The Oestrus (Estrus, Sexual, Heat, Season, “In”) Cycle describes the process through which the body prepares itself for pregnancy. Mares begin cycling at puberty, which takes place at about 1.5 years. They can become pregnant in that same year, but don’t usually become pregnant in the wild until their 3rd year. Early pregnancy is also not advised because it can stunt her growth. More on pregnancy can be found in a later article.
Horses are seasonally polyoestrus, meaning that they only cycle during a particular breeding “season”. In winter they are in anoestrus (sexually dormant). The cycles are influenced by daylight, and for this reason thoroughbreds, who are always bred early in the year, have artificial light in their stables.
After anoestrus, mares may not have regular cycles. In general cycles are 21 days, but may be up to 1 month after winter, and may not even yield functional follicles. Her behaviour may also not be typical “in season” behaviour, therefore making seasons very hard to asses!
There are two main phases to the oestrus cycle, oestrus, where the mare is ovulating, and receptive to the stallion, and dioestrus, where the mare is not ovulating, and will not accept the stallion. Oestrus lasts for 3-5 days, the last two days showing stronger signs (usually because the mare has ovulated ), and dioestrus for about 15-16 days.
As the day length increases, the hypothalamus (in the brain) releases follicle stimulating hormone (FSH)and luteneising hormone (LH) which stimulates the ovaries, and an immature ovum begins to ripen. The ripe ovum is released on the second-to-last day of oestrus, and is about 3-7cm (1.5-3.5in).
Signs of oetrus
- Pink ‘winking’ of the vulva
- Frequent urinating
- Excitement or hyperactivity
The ovum must be fertilized within four to six hours of being released. The remains of its follicle fill with blood and become the corpus luteum, a structure which secretes progesterone (a job which is taken over by the placenta during pregnancy) which is vital for maintenance of pregnancy and dioestrus. and After release, the ovum travels down the oviduct, where it waits for sperm. If its not fertilized it will degenerate in a few months.
And the cycle begins again.
- Follicle stimulating hormone (FSH) (from the hypothalamus (brain)) cause the follicles to develop
- The follicles produce oestrogen when they have developed sufficiently. This causes characteristic signs of oestrus.
- Luteinising hormone (LH) is released. This causes ovulation.
- The remainder of the follicle forms the corpus luteum, which produces progesterone and causes dioestrus (or the maintenance of pregnancy).