Female age and parity in horses: how and why does it matter?

Although puberty can occur as early as 14-15months of age, depending on breed and use, the reproductive career of mares may continue to advanced ages. Once mares are used as broodmares, they will usually produce foals once a year until they become unfertile, and their productivity can be enhanced and/or prolonged through embryo technologies. There is a general consensus that old mares are less fertile, but maternal age and parity are confounding factors because nulliparous mares are usually younger and older mares are multiparous in most studies. This review shows that age critically affects cyclicity, folliculogenesis, oocyte and embryo quality as well as presence of oviductal masses and uterine tract function. Maternal parity has a non-linear effect. Primiparity has a major influence on placental and foal development, with smaller foals at the first gestation that remain smaller postnatally. After the first gestation, endometrial quality and uterine clearance capacities decline progressively with increasing parity and age, whilst placental and foal birthweight and milk production increase. These combined effects should be carefully balanced when breeding mares, in particular when choosing and caring for recipients and their foals.