Grass fed chickens. Do they get only grass or is this supplemented?

[nuthatched]

Our son has a corn allergy and I would like to raise chickens without wheat or corn in their supplemental feed at all. We want to do all grass fed but this thread makes it sound like we need to offer them additional feed, too. Do you have options for corn and wheat free chicken feed? Can we make our own?

There are corn and wheat free feeds, but you'll have to search. You could make your own but it won't be cheap, think $8-$10 per pound of feed. Four birds eat a pound a day.
They do need more than just grass and free ranged forage.
I know a family that has serious corn and wheat allergies, they can eat eggs from any source fine, the allergens are based in the protein of the grain and the chicken convert it.