Age to Breed a Colt

Q. How young is too young to breed a colt? We have a coming 2-year-old who acts like he is ready to breed.  We don't have any mares to breed to him, but the question came up for discussion last night among a group of guys.  This colt was out there showing his stuff along the fence near the fillies.  Among us we had strong opinions about how young horses could breed, how young they should breed, and how young they could be fertile.  Some of us thought that it was okay, and others thought it might mess them up for the future to start too early. If a colt appears to be breeding, does that mean he is mature enough to impregnate the mare?


A. Most yearlings and two-year-olds can and will breed under good circumstances.  There's probably not a horse practice without a firsthand story of the pregnant filly that was only with a colt until a year of age.  So for sure, many fairly young colts can be fertile.  Some young colts are quite mature behaviorally, looking like old pros before the age of two if given access to fillies or mares. Others might be awkward and slow but given ample opportunity, particularly with young mares of their size, will have no problem mounting, inserting, and ejaculating normally.

One common attribute of young colts is that their endurance and confidence might be much less than those of a three- or four-year-old. If you hand breed them, they might be shy, easily distracted, or overly put off by correction.  That is probably the basis of concern about "messing up" a stallion by breeding him too young. It is not wise to count on a two-year-old being able to handle a busy breeding or semen collection program.

We have studied colt development in our semi-feral pony herd here at New Bolton Center. In that herd and in other herds of horses that have been studied under natural social conditions, the yearling and two-year-old males do most of the breeding of the young fillies.  The young fillies are often still living for the most part in their natal band (band in which they were born) with their sires and dams at the time of their first estrus.

As estrus begins, the young fillies wander from the family band up to several times a day to join roving bands of young bachelors.  The fillies seem to actively solicit attention from the young males.  The young males take turns breeding the fillies. They tolerate each other's awkward "schooling" mounts, and seem to wait their turn patiently. So even though stallions might take years to get their own established harem, most young males have some of this style of breeding before the age of two.

The maturation of sperm production and behavior often don't coincide.  That means a young colt might be very willing to breed and have nearly perfect form as much as a year before his testicles and sperm production have developed.  Similarly, a colt might have apparently maturing testicles but be immature behaviorally.

Another concern we hear from time to time is whether early breeding influences a colt's manners and studdishness in non-breeding situations. One episode is probably not going to mean much, but if a young colt is allowed to breed frequently, research suggests that his hormonal and behavior maturation might be accelerated.  That means he might be more "full of himself" at an earlier age.

While we're on the subject, many horse owners are alarmed when a colt mounts his mom during foal heat.  This normal behavior is seen in almost every colt at the time of foal heat. In fact, almost all of the normal sexual behaviors --teasing, marking, flehmen response, erections, mounting, sometimes weak thrusting --are seen within the first week of a colt's life.  They are sometimes not in the "adult" sequence, and they might be subtle or interspersed with action play sequences. But if you look closely, they are there. Colts do a lot of sexual play with filly and colt playmates.  They achieve erections but rarely ejaculate in the play form of sexual behavior.

Editor's Note: This is an excerpt from Understanding Your Horse's Behavior by author and equine behavior specialist Sue McDonnell, PhD, Certified AAB. The book is available from

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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