**2. Neuroendocrine control of the estrus cycle in cycling mares**

The estrous cycle is defined as the interval of time between two consecutive ovulations. The approximate length varies between 18 and 22 days, considering on average a period of 21 days [4, 5]. The current nomenclature stipulates that the estrous cycle consists of two differentiated stages: estrus or follicular phase and diestrus or luteal phase. These phases are characterized by internal modifications of the sexual organs and glandular system as well as behavioral alterations based on the levels of oestradiol (E2) and P4 [6, 7].

### **2.1 Follicular phase**

Estrus, heat or follicular phase is characterized by the presence of follicles at different stages of development, and the simultaneous increase in the secretion of E2. It has a duration of about 5–7 days, with a variability of 3–9 days related to the season. Thus, estrus is extended in autumn (7–10 days) and is shortened considerably, in late spring and early summer (4–5 days). During this period the mare is sexually receptive to the stallion genital tract and is ready to receive and transport of sperm and finally culminates with ovulation [5, 6, 8].
