The thing is, they'll eat a little bit of it and then destroy and waste the rest. But, if you grow a mat of greens then you can cut or tear off clumps and give it to them little by little to make it last longer.
After trying all the grasses and grains the birds and I decided we like alfalfa the best. Let the birds have it at three to five inches tall when it still has the tender round leaflets. The alfalfa sprouting seed is a little pricy because chickens are competing with the healthfood market...