"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 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:
2. Short pastern - top & bottom between birth and 6 mos.
3. Long pastern - top & bottom between 6 mos. and 1 yr.
4. Cannon bone - top & bottom between 8 mos. and 1.5 yrs.
5. Small bones of knee - top & bottom on each, between 1.5 and 2.5 yrs.
6. Bottom of radius-ulna - between 2 and 2.5 yrs.
7. Weight-bearing portion of glenoid notch at top of radius - between 2.5 and 3 yrs.
8. Humerus - top & bottom, between 3 and 3.5 yrs.
9. Scapula - glenoid or bottom (weight-bearing) portion - between 3.5 and 4 yrs.
10. Hindlimb - lower portions same as forelimb
11. HOCK - this joint is "late" for as low down as it is; growth plates on the tibial & fibular tarsals don't fuse until the animal is 4 yrs old! So the hocks are a known a "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.
12. Tibia - top & bottom, between 2.5 and 3 yrs.
13. Femur - bottom, between 3 and 3.5 yrs.; neck, between 3.5 and 4 yrs.; major and 3rd trochanters, between 3 and 3.5 yrs.
14. Pelvis - growth plates on the points of hip, peak of croup (tubera sacrale), and points of buttock (tuber ischii), between 3 and 4 yrs.
"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.
"The spine does not fuse until the horse is at least 5-1/2 years old. This figure applies to all horses, small scrubby, range raised horses to huge Warm Bloods. The taller your horse and the longer its neck, the later full fusion occurs. For a male (is this a surprise?) you add six months. So, for example, a 17-hand TB or Saddlebred or WB gelding may not be fully mature until his 8th year. Something that owners of such individuals have often told me that they "suspected."
"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." ~ Dr. Deb Bennett, Ph.D. is a 1984 graduate of the University of Kansas, and until 1992 was with the Smithsonian Institute. She is internationally known for her scientific conformation analysis.