When we talk about the estrous cycle we are referring to the time interval between two estruses. Estrus is considered the moment in which the female is receptive to the reproductive male, and which is also known as heat or heat.
This cycle can have an average duration of 21 days, which is divided into two phases, a luteal phase (known as Diestrus) in which the female does not accept the male, and can have a period of 13 to 16 days, At this stage we will have the presence of the Corpus Luteum in the ovary; and the second phase includes the follicular phase (called Estrus) and can last between 4 to 7 days, in which there will be follicular growth and subsequently a dominant follicle and there will be no presence of corpus luteum.
It is important to know that on the days of Estrus, is when ovulation occurs and this event can take place 24 to 48 hours before the end of heat; which we will know is over when the mare stops showing signs of acceptance to the male.
On the other hand, we can mention Anestrus, which refers to the period in which the estrous cycle of the mare is paralyzed and its reproductive organ is static, that is, without follicular activity. In the case of the equine species, whose reproductive activity is related to the photoperiod, the amount of sunlight in the day, moments of follicular inactivity are normal in countries where environmental seasons are marked. (I invite you to read my Monta seasonal article)