If your horse feed has the same as your chicken feed you have really bad horse feed that I wouldn't give to anything I want living to old age. Grain is actually really bad for horses and we do not give ours any. We use boss and beet pulp when they need supplemented with something. When we did use grain we used straight oats to balance our alfalfa hay and never more than 1 scoop which I believe was 2lbs. Chicken feeds have all sorts of cheap grains, grain by products, and it should contain some type of animal protein. Chickens are omnivores while horses are grazing herbivores. Chickens need lots of protein, lots of calcium, do ok with carbs, and minimal fiber. Horses need mostly fiber, low protein, minimal carbs, and no animal protein.