How to decide if your older mare is a good candidate for breeding and improve her chances of success

Gala Argent wanted one last foal. After 25 years breeding Foundation Appaloosas, the educator and researcher had decided to retire from breeding. To mark the end of that era, Argent chose to breed her cherished homebred mare Della. Beautiful inside and out, the leopard roan was born on Argent’s Batavia, Illinois, farm two decades earlier.

Della might have been a senior mare, but she was in excellent health, Argent says. Like her dam and sire—who lived to be 28 and 30—Della was fit, energetic, and pain-free in her golden years. Plus, she’d already foaled twice with no ­complications.

After three months at pasture in a domestic harem with a leopard stallion, Della was in foal. Nearly a full year later, at age 21, she gave birth to the biggest and most precocious foal Argent had ever seen. Scooter was standing and suckling the senior dam’s full udder within minutes, she says.

While Argent’s story had a positive outcome, the reality is it’s not always easy to breed older mares, and there’s a higher risk of complications, our sources say. In this article we’ll share expert advice on how to decide if your older mare is a good candidate for breeding and how to improve her chances of success.

15 Years & the Reproductive Plateau

Up until about age 15, healthy mares have a 50-60% chance of conceiving per estrous cycle, says Patrick McCue, DVM, PhD, Dipl. ACT, a professor of theriogenology in the College of Veterinary Medicine & Biomedical Sciences at Colorado State University (CSU), in Fort Collins.

The ability to conceive and produce a live, healthy foal declines steadily starting at age 15 and decreases even more after age 20, he says.

That’s mostly because of an aging, scarred, infection-prone uterus, says Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute for Equine Science, in Neustadt, Germany. While some mares experience ovarian senescence, a gradual decline in the ability to produce fully functional oocytes, and stop cycling at around age 25, most continue to ovulate and produce good-quality eggs that then migrate into a poorer-quality uterus. 

Getting these mares pregnant often requires extensive veterinary assistance, Aurich says. Once pregnant, they’re more likely to develop uterine infections and abort. At foaling, they have an increased risk of uterine arterial rupture, due to aging blood vessels, and a retained placenta. And if it’s the mare’s first foal, it’s likely to be small.

For these reasons, the first step in breeding an old mare is assessing whether it’s right for her, she says.

Take an Honest Look at the Mare

If you want to breed your senior mare, take an honest look at her—not as the horse she used to be, but as the horse she is today.

“It’s not always a question of when she was born, but of her biological age,” Aurich says. “Does she look like a fit, healthy mare? Or is she losing body condition and having problems moving or getting up and down?” These are particularly important questions when considering the extra 250 pounds she’ll be carrying between the foal, placenta, and fluids and that she’ll be a full year older by the time she foals.

Non-steroidal anti-inflammatory drugs (NSAIDs) can help relieve arthritic pain during those final, hardest months of pregnancy without harming the foal, Aurich says. “But if you say you have to do it from the beginning of the pregnancy, I wouldn’t breed that mare,” she says.

Owners should also exclude mares with significant lameness, musculoskeletal injuries, or chronic laminitis from their breeding program, McCue adds.

Mares with pituitary pars intermedia dysfunction (PPID, formerly equine Cushing’s disease) might be poor candidates for breeding due to treatment with pergolide, Aurich adds. As a dopamine receptor agonist, pergolide can reduce fertility and interfere with colostrum and milk production.

Even so, stopping pergolide treatment one month before the anticipated foaling date might restore lactation potential, and owners can plan to provide donated, frozen-thawed colostrum at foaling, McCue adds.

Owners should check their senior mares’ reproductive anatomy for telltale signs of perineal aging, in which the vulva forms a sort of horizontal “shelf” under the anus. In such cases fecal material can enter the vestibule or vagina, urine might pool inside the vaginal vault, and mares might aspirate air into their reproductive tract. Owners can also check muscular tone by using two fingers to gently spread the vulva open.

“If it opens easily with minimal pressure, that’s indicative of less muscular tone,” McCue says. Poor muscular tone can also allow air and fecal matter to aspirate into the reproductive tract. The resulting trapped bacteria—and that from pooled urine or aspirated air—put the mare at risk of uterine infections, which are incompatible with conceiving and maintaining pregnancy without active treatment, he says.

Strongly Consider Foaling History

Older mares that have foaled in the past—even once— have a more mature reproductive system than older maiden mares that have never been bred, our sources say.

“It makes a big difference in potential outcome and prognosis for getting a mare pregnant,” McCue says.

Maiden senior mares—often retired performance horses whose owners now hope to start breeding—usually have “old maiden mare syndrome,” explains Aurich. That’s marked by an immature uterus that’s likely to produce a relatively small foal over a long gestation period—often a full year, she says. These mares also have a tight cervix that blocks the natural passage of fluids during a breeding cycle. That means the semen, bacteria, and other breeding-related debris that enter the uterus during ­breeding—whether through live cover or artificial insemination—can’t get expulsed afterward. As a result, inflammatory fluids pool in the uterus, interfering with both conception and pregnancy.

In addition, older maiden mares might struggle with their pregnancy weight, she explains. “They have to get used to getting heavier and to having problems getting up and lying down,” she says, especially if the mare has arthritic joints. Plus, they’re more prone to stress if they’re new to breeding and undergoing multiple invasive procedures over several months. “Then maybe she finally gets pregnant late in the season, and that means she’s late pregnant the following July or August when it’s hot outside,” she says. “That may be difficult for her circulation, thermoregulation, and everything. It’s much tougher.”

Seasoned broodmares, by contrast, have already benefited from “the whole endocrinology of pregnancy” that includes the activation of glands (and hormone release), muscles, and other tissues that favor reproduction, McCue says. The foaling process itself also exercises the cervix, which opens fully and then pulls closed again. For healthy breeding, “it’s good to maintain that dynamic cervical function,” he says.

And if the mare has foaled in the past year, she’s more likely to conceive again relatively quickly, he adds.

Still, if an experienced broodmare has become more difficult to get or keep pregnant, it’s probably time to retire her rather than trying for one last foal, Aurich says.

Get a Thorough Clinical Exam

Breeding exams start with a confirmation of what the owners might have already observed themselves, making sure their senior mares have the overall physical health, musculoskeletal stamina, and body condition to support a year and a half of pregnancy and lactation, our sources say. Veterinarians will then assess the mare’s perineal anatomy and inspect the vagina with a speculum to identify pathological conditions such as scars in the cervix (that might have occurred during the previous foaling) or varicose veins that might develop in aging mares, Aurich says. They can also use a gloved hand to digitally evaluate the cervix.

Uterine ultrasounds and biopsies can help the veterinarian determine the degree to which the lining of the uterus—the ­endometrium—is still healthy and functional, she explains.

Endometrial glands produce a substance called histotroph, which provides nutrition to the developing embryo. Aurich says fibrous scar tissue often develops in the uterus of older mares, compromising those glands.

Ultrasound can reveal lymphatic cysts, which result from scar tissue deposition, says McCue. While small lymphatic cysts are relatively harmless, growth in size and number can indicate more severe endometrial scarring as well as a proliferation of microscopic glandular cysts, which compromise endometrial function.

Biopsies can confirm the presence of microscopic glandular cysts, he says. They also reveal age-related inflammatory cells or damage to uterine glands. Veterinarians grade biopsy results from Grade 1 (healthy) to 2 and 3 (moderate and severe pathologies affecting fertility).

Combined, these results can provide a prognosis for breeding success and guide decision-making, our sources say.

Manage for Optimal Fertility

Mares should be up to date on the American Association of Equine ­Practitioners’ core vaccinations—rabies, tetanus, West Nile virus, and Eastern and Western equine ­encephalomyelitis—as well as locally relevant risk-based vaccines. Vaccinations should be given at least 30 days prior to active breeding to allow for an appropriate immune response, McCue says.

Maintaining good dental care is critical in older horses but even more so in older broodmares because they’ll be eating for two, he says.

Because pregnant mares’ bodies prioritize the growing fetus, broodmares are at risk of losing body condition during pregnancy and lactation—“one of the highest nutritional demands on any horse,” McCue says.

Older horses often struggle to maintain body weight, so senior mares must start the breeding season in good body condition or even mildly overweight if their metabolic health allows it, says Aurich.

Older, metabolically healthy mares should be fed high-protein and calorie-rich forage, such as first-cut meadow hay or a combination with alfalfa, supplemented with concentrated feeds as needed, she says.

Recent research at CSU suggests diets rich in omega-3 fatty acids and antioxidants help oocytes metabolize energy and mature more efficiently, especially in senior mares. Therefore, supplementing with omega-3 fatty acids might be helpful, McCue says.

Finally, manage older mares to start cycling as early in the year as possible, he says. Older mares naturally start cycling as late as May or June and tend to have longer intervals between cycles due to slower ovarian follicle development. “I would recommend housing older mares under lights (or using blue-light therapy hoods) beginning on about Dec. 1 to advance the first ovulation of the year to mid-February so you have multiple opportunities to breed her,” he says.

Breed With Planning, Patience, and Assistance

For older mares McCue recommends selecting stallions with high fertility rates.

While live cover can be effective, breeders can also choose artificial insemination (AI). Fresh semen that is collected, processed, and inseminated within hours of collection often provides the highest pregnancy rates, he explains, though cooled storage can offer similar fertility rates for many stallions. By contrast, pregnancy rates using frozen semen are frequently 10 to 20% lower than with fresh or cooled semen. Even so, it’s still an acceptable option for breeding older mares, he says.

Regular transrectal ultrasound exams can reveal exactly when older mares are about to ovulate, says McCue. This helps you time insemination as closely as possible to ovulation, which improves conception rates. If needed, veterinarians can administer an ovulation-inducing agent, such as human chorionic gonadotropin or deslorelin, to help ensure timeliness. Follow-up ultrasounds can confirm ovulation has occurred and show whether the mare is collecting inflammatory post-breeding fluid, which oxytocin administration can often eliminate. 

Once bred, mares with poor perineal conformation should undergo a Caslick’s procedure. Also called a vulvoplasty, the procedure closes the top part of the vulva to prevent aspiration of air and bacteria as a protective mechanism against uterine infection during pregnancy, McCue explains.

Aurich says she recommends a daily dose of altrenogest starting the fifth day after ovulation to compensate for low ­progesterone—the hormone that prepares the uterus for pregnancy. “The corpus luteum function may be compromised, for example, because of the inflammatory response of the uterus to breeding, but it is also possible that older mares need more progesterone because of the ‘scarred’ endometrium,” she says. The corpus luteum is the structure formed after the ovarian follicle releases the egg; it produces progesterone.

Consider Embryo Transfer or ICSI

Older mares that are poor candidates for conceiving and carrying a pregnancy can still reproduce through advanced reproductive techniques such as embryo transfer and intracytoplasmic sperm injection (ICSI), Aurich says.

The relatively good quality of older mares’ eggs makes them good candidates for these assisted reproduction techniques, she explains.

If the mare’s endometrium and uterus are healthy enough to maintain a free-floating fertilized egg and growing embryo for at least eight days, she can undergo precisely timed AI and then have her uterus flushed to recover the embryo. That embryo then gets transferred into a younger surrogate that carries the foal to term and nurses it.

For old mares that can’t successfully conceive, scientists can harvest their eggs and perform ICSI in a laboratory before transferring embryos into surrogate mares.

An added advantage of these techniques is they allow for multiple foals from the same aging mare at the same time, in different surrogates, she says. You can even freeze and store the eggs long-term.

“If you have a mare that’s getting old and you really like her, you can actually harvest her eggs or embryos and then produce the foals at a later time,” Aurich says.

Take-Home Message

Owners can increase their chances of successfully breeding their old mares by recognizing the ones that make the best candidates, managing them appropriately, and providing the necessary therapeutic assistance, our sources say. When making decisions about breeding a beloved aging mare, it’s important to consider her health and welfare as well as the financial risks. When the mare’s general or reproductive health can’t support a pregnancy, her eggs can still be used for ICSI and/or embryo transfer into a younger surrogate.