Facts About Horse Skeletal Growth

Discussion Board - www.equinestudies.org: General Equine Discussions: Skeletal growth rates in horses

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By Anonymous on Thursday, January 08, 2004 - 08:13 pm:

How great is the variance noted in skeletal growth rates between different breeds of horses? From my experience with Arabian horses, they, in general, seem to mature skeletally more slowly than do some other breeds (specifically, the epiphyseal plates). We have noted variances between different bloodlines (e.g., Polish bloodlines seem to mature more rapidly than do English or American/Domestic bloodlines) of anywhere from weanlings/yearling ages up to 2 1/2 year olds.

By Dr. Deb on Friday, January 09, 2004 - 12:21 am:

Dear Anonymous -- Ahh, here you touch upon a field fraught with confusion. At the moment our Knowledge Base section is not up -- in the old website I would have referred you to the Conformation section and told you to read the Ranger piece. Or -- I don't know who you are because you've chosen to be anonymous -- perhaps you're an old regular here and have long ago downloaded that material.

To briefly review, the most important thing I said in the Ranger piece is that, plus or minus six months, skeletal maturation does not vary at all from breed to breed. Males mature about six months later than females. I am using the term "skeletal maturation" in the normal way, to mean fusion of the epiphyses and resorption of the "growth plates". Plus or minus six months, almost all horses reach skeletal maturity -- fusion of all growth plates on all postcranial bones -- at the age of six years. A few horses -- those that are destined to be very tall or those that have longer necks -- will not complete skeletal maturation until they are eight years old. There are no horses whatsoever, of any sex or breed, that are skeletally mature before the age of five and a half years.

All breeders and buyers need to beware of the fact that a horse can LOOK mature "on the outside" a long time before it is skeletally mature. A horse can have ceased getting measurably taller at the withers several years before all of the epiphyses on all of its postcranial bones have fused to their respective bone shafts. Particularly, it is to be noted that lengthwise growth in the horse goes on for longer, and actually produces a much greater degree of size increase from foal to maturity, than does its growth in the vertical direction. This is because there are 32 bones (vertebrae) between a horse's poll and the root of its dock, and all 32 of 'em have a physis on one end of the centrum. By contrast, in no limb are there 32 articulating segments which could contribute to height increase.

None of this is to negate, however, your observations about differences in growth in different Arabian bloodlines and in Arabians vs. other breeds. Some of them do APPEAR to be more mature sooner. But it is appearances which can do quite a lot of harm to the horse, especially where the external appearance of the animal leads the owner to think that it is OK to start riding and working the horse. Or, even worse for the animals is when we get a breed club or association whose advertising material states -- or implies -- that a two year old is all ready to start earning a paycheck. There are no such two year olds, whether in cutting, racing, or any other competitive venue in which "futurity" competition is a possibility.

In the previous incarnation of this Website, the Ranger piece prompted a fairly continuous stream of inquiries to my office from owners worried that "starting" or "breaking" their horse at a year and a half or two years old might have been the wrong thing to do. I agree that, the world being what it is, generally this is not advisable if our primary criterion is the animal's own welfare. However, a lot depends upon what you mean by "starting" a horse under saddle. If you have that attitude and approach to horsemanship which we here advocate, your first ride on the colt you bought as a weanling is liable to be the day he stands up under you (because the first thing you taught the colt, after you taught him to lead up properly, was to lie down on command). The first ride is a fine ride indeed even if it consists only of four steps -- so long as those steps are taken with the animal fully conscious and accepting of what is taking place.

However many steps are taken in that first ride, and no matter at what age those steps are taken, you will build from there. And the wise owner will build at a rate that leaves the horse in the same state of comfort, calm, and contentment each and every time you have an interaction with him. On the basis of what we know about the fusion of the epiphyses, I would prefer not to see any horse in "serious" work until he is at least four years old.

I choose this age because I think it is a reasonable compromise -- horses cost money to keep and it isn't sensible to delay getting them on the payroll any longer than necessary. The four year old year is not only the first year when I think the vertebral physes -- those which are latest-maturing and most liable to direct damage -- are reasonably "safe", but it is also the old, traditional time for a young horse to start working. He will have been introduced to all parts of whatever work he is destined for during his two and three year old years: shown it in little bits, and in a way he can understand, accept, and like. It is in the four year old year that the wise owner puts those parts together.

I digress here because I'm anticipating receiving the same sort of letters of inquiry we were receiving before with the Ranger piece. The bottom line with your own observations is the same as the old "See How She Grows" series I did 15 years ago for Modern Horse Breeding magazine, where we followed the growth of a single Arabian filly for the first two years of her life. Nepenthe grew as SHE grew -- she was Russian-bred (sire was *Napitok) -- you couldn't have mistaken her for a Quarter Horse filly. And in every bloodline the pattern of growth differs: some grow in front-end hind-end spurts and go through an "ugly" phase at about the end of their yearling year; some are pretty well-proportioned throughout their early lives. Some seem to hang back initially and then put on a lot of growth, others grow at a pretty uniform rate all the time.

As conformation is the study of proportions, and conformation study is the art of seeing and appreciating proportional differences, I believe that this is part of what you are observing -- not skeletal maturity per se but differences in body proportions, which change from month to month during the first six years of a horse's life, and which are characteristically different in different bloodlines.

Hope this answers the question. -- Best wishes -- Dr. Deb