Breeding Challenges of Older Mares

Older broodmares need a little extra attention. 

Breeding an older mare is often challenging--not only in getting her in foal, but in having her carry the pregnancy to term and produce a healthy foal. Many breeders take on this challenge, however, since new technology and better ways to handle these mares have improved the chances for success. The older the mare, the greater the probability she'll have trouble conceiving.

Natural aging makes it harder for her tract to function normally, but age alone is not the only thing that hinders normal reproduction. "The more foals she's had, the more the tissues are stretched; the tract never comes back to its original (position), and this allows more contamination to enter," explains Stephen H. Slusher, DVM, MS, a retired veterinarian who practiced at Oklahoma State University. He worked with problem mares to correct conditions that might interfere with conception and pregnancy.

"The first thing we do is completely explore the reproductive functions of a mare to see where problems might occur, then do corrective measures ... to take care of those," says Slusher.

Walter Zent, DVM, of Hagyard Equine Medical Institute in Lexington, Ky., sees a lot of problems that arise in older mares. "One of the most important things to know is her history," he states. "Has she had a foal every year, or is she an older mare that's never had a foal? We also take a uterine biopsy, to see whether there is fibrosis (scar tissue)."

Uterine Health

Fibrosis is one of the degenerative changes often seen in an older mare's uterus, and it is mainly due to aging rather than to wear and tear from pregnancies. "A 15- to 18-year-old mare that's never had a foal will still have some of these changes," says Zent. "In fact, there have been studies showing that pregnancies may be somewhat protective to the uterus. The mare who has never had foals may actually have a worse uterus because of continually coming in and out of heat. This may be one cause of degenerative changes."

Regarding older mares that have never been bred, Zent worked on some Saddlebred mares that were shown until they were 15 years old, then retired for breeding. Some of them had cervixes that would not open properly and problems with fluid in the uterus.

The cervix, which protects the uterus from contamination, is a muscular structure that requires a tight seal to maintain pregnancy, but yet it has to relax enough to allow a foal to pass through.

"Pregnancy loss is higher in older mares than in young ones," says Zent. "There's been some very good work done (at Newmarket) showing the villi in the uterus are less numerous in some older mares as a result of fibrosis. Thus the placental area (for attachment) is greatly reduced."

You end up with a fetus that's not as well-nourished as it should be. Often you work very hard to get these mares in foal, but the resulting foal is small and weak.


Endometritis is inflammation of the uterine lining. All mares experience some degree of inflammation after being bred (in response to semen, which is a foreign substance to the uterus), but most mares clear this inflammation within 24 hours. Healthy tissue puts forth enough immune response to deal swiftly with inflammation and any bacteria that were introduced with breeding, well ahead of arrival of the fertilized egg traveling down the fallopian tube (reaching the uterus on Day 5 or 6). In some mares, especially older mares, inflammation, uterine fluid, or infection remains, so the embryo does not survive and the mare comes back into heat.

Ed Squires, MS, PhD, a professor at Colorado State University, says mares can have an infectious or noninfectious endometritis. In the latter the mare simply reacts to the sperm; neutrophils come into the uterus to clear up all the products of dead sperm and any bacteria that happen to be there. The larger particles that can't be "ingested" by specialized cells are physically expelled through uterine contractions that send this drainage out through the cervix. The uterus is then clean and ready for the embryo.

"If a mare is abnormal--what we call a 'susceptible mare'--24 hours after breeding she still has fluid in her uterus, and she may take several days to clear it," explains Squires. "Some of these mares end up with bacterial infections, since fluid retention provides an ideal environment for bacteria to grow. But a lot of them are just old mares that accumulate fluid."

Uterine contractions that clear debris might be less strong in some mares, resulting in slow or nonexistent evacuation of the uterus after breeding. Mares that retain fluid often have poor external conformation of the reproductive tract (such as a tipped- inward or sunken vulva, which is common in older mares) so they tend to become contaminated and accumulate fluid.

Uterine Cysts

Endometrial cysts are also common problems in older mares. These clear, fluid-filled structures can prevent conception and/or interfere with normal pregnancy. Sometimes cysts keep an embryo from attaching to the uterine wall, but more often they are just a sign of an aging uterus, says Zent.

"Most mares will get in foal in spite of uterine cysts," he says. "The more common difficulty is in management and recognition of pregnancy. The cysts tend to get in the way of ultrasound; you can't see past them, and it's hard to know if the mare is actually pregnant. You have to follow that mare along until you can actually see a heartbeat to be sure there's a foal in there, just because there are so many cysts."

Slusher says that in some mares the cysts must be removed, if they are of significant size or number that they interfere with normal implantation or movement of the embryo. One of the surgical areas of expertise at Oklahoma State University is in dealing with endometrial cysts. "We use ultrasound to diagnose and discover them, then use an endoscope to view the cysts and a laser to remove them," Slusher says. "The laser is a small-diameter fiber that goes down the endoscope; we find and remove the cysts in one operation (using a technique developed at Oklahoma State University)."

He says mares might have a few or many, and they might not be of significant size or number to cause problems. During the first 16 to 17 days of gestation, the embryo moves around inside the uterus before it attaches. "This is part of the message to the mare, so her body knows she's pregnant (and halts her heat cycle)," says Slusher. "Sometimes cysts prevent this traveling around (of the embryo). They also tend to occur in certain areas of the uterus, and large cysts may interfere with implantation of the embryo. Sometimes (although rarely) cysts create a problem in later gestation, but the more common problem is early; the embryo drops down from the fallopian tube about Day 6, and cysts can be a problem from then until about 45 to 60 days of pregnancy."

After cysts are removed with the laser, most of the mares can conceive and have a normal pregnancy, although some mares create a new batch of cysts again later. "We have to keep checking. We examine mares seven to nine days after parturition with ultrasound for possibility of a new group of cysts," says Slusher.

Defective Embryos

Older mares often produce defective embryos, which lead to early embryonic loss. "The embryo may die so early that you don't think the mare got in foal; she returns to heat like she would if she didn't settle," says Zent.

"Ever since we started flushing mares and looking at embryos--which is something we're doing even in Thoroughbreds, since it gives us an idea of what we've got to work with, even though you can't do embryo transfers in that breed--we've seen some defective embryos," says Zent. "We may flush a problem mare in late fall to see what her embryos look like and see if we can actually obtain an embryo, or if for some reason the egg is not being fertilized. It helps us know what's going on with that mare."

Embryo transfer, in vitro fertilization (aspirating eggs from a mare, fertilizing them in the lab, then putting the embryos into healthy young recipient mares), and other reproductive techniques might not always solve the problems in getting a foal from a valuable older mare. "When you take eggs from older mares, they are often not as viable as the eggs from younger mares," says Zent. "Older mares can also be harder to flush embryos from. When you use a young maiden mare you'll get a good embryo most of the time, but with an older mare you are lucky to get one good one out of four." In addition to physical limitations, you also have constraints of breed registries that might dictate how involved you can get with manipulating a mare or her embryos to obtain a pregnancy.

More Bad News

Zent says genetics can play a role in reproductive longevity/fertility. If a mare's dam had a lot of foals, chances are she will keep producing. She's a better risk than the mare whose dam was not able to produce very many foals.

Cushing's disease is another problem in some mares; older mares are more prone to pituitary problems that can affect cyclic activity and the outcome of pregnancy. "The high cortisol level (caused by pituitary problems) affects the fetus, and it doesn't do very well," says Zent.

The mare usually carries the foal too long. Signals for birth aren't well-recognized by the mare's body. Reduced placental area seems to go hand-in-hand with this problem, so even though the mare ends up carrying the fetus 13 months, it is small/weak or dysmature (full-term but not fully developed) when it's finally born, he explains.

A diminished immune system is another common problem in older mares. "We see more uterine infections, just from the normal contaminations of breeding, and more endometritis," says Zent. "A young maiden mare can handle a lot of contamination and throw it off, but an older mare is much more vulnerable."

Performing a Caslick's procedure (stitching the vulva to keep out contamination from fecal material) is routine in almost all older mares under aggressive breeding management.

Take-Home Message

Whether you are breeding an older maiden mare, or an older mare that has been competing, there are major obstacles to getting a live, healthy foal. There are advanced reproductive techniques to help the problem mare, whether she ends up producing the foal herself or contributing an embryo for transfer. Consider the time and cost that will be associated with breeding an older, less-fertile mare versus the value of the resultant foal.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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