There have been a LOT of threads about this topic already. You might want to do a forum search for more opinions. You might also want to post where you're located. Types of feeds available vary widely from place to place.
My horses get free choice grass hay and 1.5 lbs of alfalfa (lucerne) pellets daily. Mixed in their pellets is 1/2 cup of milled flax, a complete vit/min supplement that is high in amino acids, and 1/2 cup of apple cider vinegar to make it all stick. This is enough food for even my "hard keeper" in the winter and my lesson horses.
By stopping sweet feed and horse feed & grains in general, I have seriously reduced the amount of food my horses get and their health/condition is better. Coats are shinier, better feet, less thrush and rain rot, etc. My mare's heats are less noticable, my "spooky" horse is barely phased any more, and they all have better work ethics. I'll never go back to sweet feed or commercially prepared horse feed again!
All of that said, the key to a grain-free diet is HAY! The horse needs LOTS of top quality grass hay, mixed hay, or Timothy hay.