Feed the dirty rats daily dose of mash potatoes that make them die.
Please, don't.
The problem with poison's is that they are generally water activated. Rat
eats the poison taters, gets thirsty and goes for water..dies by the creek,
or pond. Then somebodies cat, or dog, or an eagle, etc. comes along
and eats the rat..and suffers an agonizing, miserable death as a result.
Shoot 'em, trap 'em, snare 'em..kill 'em with a shovel, or a dog, but
please don't use poison.
I used to make a "gang trap", which starts with an 24' long x 18' wide x12" high
sized box. Grab some velcro tape and 8 rat Victor snap style rat taps. Space
the traps out evenly (1 in each corner and 4 in the middle) and then velcro
them in place.
Cut a 2" entrance hole in the middle of each end, bait and set the traps,
and then close the top.
Move it around in the barn to a new spot every day, and that sucker will catch
you rats every night so long as there are rats to catch. You can try different baits,
as well, so they don't identify any one in particular as being associated with the
traps.