In the USA there is a method of using poisons in enclosures, so that the predators aren't downstream because the rats don't get out of the enclosures. Rats like dark hidy holes, so it may be possible for you to trap rats and safely poison them. Something like a cinder block with steel plates and flaps that only open one way may not be too difficult to whip up from stuff you have laying about on the farm.

I have a client with bat issues. I managed to get them out of her attic by thoroughly sealing every hole bigger than a small coin, but her house is more than 50 years old and was built by her now deceased husband, so there are some unique features that present novel challenges I have to surmount to keep vermin out. I neither want to use poison on the bats, because they can't be confined to a trap and raptors would be at risk, as well as pets and local fur bearers.

Critter control is a tough row to hoe. I hope you seize on quick and easy means to resolve the matter.

Thanks!