Grass fertilizer will kill a rabbit if it eats the grass-don't think it does the same to a chicken-but to be on the safe side-I don't let mine out if I have treated the lawn-they peck all the grass anyway
I would suggest just not mixing the two for a few weeks
Lawn and chickens. Does that count as an oxymoron?
I think forget about doing anything to the lawn the chickens will do that plus make a dust bath,but they are worth it,bless.