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(1) Just about everybody has heard of the horse's "growth plates," and commonly
when I ask 'em, people tell me that the
"growth plates" are somewhere around, or in, the horse's knees (actually they're
located at the bottom of the radius-ulna bone just above the knee). This is what
gives rise to the saying that, before riding the horse, it's best to wait "until
his
knees close" (i.e., until the growth plates fuse to the bone shaft and cease to
be separated from it by a layer of slippery,
crushable cartilage). What people often don't realize is that there is a "growth
plate" on either end of EVERY bone behind the skull, and in the case of some
bones (like the pelvis, which has many "corners") there are multiple growth
plates.
So do you then have to wait until ALL these growth plates fuse? No. But the
longer you wait, the safer you'll be. Owners and trainers need to realize
there's a definite, easy-to-remember schedule of fusion - and then make their
decision as to 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 mature. This puts these horses
in jeopardy from people who are either ignorant of the closure
schedule, or more interested in their own schedule (for 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 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 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 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)
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, 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 1/2 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 full
fusion will occur. And for a male - is this a surprise? -- you add six months.
