Rice Bran and Soybean hulls are ranked with the lowest GI according to this site (Sugar Content in Feed and Forage Affects Horses' Health
). And as far as the alfalfa, make sure it's late cut hay. As that website mentions it is useful to get the hay tested for sugar content and GI. Here's an interesting article on hay as well (http://www.uky.edu/Ag/AnimalSciences/pubs/id146.pdf
I feed my horses a soybean hull based pellet which also contains rice bran. They love the taste and I can mix in my biotin supplement with a little water and there are no issues. Moorglo is another popular feed with similar ingredients.