LONG DAYS: A SEASONAL CHALLENGE

MARES are seasonal breeders and require long, light days to induce follicular activity.

Most mares in the UK begin cycling naturally from mid-March and continue until autumn, when they enter a period of sexual inactivity known as “anoestrus”. “Spring heat” is the transitional phase where follicular activity of the ovaries commences. This can cause a prolonged heat period of up to six weeks, which can be problematic at this time of year.

Here’s what’s happening:

The oestrus cycle of a mare is, on average, 21 days in length, of which five to seven days are called the follicular phase. Within her ovaries are small, fluid-filled sacs called follicles, which release increased levels of oestrogen as they mature. This is when the mare is in-oestrus and may display behaviour to show she is receptive to mating. The release of an ova (egg) from a dominant follicle, known as ovulation, occurs on days three to five.

Typical signs that a mare is in-oestrus include winking of the vulva, tail raising, more frequent urination and standing as if ready to be mounted by a stallion. These signs, often more evident when a stallion or an interested gelding is present, are due to the stimulation of her hormone receptors for oestrogen and testosterone and a decrease in circulating progesterone levels. The hormones are then thought to act on the mare’s central nervous system, resulting in this related behaviour.

After ovulation, the follicle develops into a structure that secretes progesterone to prepare the uterus for pregnancy. The uterus will recognise that the mare is not in foal around 14 days post ovulation. Hormones are then released that remove this progesterone source, allowing follicular activity to resume and oestrus to begin.