i have had exactly the same problem. alot of farmers down where i live are keeping the feed in food buckets instead of leaving it in a big pile, which has reduced the number of rats considerably. lucky for me, my regular permission is a very old old farm and has lots of big open drains. i find curshing up cheese and onion crisps and placing the pile about 5 foot from the drain lures the little suckers out.
go get them mate . Rhys