How much to grow for each chicken?

This spring I want to start growing grains and feeding most of them to my chickens. I know that I will not be able to provide all their feed from the grains but im hoping to be able to produce 50% of their diets. I don't think I will have enough room in my yard to grow enough for it to be worth it so I will probably pay to use 2 community garden plots (20' X 20' each).
How many pounds of feed would a chicken that is allowed to free range during good weather, eat per year, or per month?
What type of plants do you recommend planting for the chickens? I was thinking of growing field corn, wheat, oats and sorghum for the hens, any suggestions?