Really nice job!
I like the way you have captured the jumps so close centering the subjects without cropping details! It's no as easy as saying it, expecially if you were outside of the arena! And there aren't many disturbing elements on the background, other quite hard task while shooting in a horse show! Very well done! =)
Personally I like the most the images with the dark horse because it creates a good contrast with the background while the grey one it's a bit confused with the colour of the arena's sand.
May you could try to improve adding a bit more depth to the images, so that the attention fall only on the subjects and the rest remains only a blured and totally undefined background. Anyway, that depends from the camera you are using and don't solve all the problems: in that particular case for example there are some wonderful elements in the background that really deserve to be shown. The arena was really well prepared, with nice fences and decorations, and the blossom trees on the background are awsome! So I like your choice to show them all renoucing to field's depth (I mean expecially in the 4th photo, with the white horse jumping). Anyway I don't find that solution as much appropiate for the other jump images: if there's nothing special on the background, personally I prefer having a real strong field's depth, as in a portrait, but consider this only as my personal taste.
Then I appreciate that you have taken images even outside of the arena: many people think that the competition and the jumps are the only moments that deserve to be photographed in a horse show while there are many other scenes that can become subjects of amazing photos!