There is a very interesting article by Dr. Deb Bennett that I just read.

Thought it was particularly pertinent with the 2yo season just around the corner.

A short version and quite clearly explained about when joints etc are mature on a horse. By Dr Deb Bennett.
Some of you may have read this before but it's still very relevant especially about the joints in the back and neck.

"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. 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 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 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

Article courtesy Vichorse.

Does this explain the failure of 3yo's that raced as 2yo's?
I'm inclined to think so.

Racing stresses on a baby 2yo is like asking a toddler to cycle the Tour De France, the joints, vertebrae, even the spine and knees are not fully formed.
If they don't get shin splints or bowed tendons, there's still the issue of maximum stress to unfused bones.

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Top read CP.
Now....... how can we turn a quid from this I wonder.

__________________
Jose'.

Thanks Jose, it's an area that interests me, as not only do I have a punting interest, but I also train ex racehorses, so I come across these problems quite often.
It's something to consider when the boom 2yo's come back as 3yo's and are racing against less experienced, but older horses. Like anything there are exceptions to the rule, but as an overall approach, perhaps ratings might be more accurate if at least some impact rating (negative) were given to horses that won as a 2yo and are now racing as a 3yo.

Here are some examples for 2yo Group One winners which came back as 3yo's, here is their career record after age 2 starts / wins:

Quick Star 2/0
Road To Success 4/0
Preserve 5/0
Belle Du Jour 22/4
Assertive Lad 9/3
Reenact 16/1
Show A Heart 21/2
True Jewels 6/0
Spectatorial 6/0
Ha Ha 14/4
Viscount 7/1
Juanmo 8/0
Bel Esprit 12/3
Pillaging 0/0
Calaway Gal 16/0
Victory Vein 16/4
Lovely Jubly 11/2
Roedean 16/0
Winestock 8/0
Polar Success 10/0
Hasna 7/1
Ambulance 9/1
Picaday 17/2
Alinghi 10/6
Barely a Moment 16/2
Dance Hero 26/4

Out of 26 boom 2yo's 15 won as 3yo's, but many hotpots scored duckeggs.
Of the 15 that won as 3yo's, those that raced older than 3yo....

Belle Du Jour 12/2
Assertive Lad 4/1
Reenact 3/0
Show A Heart 9/2
Ha Ha 5/0
Viscount (retired)
Bel Esprit (retired)
Victory Vein 7/1
Lovely Jubly 5/2
Hasna (retired)
Ambulance (retired)
Picaday 10/1
Alinghi (retired)
Barely a Moment 5/2
Dance Hero 19/1

A lot of early retirements.

Something to at least consider.

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*RaceCensus now updated to 31/12/2024
Video overview of RaceCensus here:
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For sure something to consider there.
292 total starts for 40 wins.
A 13.6% WSR.
Don't know what the SP's were though.

I was actually thinking more along the lines of something like this.......

If a young horse "blunders at the start" or "knuckles at the start" or somesuch, then lay it if it starts again within say, 3 weeks.
The theory being that seeing as it's bones haven't yet knitted, it perhaps has sustained unknown/unseen injuries to unfused joints.

Dunno, may have legs.

__________________
Jose'.