Peaceful Valley Garden Supply sells a field mix that's supposed to be specifically formulated as chicken forage. It has a mix of grasses and legumes. Chickens need protein in order to lay eggs. I assume that meat birds need to eat protein as well, altho I have only layers so I dont know for sure. The more protein your girls eat, the better they will lay. Chickens probably can survive on forage alone, if it's a rural property with plenty of wild vegetation (as has been pointed out already). Farmers in my area used to grow acres of wheat specifically to feed to their layers, which was their livelihood. So I figure they must have known what they were doing.
I feed a mix of scratch, sunflower seeds (both bought and grown), and laying pellets. When I spilled some TVP (tex. veg protein i.e. soybeans) on the floor and fed it to them, they fought like tigers for it. They love bread (whole grain, not white), pasta (ditto), and greens as a supplement. I think they know what they need -- they fight the hardest for the protein-rich foods and leave the white bread alone. Smart girls.