Mare owners especially will recognize it: excessive urination, tail to the side, a lot of neighing at other horses and no longer going forwards off your leg… The mare is in season. A mare in estrus is in reproductive mode, which means she has the urge to mate. She does not get this urge just like that, she only gets this when she is fertile. The same as with women, mares have a certain fertility cycle, also called the estrus cycle. 

Mares are so-called ‘long-day seasonally polyoestrous breeders’. This means that a normal, non-pregnant mare has a regular cycle from about April to October. A mare’s estrus cycle lasts an average of 21 days. Of these 21 days, the mare is in estrus for 5 to 7 days. In the period from October to March the mare is in anestrus. This means that the follicle development is at a standstill and the mare normally does not have an estrus cycle. In the spring there are a number of natural factors that cause the mare to enter estrus. These are the higher temperatures, the longer days (more daylight) and better quality of the grass. You can artificially influence these factors to get mares in estrus earlier or even year round. 

The estrus cycle starts at the moment of ovulation, which is also the end of the previous cycle. Ovulation is the release of the egg (ovum) from the ovary. After ovulation, the empty follicle becomes a corpus luteum. This corpus luteum produces the hormone progesterone, also known as the pregnancy hormone. Under the influence of this hormone, the uterine lining is prepared for possible pregnancy. When the mare indeed becomes pregnant, the embryo ensures that the corpus luteum remains intact. During the first 150 days of pregnancy, the entire body is responsible for the production of progesterone, after which the placenta takes over.

If no fertilization has taken place and the mare is not pregnant, the hormone prostaglandin is released. This breaks down the corpus luteum and thus stops the production of progesterone. This is the signal for the pituitary gland in the brain to make the hormones FSH and LH. FSH is the Follicle Stimulating Hormone; as its name suggests, this hormone ensures the maturation of follicles. LH stands for Luteinizing Hormone, it stimulates ovulation and ensures the development of the corpus luteum. Because of these two hormones, one ovum starts to grow. With this growth, another hormone is released: estrogen. Estrogen is also known as the reproductive hormone because it causes the mare to come into estrus. The follicle continues to grow until it has matured and ovulation takes place. Then the cycle starts again. 

Source: HMV module