http://www.equinestudies.org/knowledge_base/ranger.htm. Dr. Deb Bennet
The Schedule of Growth-Plate Conversion to Bone
The process of converting the growth plates to bone goes from the bottom of the animal up. In other words, the lower down toward the hoofs you look, the earlier the growth plates will have fused; and the higher up toward the animal's back you look, the later. The growth plate at the top of the coffin bone (the most distal bone of the limb) is fused at birth. What that 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 - top and bottom between birth and 6 months.
Long pastern - top and bottom between 6 months and one year.
Cannon bone - top and bottom between 8 months and 1.5 years
Small bones of the knee - top and bottom of each, between 1.5 and 2.5 years
Bottom of radius-ulna - between 2 and 2.5 years
Weight-bearing portion of glenoid notch at top of radius - between 2.5 and 3 years
Humerus - top and bottom, between 3 and 3.5 years
Scapula - glenoid or bottom (weight-bearing) portion – between 3.5 and 4 years
Hindlimb - lower portions same as forelimb
Hock - this joint is "late" for as low down as it is; growth plates on the tibial and fibular tarsals don't fuse until the animal is four (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 - top and bottom, between 3 and 3.5 years
Femur - bottom, between 3 and 3.5 years; neck, between 2.5 and 3 years; major and 3rd trochanters, between 2.5 and 3 years
Pelvis - growth plates on the points of hip, peak of croup (tubera sacrale), and points of buttock (tuber ischii), between 3 and 4 years.
And what do you think is last? The vertebral column, 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 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 or 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 to the stress of the load passing through them, while those of the vertebral chain are oriented parallel to weight placed upon the horse's back. Bottom line: you can sprain a horse's back (i.e. displace the vertebral physes - see Figs. 5 and 8) a lot more easily than you can displace those located in the limbs.
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 years to achieve full maturity - it's the base of his neck that is still growing). So you 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 tie before you ever tie him up, so that there will be no likelihood of him ever pulling back hard. For more on this, see separate article in this issue).
