The equine species has relatively poor fertility compared to other livestock species. Seasonality of reproduction, a long gestation period and above all, human interference in the physiology, may reduce the effectiveness of getting the mare in foal.
Part 1: PHYSIOLOGY
The first oestrus (coming into puberty) and ovulation in mares usually occurs between 12 and 18 months of age (on average 15 months) but varies from 8 to over 27 months. Such substantial differences depend on many factors such as the month of the mare’s birth, her breed and physical condition and her location (stable vs. paddock).
Puberty is not consistent with sexual maturity; the latter being defined as the capability of carrying a full-term gestation and giving normal birth (parturition). In practice, mares are often served / put in foal between three and 4 years of age, depending on their sport and race careers.
There are no age limits in equine reproduction or equivalent of the human “menopause”. Mares can be successfully bred into their 20s, if they haven’t experienced significant health issues or had difficulties foaling. However, we must take into consideration, that fertility rates and fertile ovulations may decrease in older mares.
The equine reproductive cyclicity is strongly influenced by changes in the photoperiod (the length of the day), impacting on hormonal secretions. Horses are “long-day breeders” which means that the reproductive (breeding) season occurs during the longer days in spring and summer, while during autumn and winter the majority of mares have inhibited cyclicity and oestrus behaviour (seasonal anoestrus)
In the Southern hemisphere the reproductive season starts at the beginning of September and lasts until late February/ beginning of March, with a little variation depending on the latitude.