Wonderful photograph, especially with regards to your lighting and use of space.
Your image composition is done very tastefully with enough action taking place for the image to be interesting but at the same time enough textured negative space to help simplify the scene.
Because you’re working with a larger space it’s important that you think of ways to simplify the image. There is a big difference between “negative space” and “dead space”, and I think you’ve used your surrounding area very well. The grass which acts as your negative space is lit so nicely that it is full of texture and detail. Therefore, even though it’s a secondary element that helps to provide context, it’s nevertheless and interesting object to examine on it’s own.
For example look at the two different uses of negative space in these two images
http://plateauphotography.net/blog2/wp-content/uploads/2008/04/dscf1590.jpg
and
http://www.2dive4.net/oldsite/photogallery/10012006/Tourguide.JPG
Notice how the first image even though it’s primarily an image composed of sand, it still has an abundance of texture and life. The second image on the other hand lacks that texture and therefore the space, rather than just being “negative space” is also “dead space”. The second image looks over simplified. You’ve done a great job of avoiding this pitfall. Great work!
Just be careful of that right wall. You’ve amputated a small part of the horse.
Again, great work!