Susan Garlinghouse has a web site that disucsses equine nutrition.  Primarily for distance and endurance horse.  But if you understand the principles of what she discusses, You can adapt them to the type of exercise your horses are doing.

Her Website

http://www.shady-acres.com/susan/index.shtml

And an article about energy balance

http://www.shady-acres.com/susan/metabolicpull3.shtml

And this article about insoluable and soluable carbs

http://www.shady-acres.com/susan/probiotics.shtml