AMost yearlings and 2-year-olds can and will breed under good circumstances. There’s probably not a horse practice without a first-hand 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 an old pro 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 that of a 3- or 4-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 2-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 horses that have been studied under natural social conditions, the yearling and 2-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