Appropriate Breeding Age

Q: What is the correct age to breed a mare for the first time? I have strongly considered breeding my Pure Polish Arabian filly before she begins a serious dressage career. I would keep and train the foal. The filly is currently 3 years old. I have been told many ages of when I should first breed her: 4, 5, 6, and even 10! I would like a professional's opinion and would really appreciate what you have to say.

Jasmine, California

A: I'll start by commenting on behavioral considerations and share what I know about natural horse reproduction and behavior. Dirk Vanderwall, DVM, PhD, Dipl. ACT, from our reproduction center here at New Bolton Center, will comment from the veterinary reproduction specialist's perspective.

Under natural herd conditions mares that are in good nutrition typically start cycling, are bred, and become pregnant in their yearling spring, often even before their first birthday. Some of these young mares will lose their pregnancies, but most go on to foal at about the age of 2. Their first foal might be small, reflecting the dam's not yet fully grown size. The milk of a 2-year-old filly can be a bit slower to come in than that of older maiden mares, but it usually comes in strong within a day or so after the foal is born. Similarly, maternal behavior is variable and often a bit delayed in some 2- and 3-year-old first-time dams.

In a natural herd situation the dam and foal have the social support of the other herd members, especially the older mares and the harem stallion. Harem stallions in nature are diligent dads, often picking up the slack of parenting, by looking after new foals and keeping the neonates alongside ambivalent young dams until their maternal behavior kicks in--usually within a few hours. Under natural conditions, fillies that first foal as 2-year-olds usually do not grow to be as big as mares that do not foal until 3 years of age, and their first foals tend to be smaller adults as well.

While most domestic breeders would consider yearlings and even 2-year-olds too young for breeding, certainly there are populations of domestic horses around the world in which breeding fillies at that age is understood as normal.

From that perspective my opinion is that for well-managed domestic mares, anytime after their yearling year is fine to breed. Your mare is 3 years old and would be foaling at 4 years or older. She would be well on her way to being fully grown physically, so you should have no concern for her ending up smaller as an adult or having an especially small first foal. For a young mare, having a foal before training can have distinct benefits. From having known and worked regularly with dozens of mares that foaled young, it is my impression that most seem to mature by leaps and bounds with foaling and caring for a foal. They tend to mellow with regard to human handling and training, seem less fearful and distracted, and are better able to focus on human direction. So breeding could be an ideal interlude between basic training and serious dressage training.

According to Dr. Vanderwall: As noted, fillies generally go through puberty as yearlings and, therefore, by definition can be considered "fertile" at that time. However, the general consensus within the horse/veterinary community has been to give fillies additional time to mature by allowing them to reach 3 years of age before breeding to carry a pregnancy to term. Dr. McDonnell's findings that under natural conditions fillies that foal for the first time as 2-year-olds (as well as their offspring) usually do not grow as big as mares that do not foal for the first time until 3 years of age supports that rationale. In addition, a study published in the 1970s demonstrated there can be very high pregnancy loss/abortion rates (46%) in yearling fillies.

An alternative to waiting to breed a mare until she is 3 years old is embryo transfer, which has been used to produce offspring from 2-year-old fillies since the 1980s and even more recently has been performed successfully with yearlings, though the embryo recovery rate was lower in yearling fillies compared to 2-year-old and older mares. If young mares are used repeatedly and extensively as embryo donors, many believe the mares' long-term fertility might benefit from being allowed to occasionally carry a pregnancy to term.

There are additional factors to consider when breeding an older mare for the first time. Mares that are not bred for the first time until their teenage years may demonstrate characteristics that are referred to as "aged maiden mare syndrome." One feature of this syndrome is failure of the cervix to relax when the mare is in estrus, predisposing the mare to retaining fluid in the uterus after breeding. This results in persistent mating-induced endometritis (failure of the uterus to clear foreign contaminants, resulting in inflammation of the inner lining of the uterus), which requires appropriate treatment such as uterine lavage with a large volume of sterile saline along with the use of "ecbolic" agents--for example, oxytocin--to stimulate uterine contractions that will help evacuate fluid from the uterus.

Although some broodmares have an unblemished fertility record and produce foals year after year throughout their lifetime, such mares are the exception rather than the rule, particularly once they reach their middle teen years and beyond. Older mares tend to become more susceptible to problems such as persistent mating-induced endometritis or infectious ¬endometritis, both of which make breeding these mares more challenging; however, possibly the biggest hurdle to successfully breeding aged mares is declining egg (oocyte) quality.

It is now well-documented that the eggs of mares over 18 years old have a very high incidence of inherent defects that result in a high rate of early pregnancy loss (20-30% or higher). In women, an age-related decrease in egg quality is associated with chromosome abnormalities characterized by too few or too many chromosomes (called aneuploidy) that cause pregnancy failure and congenital birth defects such as Down syndrome (extra chromosome 21). Although it is plausible that similar chromosomal abnormalities are responsible for decreased egg quality (and higher pregnancy loss rates) in aged mares, the exact reason for declining egg quality in mares is currently unknown. Ultimately, for mares that live long enough, they may reach a point at which they undergo a complete cessation of reproductive activity, which is referred to as age-related reproductive senescence. This occurs in some mares when they reach their mid-20s or older. So considering all of the above factors, the optimal age to breed a mare for the first time is when she's 3 to 10 years old.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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