Ask the Vet: Reproduction

Courtesy of AAEP
Answered by Madison Seamans, DVM, MS, Kuna, Idaho

Question: How old is too old to breed a mare for the first time?  

Answer: Mares do not typically go through menopause, as they will usually cycle throughout their entire lives, but their fertility starts to decline after about 13 years as their eggs and reproductive organs age. Mares that have had foals regularly up to that age, may be easier to get in foal, so their productivity can extend into later years. However, older, “maiden” mares can be a real challenge to breed. There is no greater danger to the mare due to advancing age, but conception is often more difficult.

Question: I have a 16-year-old mare that has had two foals after we purchased her four years ago. We have tried to artificially inseminate (AI) her to no avail. She spent a week at Auburn University in which they could not find a reason why she would not breed. Any suggestions?  

Answer: Mares are remarkably fertile animals. However, their productivity declines after the age of about 13. The fact that your mare has had two foals in the last four years is encouraging, but without more details of her breeding history, it will be difficult to make specific suggestions here. In young, “normal” mares, pregnancy occurs in about 60% of cycles. During the course of the average breeding season, though, we would expect 90% or more to conceive after breeding through four or five cycles unless infection or other pathology is involved. Even though she has had foals in previous years, she has eggs and a reproductive tract that are 16 years old, and many mares of this age can be challenging.

Managing older mares often requires diagnostic tests. Culture, biopsy and ultrasound images can detect abnormalities that may impair fertility, but our understanding of the amazingly complex intricacies of reproduction is still very limited. Although the standard diagnostic tests may come back completely “normal”, these observations are a minuscule segment of the entire process. There are many things we can measure, but the microscopic, cellular events that must take place for pregnancy to be established and maintained to term are still a mystery . Some mares just “skip years”. It is not uncommon, despite exhaustive measures, to fail to get the mare in foal this year, but she easily conceives the first time next year. Artificial insemination (AI) can improve fertility in older mares because it reduces the chance of infection, but many mares–even difficult cases–will “take” when they are simply turned out with a stallion.

Although stallion owners may be reluctant to do this due to concerns for safety, horses have been making foals a lot longer than there have been people to manage them. You may try changing stallions. There are aspects of the immune system that may impair fertility in mares bred repeatedly to the same stallion. In addition, there are some breed specific genetic disorders that can have effects on fertility. These tests are readily available through your AAEP member veterinarian.