Mixed grains will not be sufficient for health. They will need a nice high protein source to go along with that...seeds can accomplish much but really I recommend buying a poultry nutrition book (Heuser's book by Norton Creek Press is good) or researching extensively on internet first.
During summer if they are digging up lots of earthworms and free ranging, it is different than being penned with nothing but grains. Roasted soy protein today has become the gold standard for poultry feed, since it provides a cheap source of protein. Clabbered (from what I read must be raw) milk, meat scraps, seeds, and the like are also good but more expensive unless you have a steady supply available in your household anyway.
Soy needs to be roasted otherwise is toxic to poultry.
Peas:
http://www.extension.org/pages/67359/feeding-field-peas-to-poultry#.VNrZYPnF_EY
Google "barley poultry sticky droppings necrotic enteritis" so don't give too much of that but they do love it.
http://www2.ca.uky.edu/smallflocks/Nutrition-Barley.html
Black oil sun. seeds, split peas, wheat, millet, corn, barley, oats (the hulls can be irritating to the crop so in moderation unless they have no hulls) etc. are good.
They will need grit and oyster shell. My chickens lay better with some soy. So I have gone to using chick crumbles (unmedicated) as the high protein source.
Protein levels:
wheat 12-14%
millet 11%
corn 7-9%
oats ? don't remember
peas 24.5%
barley ?don't remember
Black oil sun. seeds 16%
Have fun and look for Harvey Ussery's articles on internet too on mixing feed.
