Middle-aged Mums

Tempted to breed from an older mare? Emma Houghton  MRCVS outlines the costs, complications and considerations to bear in mind.

- Emma Houghton  MRCVS

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.