I would also go with a dirt floor. For storing hay/feed, I'm assuming small square bales? If so, stack them on wooden pallets. It keeps them off the ground so they don't get wet. For feed I would use plastic garbage cans or plastic tubs/bins, depending on how much you use and if the feed isn't 'damp' (sweet feed types). Plastic bins/tubs/cans tend to cause damp type feeds to mold quicker when it gets hot.