You can use metal storage bins to hold horse feed. When you use the bins for this application, you need to make sure that they are appropriate to both store food and to be kept outdoors. Some metal bins will not do because they do not sufficiently keep out rain and other moisture. If the horse feed gets wet, it will not be suitable for the horses to eat, so you must either buy bins that come with weather stripping or install some yourself. You also need the bins to be sealed up enough so that other stray animals will not get into them and eat all of the food. The bins should also be made of a thick, durable metal.