Hey, you've got a great image here, nice one.
A few minor crits/suggestions if you wanted to do a bit more on it:
- The cast shadows from the horses could be a little softer and maybe lean towards the blue shadows in the background
- The jockey positions look great. I'm sure you've got some good reference for the horses but the leg positions feel a bit more like a horse canter than a gallop. Their neck position are good but my intial reaction was that all the feet are fairly close to the ground and none of the limbs are in extreme positions. Feel free to disagree though, I'm sure you can probably find ref where they look like yours. It might add to it though to take a look at the poses below:

-the background is quite heavily blurred so you could probably get away with softening your midground (theres a slight border on the grass), also some motion blur could really help on the horses.
Great piece overall,
Dave
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//slowgolde