By about day 17, the luteal phase of the estrous cycle comes to an end. The follicular phase begins with the removal of the blocking action of progesterone, which allows for greater amplitude and frequency of GnRH pulses.

How long does the estrous cycle last?

Estrus is the stage when the female is receptive to the male. This stage will last an average of 9 days based on behavioral signs, but can range from 4 to 24 days. The fertile period occurs during this time.

How long is the estrous cycle in horses?

Although the mare continues to ovulate regularly every 21 days throughout the breeding season, the length of estrus (sexual receptivity) varies from 2–8 days, and the length of diestrus varies accordingly to maintain a 21-day interval.