I caught 44 rats in under an hour using a few propositioned cockatiel nest boxes, some corn shucks, and some duct tape. Hung the boxes then the next day checked them to see what moved in overnight. Rats, especially the females, can’t resist bird nest boxes hung high. You can tell when they are in the box by the corn shucks dangling out the entrance hole after they stuff them in the boxes to make their nest. You can also tell just by the weight of the box when you grab it. So when you have an occupied box you slap some duct tape over the hole and carry the box to a 55 gallon drum with two feet of water in it and drop the box in. Push it under and leave it there for 10 minutes. Then pull it out and see how many rats were in it. Often it is 5 to 7... Did not take long to clear the barn.
While on the subject of rat killings, here is the great Jerry Clower
https://www.youtube.com/watch?v=9Z0q2p8tVpM