Another vote for beet pulp and rice bran
My barn feeds a nightly mash of beet pulp and rice bran (in a 4:1 ratio), which the horses love. You can safely feed rice bran up to 2 lbs/day, and no real upper limit on beet pulp- just be sure to soak it.
My boy is an easy keeper, but he was underweight when I bought him. Rice bran did wonders for putting the weight back on him.