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

The research article discusses the effect of age and number of births (parity) on female horses, touching on how factors such as fertility, reproductive health, and offspring development are influenced by these variables.

Understanding the Factors at Play

• The study begins by addressing the evolutionary and physiological aspects of horse aging and reproduction. The researchers note that puberty can occur in mares as early as 14-15 months and their reproduction lifespan may stretch to advanced ages, depending on the breed and use of the horse.
• The general assumption is that older mares are less fertile, but the researchers propose that this is intertwined with the factor of parity; older mares have typically given birth more times which might influence fertility levels. Nulliparous mares (those that have never given birth) are usually younger and therefore also potentially more fertile.

Impact of Age on Reproduction

• The paper explores findings that show critical age impacts on various aspects of reproduction in mares. These include cyclicity (the recurring cycle of estrous), folliculogenesis (the process of ovarian follicle development), quality of oocytes (female reproductive cells) and embryos, and output in terms of uterine tract function and presence of oviductal masses.

How Parity Influences Offspring Development

• The study also investigates how the number of times a mare has given birth influences the development of offspring. The researchers found a non-linear effect of parity on maternal and foal development.
• This means that first-time mothers (primiparous mares) tend to have smaller foals that also remain smaller after birth. But with each subsequent gestation, there is a decline in endometrial quality and uterine clearance capacities.
• Interestingly, the researchers also found out that as mares continue to give birth, there is an increase in foal birthweight, milk production and placental weight. This shows that the mare’s body adapts and improves in certain aspects with each birth.

Implications for Breeding Mares

• The data presented in the study provides valuable insights for breeding mares. It emphasizes the need to carefully consider the age and parity of a mare when selecting and caring for recipients and their offspring.