Chickens will eat about any kind of seed, bug, worm or what ever they can get into their beaks. If you want them to have fresh greens, you may have to fence them out of the newly planted area, or at least part of it, until the green stuff has a chance to grow up a good bit.

Make sure any seed you plant is free of insecticide and fungicide unless you want bug free chickens with no fungus on them. Even if you cover the seed, they may get into some of it and it may not be good for them.