Every single weeds I pick out of my 2.5 acres garden.The girls get lots to choose from so they are pretty picky. They love clover and chickweed but that is only a small proportion of what they get. They don't get much in the way of grass clippings. I use a ride on so the clippings go straight on...
What is your coop like? That will determine what will work for you. If you are designing specifically to harvest poop then I think the best thing for the roosts seem to be a pit under them, with a wire screen over it - where you would have a poop board but it doesn't need to be cleaned out so...