Breeding from older mares: is it a good idea and what are the best methods?

Equine breeding veterinary specialist Emma Houghton MRCVS from Bell Equine in Kent, explains what owners need to know before attempting to breed from older mares, while Selene Scarsi speaks to breeders who have done so successfully

Maybe your mare has finished a long competitive career, or you can only now afford to put her in foal. But is she still a good prospect for breeding if she’s in her teenage years or beyond? Breeding from older mares comes at a cost in terms of fertility, which peaks at six- or seven-years-old. Studies have shown that both pregnancy and foaling rates decline markedly after the age of 12-13.

Mares very rarely progress as far as a reproductive senescence (menopause). But as they age, the interovulatory period (time between seasons) increases, and their oocytes (eggs) are of poorer quality.

The 60% chance per cycle of a healthy mare becoming pregnant when mated with a fertile stallion reduces to 20-40% if she is older. This means that she may need to be covered over more cycles to achieve a viable pregnancy, which increases veterinary, stud and semen costs.

Reasons for reduced fertility

•  Older mares are more likely to suffer from pregnancy loss during gestation. A significant number occur in the first 40 days (known as early embryonic loss); a mare aged 11-plus is up to four times more likely to lose a pregnancy than one aged two-four. Another study has shown incidence to be as high as 20% for mares over 18 years (compared with a 6-15% occurrence in younger mares). Chromosomal abnormalities of the embryo, aged oocytes and poor uterine conditions are the main cause of this embryonic loss.

•  Poor conformation is another factor. Older mares may have reduced integrity of the protective barriers of the reproductive tract — including the vulva, vestibular seal and cervix. Their vulva can tilt, which, alongside a sunken anus, may result in defecation onto the vulva, causing contamination of the reproductive tract and the risk of uterine infection.

•  Older, maiden (first-time) mares are more likely to have a tight, fibrotic cervix. All mares develop a transient inflammatory response to breeding, but this is usually cleared naturally by healthy, younger mares. In older mares with a fibrotic cervix, the fluid is unable to drain as efficiently and can accumulate, requiring veterinary treatment and management.

•  The mare’s uterus may have a poor ability to nurture the pregnancy due to age-related development of endometrosis (uterine degeneration). This can occur in both maiden mares and those that have previously foaled.

•  Ovulation failure is seen in approximately 5% of cycles, but increases to 13% in older mares.

•  Uterine cysts can be identified in 55% of older mares that have been previously bred. While small cysts are unlikely to cause a problem, numerous large cysts may inhibit normal embryo development and require surgical removal.

•  Older mares may be suffering from age-related conditions, such as Cushing’s disease, which can cause irregular or absent oestrus cycles.

Should I breed from my older mare?

There are other points to consider.

The mare should be in a good body condition, or on a rising plane of nutrition. She should also be sound, as any chronic lameness may worsen when the foetus becomes heavy.

If she’s a maiden mare or has just foaled, she is more likely to become pregnant than if she is barren (not covered or failed to conceive in the last breeding season). In addition, knowing the reproductive history is important as 33% of mares that have experienced abortion subsequently aborted in the following pregnancy. Any previous reproductive trauma or infections should be taken into consideration.

Choosing a fertile stallion is essential. In the UK thoroughbred population, 20% of stallions have been associated with increased early embryonic losses — this may be due to management, venereal diseases and mare-stallion gene incompatibility, but also due to genetic abnormalities of the stallion.

While your breeding method may be dictated by stallion choice, artificial insemination (AI) is the preferred option for older mares. With natural cover, a significant amount of contamination can occur, which could lead to fluid accumulation or infection.

Mares can also experience chromosomal problems: an aged oocyte with a chromosomal problem may either not develop or develop to an early stage before it then regresses, resulting in pregnancy loss. And finally, an older mare may produce a smaller, poorer-quality foal because her uterus has developed age-related changes that result in poor placental development.

Improving the odds

Practical measures can increase your chances of success when breeding from older mares.

It is important to screen for venereal diseases by testing for equine viral arteritis (EVA), equine infectious anaemia (EIA) and contagious equine metritis (CEM). Your vet will also perform a thorough gynaecological examination to assess her perineal anatomy, before performing ultrasonography of the uterus and ovaries.

Once she is in season, an endometrial swab should be taken to test for uterine infection (endometritis) — and any infection should be treated before breeding continues. Due to the higher risk of endometrosis in older mares, a uterine biopsy may be performed to help determine the degree of degenerative uterine changes present and assess the prognosis of a successful breeding.

Ideally, older mares should only be inseminated once per cycle due to their increased susceptibility to post-breeding inflammation. Plan the procedure as close to ovulation as possible. Ovulation-inducing drugs such as Chorulon or Ovuplant are frequently used to assist with this.

Another examination is required post-breeding, to ensure that ovulation has occurred and to assess any fluid accumulation. It is vital that fluid is cleared from the uterus before the embryo descends from the oviduct into the uterus, at around day six. If necessary, uterine lavage and drugs such as oxytocin may be used.

For mares with poor perineal anatomy, a Caslick’s procedure can be performed. This involves suturing the vulval lips together to prevent air and infection entering the vagina.

Breeding from an older mare can be rewarding, but risks and lower fertility rates must be taken into account. It is recommended to consult vets that are registered on the British Equine Veterinary Association (BEVA) Approved AI list, as these vets will have been on regular reproductive courses and are therefore up to date with the latest research in this area.

Breeding from older mares: what experienced breeders say

By Selene Scarsi 

Caroline Ironside of MFS Studfarm in north-east Scotland has successfully bred from several older mares, all of whom have been at the stud for many years.

“Most of the mares at the stud are over 14, and all have been with us for a long, long time. This means that we have a really good idea of what they match well with; we know what does and doesn’t work with them, and the type of stallion they can go to,” says Caroline.

“When you see their stock under saddle, you get a much better idea of temperament and rideability – there is less unknown, whereas often with a young mare it’s a complete guess in terms of what they’ll produce.”

Of course, the older the mare the more difficult it is to get her in foal, but Caroline ensures a daughter of her mares remains at the stud, in order to keep the line going.

“Once they are in their 20s, most will have a foal every second year. We base it on the mare and how they’re looking health-wise,” says Caroline. “If they don’t go in foal then we’ll leave it and try the next year. If they can’t, then we retire them.”

In terms of management, special attention is paid to nutrition. “Some mares are good-doers and others lose condition quite easily, but you do tend to notice a greater loss of condition as they age – carrying a foal takes more out of them, so we need to supplement a little bit more,” says Caroline. “We take blood samples from older mares, just to check they aren’t missing any essential vitamins or minerals.

“We home-grow our hay and haylage, and the feed is tailored to balance that, so there’s plenty of attention to feeding regardless of age. However, the older mares will have a slightly more tailored diet to be triple sure there are no inconsistencies or deficiencies.”

Emma Blundell of Mount St John Stud adds: “An older mare might not have as much milk, the milk might start losing some of the nutrients, or the colostrum might be of lesser quality. We test all the colostrum irrespective of age, as sometimes it’s not as good and foals will require supplementation, but this is more likely to happen with older mares.”

Independent equine nutritionist Fiona Watkins agrees: “Once the mare’s forage and concentrate requirements are met in full, a breeder may want to look into specific supplementation that may affect milk and colostrum quality. This includes omega-3 fatty acids, vitamin E – particularly beneficial for its antioxidant properties – live yeast and dietary protein.”

Conception rates

When it comes to breeding from older mares, Emma insists that the key is treating each horse as an individual.

“Some mares at 21 look like they’re 16, others less so. They will tell you when they’ve had enough. The conception rates will be lower with older mares: some might get in foal, but will struggle to maintain the foal because of the condition of the uterus,” she says.

“We ensure all of our mares are in top shape before delivery, with regular turnout and time on the horse walker, but I cannot stress enough how much of a physical exertion having a foal involves. There’s lots of extra weight – not just the unborn foal itself but the fluid around it, too – and it’s a lot to carry around all the time, especially towards the end. This inevitably takes its toll much more on an older mare,” explains Emma.

For those who are able, embryo transfer (ET) – the technique of removing (flushing) an embryo from the uterus of one mare (the biological dam) and placing it in the uterus of a younger recipient mare – can be the best of both worlds.

“It’s such a great way of ensuring you hold on to the really special genetics, but are then able to give the pregnancy to a younger mare who can run around the field and enjoy the process a bit more,” says Emma, who uses ET for around half of her foals.

“We’ve had lots of mares come to us at 18, after their previous owners couldn’t get them in foal, and they’re now still breeding in their 20s by ET with no issues.”

“Tricky to handle”

International grand prix rider Nikki Crisp put her retired international grand prix mare Pasoa in foal to the up-and-coming grand prix stallion Durable, when she was 21. In May 2019, aged 22, Pasoa gave birth to her first foal, a healthy big colt named Danoa (William).

“Historically Pasoa was so tricky to handle that I never seriously considered ET when she was competing, or inseminating her when she retired at 18; she didn’t owe me anything, and I didn’t want to traumatise her,” say Nikki.

“It was only a few years later as we were having another mare inseminated that I decided to give it a try, almost on a whim, and with the idea that we wouldn’t force it – we wouldn’t induce a cycle, for instance. Surprisingly, she was totally fine with the entire process: because the repro vet was busy with her behind as opposed to her head or neck, she couldn’t have cared less.

“Because of her age there was possibly some loss of elasticity in the uterus and we went for a ‘cross every T and dot every I’ approach, flushing her for a full week, really cleaning her out, and we also had her on Regumate to support the developing embryo. But she scanned in foal first time and gave birth to a huge, healthy colt and she’s clearly adored being a mum.”

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