hello SB,
apart from any veggie waste from the kitchen, as well as that grown specifically for them , you could also grow some wheat, barley etc. It depends on how much room you have spare . But once sown , wheat , barley , oats , are not really any trouble and will each produce about 1lb of seed per square yard. As with anything that you grow , this figure can go up a bit or down to nothing ...lol...it is just luck to a certain extent . It is very easy to work out how much grain you need though , and on the plus side , you can use some wheat for your own bread etc, barley for making your own beer, and oats for your own porridge. The surplus going , plus any waste, to the chickens. Comfrey is very good for any animal as it has a high protien content and many vitamins and minerals. You could also grow a patch of lucerne/alfalfa, which you can crop like comfrey 4-5 times a year. Really the only limit on what you can grow is time and space. Good luck with what ever you grow, and with the chickens.
cheers
Russ