i pulled up some patches of grass around my planters and threw the whole clump, dirt and all in their run. they loved picking through it and the rabbit (maybe the chickens too) ate the actual grass and weeds n junk... i imagine if they can free range.. there shouldnt be an issue with weeds being thrown in. do you treat the weeds with any chemicals? that might cause a problem?
It is safe and will save you costs on feed. I do this all the time. The only exception I make is not giving them poke berries but that is just me being paranoid because the freer ranging birds certainly have access to poke berries and wither eat them and do fine or know to avoid them. Ditto of farm mom's chemical treating comment.