Most mares only come into estrus, also known as heat, during the spring and summer. However, some mares cycle year-round, and thus do not have a defined breeding season.
Mares can only get pregnant when they are in estrus, and this is typically also the only time that they let stallions mate with them. Their cycles are triggered by changes in the light. As days get longer, the mares start to cycle, while as days get shorter, they tend to stop. Mares living along the equator may be more likely to cycle year-round.
People who breed horses often attempt to time the breeding depending on when they want the foal to be born. Mares are pregnant for about 11 months. Many show and racing breeders strive for foals born earlier in the year, which means the foal is more developed for competitions as a 2- or 3-year-old. People who do not plan to compete with their horses at that young age often try for foals born in late spring, particularly April and May. This is usually the best time of year for mild weather, which is safer for the mare and foal. Foals born later in the summer are often difficult to wean because they may have to share the same barn as their mothers, and the mares are often more uncomfortable while pregnant during the hot summer months.