Although protein is listed as a percentage on feed tags, the National Research Council’s (NRC) latest recommendation for horses lists the protein requirement unit in grams. To understand how much protein to feed a horse, we need to do some math and further reading.Many people think that a 30 percent protein feed is way too much for a horse to handle. If one looks a little closer, the tag of Hubbard Life’s 30% Supplement indicates that a 1,100-pound horse should consume one pound per day.Thirty percent of one pound is 136 grams of protein.verage mixed hay is about 17 percent protein. Feeding 15 pounds of hay alone can deliver 1,157 grams of protein to a horse. Understanding the protein content and feeding rates of all the feed sources a horse has access to is key to achieving the proper protein ratio.