I prefer alfalfa pellets or cubes over beet pulp. Wet them down some, add a vit/min supplement, and some flax meal (TSC carries Omega Horseshine, 1/2 to 1 cup a day), and you're good to go. You can better control the horse's calorie intake without sacrificing his nutrition (the vit/min). TSC should have GrandVite. That's my favorite "off the shelf" overall vitamin mineral supplement.
Bagged feeds usually contain corn, molasses, wheat middlings, grain "sweepings," grain byproducts, and other questionable ingredients, none of which are good for horses. A natural, forrage-based diet is healthiest for your horse. The alfalfa pellets have quality digestible protein and amino acids, which are key in muscle development.