This is a subject wrought with ignorance and misinformation. So as you don't need to doubt what I'm about to tell you, you should contact your local university extension office of state lab scientists, and ask them about herbicides and safety for chickens.
First, herbicides, while considered a pesticide just as insecticides, are not the same. Insecticides cause deaths in chickens very quickly. Herbicides do no such thing.
While you do not want to put chickens on a freshly treated pasture, herbicides break down very quickly in the environment and there are really no residual substances to cause sickness and injury to chickens. 2-4D is very safe, probably the safest, to use around chickens. It is what's in your Scott's fertilizer. After a week and repeated waterings, I wouldn't have any worries at all about chickens being put out to free range on it.
Then how are there herbicides that claim to work for four months if they break down so quickly? I'm just curious.