Mares are usually considered seasonal breeders, that is they will usually show oestrus (‘in season’) behaviour in the warmer spring/summer months. ‘In season’ behaviour may include: increased attachment to other horses, increased whinnying to other horses, squatting and urination at the approach of other horses, ‘winking’ of her vulva when approached by other horses. This behaviour usually lasts for about 5 days.
Mares usually have an approximately 3 week reproductive cycle. As the mare is about to cycle, one or more follicles develop on her ovaries. During oestrus the follicles grow in size until they are approximately 4-6 cm in diameter, when they ovulate and release the egg into the fallopian tubes. To achieve pregnancy it is desirable to serve or inseminate the mare as close as possible before ovulation occurs. Once ovulation has occurred a structure called a corpus luteum forms which secretes progesterone and stops the mare coming back into season. From there 2 things may occur:
- If the mare is served or inseminated at the appropriate time, the sperm fertilizes the egg in the fallopian tubes, and the fertilized egg gradually travels down the fallopian tubes until it enters the uterus approximately 5 days after fertilization. If pregnancy occurs the corpus luteum remains as the main hormonal support for the pregnancy for the first 35 days of gestation.
- If the mare is not pregnant the corpus luteum disappears approximately 2 weeks after ovulation to and the mare will come back in season.
Most breed societies in Australia have determined an arbitrary date of 1 August as the birthday for horses (for standardbreds it is the 1 September). To fit with this date it is usual to start serving mares in the first week of September (October for standardbreds). There is pressure on breeders to have their mares foal earlier rather than later in the season, especially if they are planning to sell the foals as weanlings to 2 year olds (for example most thoroughbred foals) as foals born earlier in the season may be bigger by sale time than those born late in the season. This arbitrary date often does not coincide with the peak fertile period for horses, especially in southern states of Australia. There are management practices and medication that may help to encourage mares to breed earlier in the season which may be discussed with you if you choose to breed from your mare.