foal to term. Mares with moderate changes (Grade II) drop to around 55%. Those with severe scarring (Grade III) have only a 35% foaling rate. Age correlates strongly with biopsy grade, but the biopsy itself is a far better predictor of success than age alone. A well-maintained 18-year-old mare with a Grade I uterus is a better candidate than a neglected 12-year-old with a Grade III.
The Older Maiden Mare Problem
Mares that have never been bred face a unique challenge if you wait too long to start. A maiden mare’s cervix and uterus haven’t been stretched by pregnancy, and over time the uterine environment can become less hospitable without ever having been “used.” Older maiden mares tend to have more difficulty clearing fluid and bacteria from the uterus after breeding, which leads to persistent infections that prevent embryos from implanting.
If you’re considering breeding a mare for the first time and she’s already in her mid-teens, expect that she’ll need more veterinary intervention to conceive and maintain a pregnancy than a younger mare would. That said, it’s not impossible. The decision should be guided by a reproductive exam, including a uterine biopsy and ultrasound, rather than age alone.
Physical Risks After Age 15
Pregnancy and foaling place enormous cardiovascular stress on a mare, and the risks climb steeply with age. The most serious complication is uterine artery rupture, where a major blood vessel tears during or near the time of foaling. A necropsy study of over 500 mares that died from this condition found that 78% were 15 years of age or older. The uterine artery is the vessel most commonly involved, though the ovarian and iliac arteries can also rupture.
Most ruptures happen during active labor, but about 30% occur before the mare even starts foaling, and a smaller number happen in the hours afterward. The mare typically becomes painful or colicky, and the outcome depends on the size and location of the tear. Some mares survive a contained hemorrhage, but many do not. This risk alone is a major reason breeders and veterinarians grow cautious about breeding mares in their late teens and twenties.
Mares over 20 also face a significantly higher chance of pregnancy loss. Research from the University of Minnesota found that mares around age 20 have a 21% chance of aborting before day 40 of pregnancy and a 15% chance of losing the pregnancy after day 40. For comparison, younger mares lose pregnancies at rates well under 10%.
Cushing’s Disease and Breeding
Pituitary pars intermedia dysfunction, commonly known as Cushing’s disease, is an age-related hormonal condition that becomes increasingly common in horses over 15. It directly undermines fertility. The condition disrupts the hormones that regulate the estrous cycle, leading to irregular or suppressed heat cycles. It also chronically weakens the immune system, which makes older mares more vulnerable to persistent uterine infections that prevent pregnancy.
Research comparing mares with and without Cushing’s disease found that affected mares had significantly lower pregnancy rates, and the gap widened with each additional year of age. Interestingly, age-matched mares without Cushing’s didn’t show the same steep fertility decline, suggesting the disease itself is a major driver of reproductive failure in older horses rather than age alone. Treatment with medication improved pregnancy rates considerably: 80% of treated mares conceived compared to just 44% of untreated mares. If you’re considering breeding an older mare, testing for Cushing’s disease and treating it if present can meaningfully improve your chances.
Supporting an Older Pregnant Mare
If you do breed an older mare and she conceives, the pregnancy requires closer monitoring than it would in a younger animal. Progesterone levels are a key concern. When blood progesterone drops too low, the pregnancy becomes unstable. Veterinarians typically monitor levels in older mares or those with a history of pregnancy loss, and supplementation with a synthetic or natural progesterone can be given daily or weekly if levels fall below the safe threshold.
Nutrition also needs adjustment, particularly in the last trimester when the foal is growing rapidly. Protein requirements jump from about 8% in early pregnancy to 11 or 12% in late pregnancy. Free access to high-quality grass or alfalfa hay generally covers energy and protein needs, but neither provides enough phosphorus, and grass hay will also fall short on calcium. A daily mineral supplement formulated for broodmares fills those gaps. Older mares may also have dental issues or reduced digestive efficiency that make it harder to maintain body condition, so you may need to feed more concentrated calories through grain or fat supplements.
Making the Decision
Rather than relying on a single age cutoff, the decision to breed or retire a mare should weigh several factors together. A reproductive exam gives you the most useful information: uterine biopsy grade, ultrasound findings, hormone levels, and overall health status. A healthy, well-maintained mare at 17 with a clean uterine biopsy is a reasonable breeding candidate. A 15-year-old with severe uterine scarring, Cushing’s disease, and a history of pregnancy loss is not.
As a general framework, most breeders consider 15 to be the age where extra caution and veterinary involvement become necessary. By 20, the combination of declining egg quality, rising abortion rates, uterine deterioration, and the real danger of arterial rupture makes breeding a genuinely risky proposition for the mare’s life. Beyond the mid-twenties, pregnancy is rare and carries risks that few veterinarians would encourage an owner to accept.