In South Africa, the mating season for horses officially starts on 1 August. Stallions will mate all year long, but mares ovulate only during the summer months. ‘Seasonal anoestrus’, as it is called, is linked to the length of daylight. Interestingly, mares kept in lighted stables may keep cycling throughout winter.
Often, yearling colts who have been in the herd and behaving well turn into breeding stallions overnight when the mares start cycling. It’s important, then, to know when your mares are coming into season and ensure that they are not being kept too close to ungelded colts or stallions, who may jump the fence to visit them.
The first sign a mare is coming into season is that she moves away from the herd to graze close to geldings or stallions, even if they are on the other side of the fence.The mare tends to be less settled under saddle and more aggressive to other horses, often kicking out or biting. This is known as ‘marish behaviour’.
Early in the heat period, a mare will be attracted to a stallion, but will lay back its ears, squeal and kick out if the male comes too close. It is receptive to the stallion if it stands still while being mated. At this stage it will squat and emit urine whenever it sees a stallion. A mare that is being ridden may stop and run backwards into the path of a stallion. If a stallion is being ridden in showing classes, mares in season can be a serious problem. Fortunately, a rider can usually notice them in time as the stallion reacts when it smells their scent.
In the southern hemisphere, most mares come into season every three weeks between August and April. In early spring and late autumn, mares remain in season for up to 10 days; this becomes only two to four days in mid-summer.
A mare is usually in season for about six days and she usually ovulates 24 hours to 48 hours before her season ends. She will most likely become pregnant if mated around the time of ovulation, although she may accept the stallion at any time during the six days she is in season.
How to tell
Marish behaviour is the result of changing hormones during the oestrus cycle, which is made up of two parts: oestrus (in season) and diestrous (not in season). Oestrus usually lasts for six days, varying between four and 10 days. Diestrus lasts between 12 and 18 days (usually 15). The oestrus cycle is initiated in spring when increased daylight stimulates the pituitary gland to secrete follicle-stimulating hormones. Follicles start growing in the ovaries. Each contains an ovum (egg cell) and as each grows, it starts secreting oestrogen.
It’s this that results in ‘marish’ behaviour and other signs of oestrus as it prepares the reproductive tract for pregnancy. Increasing oestrogen stimulates the pituitary gland to secrete luteinising hormone, which causes the follicle to rupture (ovulation). Once the mare has ovulated, the ovum moves down the Fallopian tube (oviduct) into the uterus.
The ruptured follicle hardens into a corpus luteum in the ovary and starts secreting progesterone. Progesterone stimulates the pituitary gland to secrete luteinising hormone. If the mare does not conceive, the uterus secretes prostaglandins, which cause the corpus luteum to stop producing progesterone after about 12 days, and the cycle starts again.