It needs to be a constant maintenance program of keeping bait stations around the farm I find. That way when one rat moves in, it gets the poison and dies before reproducing. One you have an infestation like you describe it is going to take a while and some work to get rid of them. Better to keep those blue blocks nailed to the posts in every shed all year round and that way they never get established. I rarely see a mouse anymore either. Those blue blocks are effective and I consider it cheap insurance compared to the damage rodents can do on the farm. Now if I could just find a similar method for raccoons.