I have to vote with the Jack Russell Extermination Team program. You don't need to leave them for days. Just toss a Russell into each container for a few minutes, they'll take care of the current rats and leave a predator smell behind. Then put your feed into galvanized metal trash cans with tight bungee cords and you'll pretty much have solved the problem. Would the barn owner mind if you dropped off a few Guinea Hens? They clean up any spilled grain and keep the area pretty much snake & tick free too.
Between 3 JRT's, 6 feral barn cats (the JRT's have a fenced area they patrol and the cats have figured out that to go into the dog yard is to die immediately) and about 30 guineas, I have no pests. No mice, moles, voles, rats, snakes, ticks and the fly population stays waaaay down. I'm always asked how I keep the flies away from a horse ranch and I just point to the guineas. Oh and they are excellent 'guard dogs', they squawck more and louder than the Russells bark when a stranger appears.