Keep the feed in tin garbage cans (I personally don't dump the food in, but place the whole bag inside the can and just open the top of the bag). Make sure the lid fits good.
Hardware cloth (the metal fencing-like stuff with 1/4 inch square holes) works real well against most anything. If rats are bound and determined, they will chew holes in wood or plastic to get to what they want. If they start doing that, lots of rat traps need to be set out. take a silver dollar sized circle out of the center of a fresh piece of bread and wad it up real good, like a dumpling, then kneed it deep into the bait-holder part of the trap (they love fresh bread). I also used to sit out late at night with my .22 pistol and a flashlight. When I'd hear a rat starting to chew at something, I'd quickly turn on the flashlight and blow the critter away.
A cat will work wonders too. The drawback - they'll also kill any wild rabbits, squirrels, birds. Our outdoor cat never bothered the hens with any chicks (I think it learned real quick that it didn't like a big bird coming at it trying to kill it). If it was me, I'd go the rat traps and shooting them. I hate that now I don't have anywhere near all the squirrels and rabbits we used to have.
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