I'm curious why shoot at such a wide aperture, such a low ISO and low shutter speed? Was it to show more motion in the horses? Exposures are OK, although it's tough because you lose details in the horse to shadow compared to skin of riders. But the biggest issue is faces in the first 3 aren't sharp. I suspect motion blur playing a part as your shutter speeds are only 1/200-1/320. Also difficult with the shallow DOF. In lighting that good, crank up the ISO and use narrower app and still get faster shutters. Faster shutter and narrower app = sharper faces.
Also, watch your horizons. A number of these are a bit crooked. Easily fixed in post processig. My suggestion is to use vertical lines as your guide in-camera
In the end, if your intent was to show motion - you'll have to balance that with sharpness concerns. Unlike a racecar, the motion here isn't as linear so it is going to be tough I would think to strike a balance.