I found a really good idea on this forum.. I made some raised beds out of cedar boards (roughly 3' x 5). I covered the top of them with quarter inch hardware cloth. I threw in some organic compost and bird seed, gave it a little water and now they have a couple grass beds that they can pick at, but not scratch up and kill the plants. In the winter, I'll use a cover crop, such as Cereal Rye or Vetch. They get to free range in the afternoon and get a lot of garden scraps, but at least they have some sort of greens day in and day out. You can also make a deeper bed (about 2 feet) and plant radishes, turnips and beets. Cover it with chicken wire and then the chickens can peck at the greens of the plants as they grow and when your ready, you can pull the root crop out. Just make sure you don't make the framework of the beds too heavy, as lifting them up will be needed at some point.