Although antioxidants are added to commercial chicken feed to postpone deterioration, heat, sunshine, oxygen, and a number of microorganisms that can spoil feed eventually degrade nutrition. During extended storage, even the greatest chicken feed loses nutritional value.
I suggest that you don’t stock up on bags of feed. Only buy what your flock can consume in the next couple of weeks (6 weeks maximum). This is so the feed doesn’t go stale, which can happen fast enough. (Of course, the feed will last long in cold weather.)
Storing feed in a cool, dry place and in a closed container, like a galvanized steel can with a locking lid (Amazon), slows the rate at which it gets stale.