Okay, first off, amazing picture. You have the jockey who is nicely placed on top of the horse's back. The horse is extending and giving it all he's got. Perfect angle shot as well. Both horse and rider look not only determined to win, but very trustworthy of each other. The horse is relying 100% on the jockey to guide him in the right direction.
But, since you asked me for a critique, here is what I think and suggest. (Bear in mind I have never decided to critique anyone's art, because I find it all to be great.)
The first thing I notice is the yellow bit, though you might be trying to match the shadow-roll there, it doesn't look realistic, it's nice but if you want to make it look realistic, you might want to make it silver or a mid rust color. Still awesome looking though.
Secondly, the jockey's toe looks as if he might end up making a wrong move and wind up off his mount. You might want to place his foot just a little more into the stirrup iron to make it look like he's safer.
Lastly, the thing I also noticed was the fact the horse's throat latch was just hanging there. In my opinion it probably needs to fit a little tighter.
Overall this picture is truly amazing. I see great depths and details to everything about it. You really outdid yourself. The thing that I must say I am favoring about this lovely piece, is the fact the devotion gleams from both horse and rider. And that is truly something special if you ask me.