You should have taken this shot from further to the left than you did, to catch the horses coming straight at you. Either that, or waited until all of the horses entered the turn and your shot is more perpendicular to the horses path.
Get rid of the rails in the foreground; it's too great of a distraction.
Zoom in, use a larger telephoto lens or move closer to make your subject more of the picture. Or a combination of all three.
How did you set exposure? You could have used the green grass as an approximate 18% reflectance surface to determine your exposure.
And did I say to get rid of the rails?
WM
BTW, was this taken at Louisiana Downs in Bossier Parrish?