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 an older mare 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.

A foetus can be aborted from all mares, but older mares are at a higher risk

Should I breed from my 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 mare’s chances of success.

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.

Poor perineal conformation allowing contamination to the reproductive tract. This can be improved with a ‘Caslick’s’ procedure

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.

Good perineal conformation

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.

Is embryo transfer an option?

Embryo transfer (ET) has proved a useful solution for competition horses too busy for a career break. Lucinda Fredericks’ mare Headley Britannia was perhaps the ultimate working mum, producing seven ET foals throughout her teens.

The procedure also enables older, sub-fertile mares to donate embryos to reproductively healthy recipients and so be spared the process of carrying and delivering a foal. The recipient mare’s age can vary (between four- and 15-years-old) as long as they are proven to be good breeders and have a reproductively sound uterus capable of supporting an embryo.

Embryo recovery rates as high as 75% can be achieved in healthy, young donor mares — and remain around 54% with older donors of 13-25 years. But early embryonic loss is still significant, so obtain a clear idea of the costs involved before embarking on this process.

The vet

Emma Houghton MRCVS is based at Bell Equine Veterinary Clinic in Kent, where she is responsible for the reproductive caseload. Her particular interests are breeding and neonatal care. She has completed the certificate in equine medicine and is studying for a certificate in stud medicine. bellequine.co.uk, 01622 813700

Ref: Horse & Hound; 2 March 2017