I store my feed in my attached (but unheated) garage in large plastic garbage cans. If I had a rodent problem, then I'd use metal garbage cans. I used to buy feed on sale and stock up for 4-6 months, but lately there has been no good sales on feed, so I only buy a bag of feed as needed.
Pro tip: My farm store will mark down bags of feed that have ripped or broken open in handling. Standard markdown is about 25%. So I always check the feed aisle for those broken bags even if I don't need more feed at the time. In the last 3 months, I got one bag for 10% off, 1 bag at 25% off, and 1 bag at 50% off. Mostly, at my store, it depends on who is on duty that day and what mood they are in when you talk to them. The bags of feed I bought had rips, but hardly any feed was spilled out. So, good deal for me. Sometimes they already mark down the bags on display, but most often I have to ask for the discount on the ripped bag.