I've been successful using a 50/50 mix of wheat germ mixed with concrete powder and one drop of anise oil. (Rats seem to be attracted to that anise smell.)
This mix is not a poison, is slow acting, and the rats never build up a tolerance or suspicion to it. But, try not to touch the bowl or mix with you bare hands so as to avoid adding your human scent which may make the rats suspicious.
It is useful to place a second bowl filled wth water nearby as they are thirsty after consuming the mixture.
OK — HTH
Paul M.
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