Many think that a 11-14% protein diet high in fat helps. Really it is quite the opposite. You need to find a high protein, low fat, medium fiber feed. It may be a slower process, but quite a bit more healthy gain.
Think of it this way.
You have two options, fat or protein. McDonald's or home-baked chicken breasts. Sure, you'll gain faster by eating at McDonald's versus the real meal, but imagine how much harder your body has to focus on processing that fat into energy and turning it into muscle, instead of starting with protein, eliminating the body's need to process all the fats. If you'll start with the protein, the body absorbs and uses it for energy that in turn builds HEALTHY bulk faster than packing on fat and trying to turn it into muscle.
The feed I give my mare and gelding is a 32% protein, 7% fiber, and 5%fat. They are high activity, and are NOT hotter on this feed. (they feel better, act better, and look better. They are truly themselves.)