how do you determine what kind of grain certain horses need?
The best way to determine what grain different horses need to is take a look at the type (use0 and age of your horse. Different horses of differing ages and uses will need different energy amounts from the feed you are giving them. A "back yard" horse will need a different energy than a race horse or even a weanling. The University of Missouri has a great article on figuring out rations to feed horses of any age. You can find that article here: http://extension.missouri.edu/p/G2807. Another great resource is this article from the University of Illinois: http://www.aces.uiuc.edu/vista/html_pubs/horse/horse.html.