I've never understood the concept of putting stuff like roundup on a lawn. Sure, you'll have the prettiest lawn around if it's not dotted with dandalions (sp?), but at the expense of exposing your family, pets and the earth to chemicals? No thanks, I keep the weeds.
ETA: nwgirl, I once read...
I've been letting mine run around outside since they were 2 1/2 weeks old. Now as adults they go anywhere they want on 80 acres.
I have yet to have a chicken poisoned from eating something they shouldn't have and believe me, the so-called poisonous plants are here - along with the mushrooms...