Young Guns

Q: I would like to know at what age a colt could breed a mare. I have a 10-month-old colt that has been in with mares until recently, and I was told he could have bred my mares.

A: There are many examples of colts as young as yours siring offspring, so there is good reason to believe he might have established some pregnancies. For research purposes, there have been many definitions of this point of fertility (puberty). One common definition is the point that the ejaculate contains at least a half-billion sperm.

The textbook age at which most colts achieve these criteria is generally 12-18 months. The figure of a half-billion sperm comes from what is believed to be a sufficient breeding dose under average conditions. But under excellent conditions with fertile mares and good sperm near ovulation it might not take nearly that many sperm. Sometimes managers assume the physical difference in size will preclude a yearling from breeding mature mares of the same breed. But, where there is a will, there's usually a way. And, if a colt is the only available male--and sometimes even if he's not the only available male--most estrous mares will not only tolerate a youngster, but they'll work to accommodate his stature and awkward novice behavior.

No matter the criteria, the age at which a colt can produce a fertile ejaculate and sire a foal varies with many factors, including breed, the month in which he was born, and his social condition. Most colts reach puberty in April or May of their yearling year. The maturation process starts in response to the lengthening days through the winter and spring of their yearling year, as opposed to starting at a specific age. As long as a colt is at least six months old at the time the days start lengthening, maturation usually goes forward. So if the colt was born later in the year, he will reach puberty at an earlier age than colts born earlier in the season. As an example, a July-born colt will only be 9 months old in the April of his yearling year, while a January colt will be 15 months.

By coincidence, at the time your e-mail question bleeped onto my cell phone, I was working with some students in our semiferal herd of ponies that we keep for research and teaching. We were watching one of the yearlings, Clarence (who is not quite 10 months old), trying to cover Dorothy, a 2-year-old filly who was out and about from her natal band during her foal heat. It took Clarence a few awkward tries, with a progression not unlike a novice Thoroughbred stallion with which we had worked in the clinic earlier that morning. With some patient and well-aimed adjustments by Dorothy, things got going well and together they got the job done. As Clarence dismounted, we could see that there was plenty of gel, so we were sure that there was an ejaculate.

One of the students asked more or less the same question as yours--could Dorothy get pregnant from a yearling, or is that just for practice?

We know that production of gel from the vesicular glands typically follows testicle maturation and production of viable sperm, so even though not all 10-month-old colts would be this far along in their reproductive development, in this case it is likely he was fertile.

We often see yearlings mounting mares, sometimes very awkwardly and without complete insertion or signs of ejaculation. But as with Clarence this day, ejaculation can occur. Sometimes the colt is seen breeding before he is producing much semen or before that semen has viable sperm. In this herd we do DNA parentage testing to identify specific sires, and we have had numerous examples of sires in the range of 9 to 12 months. One year, our most productive sire was one of these youngsters, Rolo. He kept very busy with all the yearling fillies and even sired foals for some of the 2-year-olds. Harem stallions rarely breed any mares other than their established harem mares, which in the case of our herd rarely exceeds four or five mares.

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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