Understanding the basic physiology and anatomy of the brood mare will provide owners with information that is vital for successfully breeding healthy foals.

Anatomy

The mare has two ovaries that lie in the dorsal (upper) part of the abdomen, behind the kidneys. The ovaries are the female gonads and produce and release eggs. They are also the site of production of the female sex hormones oestrogen and progesterone. The uterus is comprised of two horns and a body, all being suspended from the roof of the abdomen by the broad ligament. The cervix guards the entrance into the uterus from the vagina.

The Oestrus Cycle

Mares are ‘long day’ breeders and in the Northern hemisphere, generally, are most sexually receptive from April through to September. During winter the mares become sexually inactive and are in a state of anoestrus. As spring approaches and day length increases, a gland within the brain, the pineal gland, produces less of a substance called melatonin and the mare enters a transitional oestrus period. In response to rising hormone levels, follicles are stimulated to grow within the ovaries. Initially lots of follicles grow simultaneously until one becomes dominant and ovulates. After this, regular oestrus cycles will occur through the summer months. Each oestrus cycle lasts around 21 days with the mare being in season (sexually receptive) for 3-8 days. During this time the mare will show variable behavioural changes. The mare will also be receptive to the stallion, allowing him to nuzzle her, whilst she squats and urinates and winks her clitoris.

These behavioural changes are due to increasing levels of the hormone oestrogen which is released from the follicles. Ovulation usually occurs 24-48 hours before the mare goes out of season. The follicle that has ovulated forms a structure called the corpus luteum within the ovary, which produces another hormone called progesterone. Progesterone, in simple terms, has opposite effects to oestrogen. The mare will now be out of season and aggressive towards the advances of the stallion. At around 16-17 days, the lining of the uterus produces a hormone called prostaglandin destroying the corpus luteum. This reduction in progesterone allows follicles to develop again and the mare to return to season. If the mare is pregnant the corpus luteum remains allowing pregnancy to continue suppressing further returns to cycle.