I thought about spraying however chemicals near a feeder or inside a feeder pen can't be good or can it?
Roundup is commonly sprayed on "Roundup Ready" corn and other crops to kill weeds. The active ingredient in Roundup and comparable products is absorbed through the leaves to kill the plant by destroying the roots. It's my understanding there is little trace of it after a few days.
Tractor Supply sells a comparable product with a higher percentage of the active ingredient at a much lower cost per volume.