The oestrus cycle is also referred to simply as the controlled estrus when a mare is acceptable to a stallion, and helps the body prepare the mare for conception.
Usually mares are male (in heat) once a year, this usually happens in spring or summer. The period of the mare is normally heat-resistant in warmer weather. In temperate regions, non-pregnant equine females generally have a seasonal estrous cycle, from early spring to autumn. The majority of females will go into a state of cessation of heat during winter and are therefore not able to conceive / give birth during this period. The reproductive cycle is controlled by cycle of lighting (daytime time length), with oestrus stimulated as the length of the daytime time lengthens.
Cessation of estrus prevents a female from conceiving during the winter months, so the chances of survival of the offspring are very low during the harshest times of the year. However, when they live near the equator, the variation in daytime time is negligible, the females do not have a period of cessation of heat, about 20% of domesticated mares in the Northern Hemisphere also in heat year-round due to loss of sensation toward melatonin.
Ovulation in females is generally one egg, although about 24-26% is multiple (99% of which is two). The interval between ovulation sessions is 1 day. Levels of progesterone (the sex hormone that maintains pregnancy) will increase after the second ovulation. The estrus cycles of a horse occur about every 19-22 days, usually between early spring and autumn. Most horses do not go into heat during winter, and hippocampus does not occur. Horses as young as 18 months old are sexually and reproductive (perhaps that is why the expression “slut horse” indicates fullness), but it is common for a stallion to be over three years old. Only four years old will truly mature in horses.