Excerpt below is taken from "Timing & Rate of Skeletal Maturation in Horses" by Dr. Deb Bennett, Ph.D
The full article has much more information, but here's a taste of her message:
"Owners and trainers need to realize there's a definite, easy-to-remember schedule of bone fusion. Make a decision when to ride the horse based on that rather than on the external appearance of the horse.. For there are some breeds of horse--the Quarter Horse is the premier among these--which have been bred in such a manner as to LOOK mature LONG before they actually ARE. This puts these horses in jeopardy from people who are either ignorant of the closure schedule, or more interested in their own schedule (racing, jumping, futurities or other competitions) than they are in the welfare of the animal.
The Schedule of Growth-Plate Conversion to Bone
The process of fusion goes from the bottom up. In other words, the lower down toward the hooves, the earlier the growth plates will fuse--the higher up toward the animal's back you look, the later. The growth plate at the top of the coffin bone, in the hoof, is fused at birth. What this means is that the coffin bones get no TALLER after birth (they get much larger around, though, by another mechanism). That's the first one. In order after that:
Short pastern – bottom before birth; top between 9-12 months.
Long pastern – bottom unites with shaft at or shortly before birth; top 13 to 15 mos.
Cannon bone – top unites with shaft at or shortly before birth; bottom unites with shaft at about 18 mos.
Small bones of the knee – top and bottom of each, between 18 mos. and 2 years
Radius-ulna – upper weightbearing surface, between 15-18 mos.; distal surfaces, between 3 and 3.5 years
Humerus – bottom, between 1.5 and 2 years; top, between 3 and 3.5 years
Scapula – glenoid or bottom (weight-bearing) portion – between 3 and 3.5 years
Hindlimb – cannon bone, coffin bone, andpasterns same as forelimb
Hock – this joint is “late” for as low down asit is; growth plates on the tibial and fibulartarsals don’t fuse until the animal is 3-3.5(so the hocks are a known “weak point” –even the 18th-century literature warns against driving young horses in plow or other deep or sticky footing, or jumping them up into a heavy load, for danger of spraining their hocks).
Tibia – bottom, between 20 mos. and 2years; top, between 3 and 3.5 years
Femur – there are 4 major epiphyses on this bone, including the head that goes into the hip socket; they fuse between 3 - 4 years.
Pelvis – the hip socket is firm between 18mos. and 2 years, but the rest of the bone does not stop growing until the horse is 5 or more years old.
And what do you think is last? The vertebral column (spine) of course. A normal horse has 32 vertebrae between the back of the skull and the root of the dock, and there are several growth plates on each one, the most important of which is the one capping the centrum.
These do not finally fuse until the horse is at least 5 ½ years old (and this figure applies to a small-sized, scrubby, range-raised mare. The taller your horse and the longer its neck, the later the last fusions will occur. And for a male – is this a surprise? – you add six months. So, for example, a 17-hand Thoroughbred, Saddlebred or Warmblood gelding may not be fully mature until his 8th year – something that owners of such individuals have often told me that they “suspected”)
Significance of the Closure Schedule for Injuries to Back and Neck vs. Limbs
"The lateness of vertebral "closure" is most significant for two reasons. One: in no limb are there 32 growth plates! Two: The growth plates in the limbs are (more or less) oriented perpendicular (up and down) to the stress of the load passing through them, while those of the vertebral chain are oriented parallel (horizontal) to weight placed upon the horse's back. Bottom line: you can sprain a horse's back (i.e., displace the vertebral growth plates) a lot more easily than you can sprain those located in the limbs.
And here's another little fact: within the chain of vertebrae, the last to fully "close" are those at the base of the animal's neck--that's why the long-necked individual may go past 6 yrs. to achieve full maturity. So you also have to be careful--very careful--not to yank the neck around on your young horse, or get him in any situation where he strains his neck (i.e., better learn how to get a horse broke to tiebefore you ever tie him up, so that there will be no likelihood of him ever pulling back hard)."....