Breeding Older Mares

Breeding Older Mares

Tuesdays with Tony

Getting older is really tough. Every day it seems like some other body part is aching or something else is sagging. I’ve heard the humans talking about it forever and now, unfortunately, I’m starting to feel it myself. Since we’re in the midst of breeding season, I thought it would be appropriate to discuss the senior mare you wish to breed. Quick reminder, breeding is not for the faint of heart, even when everything goes absolutely perfectly. Which is basically never.


Think about people for a second. Yes, I know, people are gross, but indulge me. What is “peak breeding age” for people? Twenties to early thirties, right? This does NOT mean twenty to thirty is the peak breeding age for your mare. Horses have a much shorter life expectancy than people and reach sexual maturity much earlier. We can talk about the thoroughbred racing industry another time, but I do find it a very good example for when to breed a mare. Most thoroughbred racehorses have completed their career by the time they are 5-6 years old. Then, if they are a mare, they go to the breeding shed. This is when they are most fertile, most likely to conceive, most likely to carry a foal to term, and most likely to have the fewest complications while foaling. 

A young mare, 3-8 years old, is in her prime for breeding. I know what you all are saying, “but Tony, I’m still riding my mare, I can’t breed her yet.” Totally fair statement. However, there are options which we will get into in just a bit.

A young mare has a pregnancy rate of 55% which means they have a 55% chance of becoming pregnant, maintaining a pregnancy and foaling normally. It can often take three tries to get even the youngest, most fertile mare in foal. That’s the full breeding expense times three, with no guarantee.

As your mare ages, her uterus also ages. A 9–13-year-old mare who is in her prime competition/riding age only has a 30% pregnancy rate. In 14–18-year-old mares that rate decreases to 10%, and if the mare is over 18 years old, we are looking a 2% pregnancy rate. Age is more than just a number when it comes to breeding your mare.

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Your mare’s uterus is an extremely important player in her ability to get pregnant and maintain pregnancy. The uterus becomes the foal’s waterbed for at least 11 months, sometimes longer. A safe, healthy uterus is essential to having a safe, healthy foal. Back to this whole, body parts like to start sagging as we age thing. The uterus in mares is no different. As they age, the stretchy connective tissues of the uterus and body lose some of that elasticity. 

During your mare’s reproductive examination, you may hear my docs mention that a mare has a “dependent” uterus. This means that it sags. Instead of being up in pelvic region, the uterus dips down into the abdomen. As a uterus and its connective tissues loose elasticity, strain is placed on nearby blood vessels. When an older mare becomes pregnant the weight of the foal puts tension on the tissues and blood vessels. This makes her significantly more prone to blood vessel rupture and uterine torsion during the foaling process. Both of these scenarios are life-threatening to your mare and the foal.

Older mares also tend to develop cysts within their uterus. These cysts are fluid-filled sacs that extend from the endometrial tissue into the lumen of the uterus. Cysts often contribute to fertility issues. The presence of cyst affects the mare’s uterine biopsy grade, suggesting that mares with cysts are less likely to become pregnant and maintain pregnancy to term.

It’s not uncommon for my docs to see a mare with a few small cysts present in their uterus. They take care to document these cysts as often they can look like embryos. Knowing that your mare has cysts allows you and the docs to discuss the likelihood of pregnancy. Mares with multiple, large cysts throughout their uterus have an even lower chance of pregnancy and it is almost always recommended to not breed these mares.

A uterine biopsy is highly recommended when there is the desire to breed an older mare. The biopsy gives the veterinarian an inside look at the uterus and its ability to carry a foal to term. It looks at the tissue of the uterus on a microscopic level. Inflammation, infection, and fibrosis, or lack of stretchiness, is evaluated. As the mare ages her uterine biopsy score increases. We do not want high uterine biopsy score. A biopsy grade of 1 indicates a greater than 80% chance of pregnancy to term. Grade 2a gives the mare a 50-80% chance of pregnancy to term. Grade 2b, we’re at a 10-50% chance and a Grade 3 uterus has less than 10% chance of becoming pregnant and carrying the foal to term. The numbers don’t lie, and it can be extremely difficult to make a decision on what to do when you want to breed your older mare.


Fortunately, unlike people, mares continue to cycle their whole lives. A mare is born with every immature egg she will ever have. That means that if you’re trying to breed your 25-year-old mare, the egg she ovulates is also 25 years old. A 25-year-old egg has been sitting there, waiting to do its thing. During that time, there is a chance that the egg will suffer some kind of damage, thus, reducing its fertility.

Think about it this way: how many scars does a 5-year-old horse have compared to a 25-year-old? Same with the 5-year-old eggs. They’re bright, happy, full of life, and have yet to be beaten down by the world, whereas a 25-year-old egg has been around the block and seen some stuff and is maybe thinking it would rather retire and just relax than make a baby. This doesn’t mean a 25-year-old egg can’t be fertile, it just means it’ll probably take multiple 25-year-old eggs before one decides it wants to be a baby.

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I know what you are all saying, “but Tony, I love this mare! She’s been the best for me, she has the best personality, and she would make the best mom.” I believe you; I really do. So what are our options for having a foal from your mare that does not put your mare at risk and will also get you the baby you desire?

There is always the (less desirable) option to try, knowing the risks associated with breeding the older mare. Remember, it doesn’t always work and definitely doesn’t always work on the first try. It’s probably going to cost a lot more than you realize, it’s high-risk, and there aren’t any guarantees. If you decide this is the route you want to take, please listen to my docs when they tell you that the mare needs to foal out at the clinic where I can keep an eye on her 24/7. They may even recommend that she foal out at a referral clinic just in case a cesarian section becomes necessary. I promise, my docs know what they’re talking about and want your mare and foal to come home safe, happy and healthy. They won’t steer you wrong.

The other option is embryo transfer. Highly, highly recommended for the older mare. The foal will have the genes from your prized mare and the stallion you picked, but a surrogate mare will do the heavy lifting. The surrogate mares have been handpicked for this job. They are not a backyard mare that your neighbor said you can use. These mares live their lives to carry babies for other mares. They are monitored daily, their cycles are set to match up exactly with your mare’s cycle so that once your mare is bred, the embryo can be taken from your mare and placed directly into the recipient mare.

Embryo transfer is a complicated process, but essentially, the way it works is, your mare is bred as if she is going to get pregnant and carry the baby. However, 7-8 days after your mare ovulates, a process is performed where her uterus is flushed, and the embryo is removed. Once an embryo is located, that embryo is placed into the recipient mare who will then carry the foal to term. It also doesn’t always work and definitely doesn’t always work on the first try, however, it’s the best option for breeding an older mare without risk to her.

I’m sure you have plenty of questions about breeding your older mare. That is what the docs are here for. Feel free to call and schedule a pre-breeding examination for your mare and the docs will be happy to talk about all the ifs, ands, or buts. I’ve said it before and I will say it again: breeding is not for the faint of heart! It’s a tough job, and it can get expensive. But my docs are here for you and your mare and are arm deep in it every step of the way with you.

Until next week,


P.S. Don’t take my word for it! Go on over to the Podcast Page and listen to some of the breeding episodes. There are 4 or 5 to pick from, although, if you’re serious about breeding, you should probably listen to all of them. And if you aren’t trying to breed (that’s a good human!) there are a lot of other episodes you’ll want to check out. It’s the easiest free education on horses you’ll ever get. You just have to listen while you drive around shopping for cat treats. It’s hard to beat that!

Tuesdays with Tony is the official blog of Tony the Clinic Cat at Springhill Equine Veterinary Clinic in Newberry, Florida. If you liked this blog, please subscribe below, and share it with your friends on social media! For more information, please call us at (352) 472-1620, visit our website at, or follow us on Facebook!

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