I have been mixing my own feed since I got chickens, only four years, but that is long enough to see problems. They are free range, but during the winter all free range is under snow.
They are mellow and seem quite healthy. My pullets have been laying all winter.
I feel that there is a lot more lee way than one thinks, PROVIDED a person takes their essential enzyme needs into account by a careful researching of what chickens need at a minimum. There are threads here that can give you all the info you need, with charts that can give you the protein content of ingredients by weight, so you can easily calculate what you need to get the protein level you want.
Bottom line is you should have some animal protein in there some place, and that is easily supplied by fish meal.
I have a local wheat peas bulk mix I use for a base, and add fish meal, alfalfa, and kelp to it.
I have added a link to research about the normal diet of game fowl in the wild.
Ask serious breeders if they use 16% layer feed, yet this is sold as a complete ration.
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The Welsummer and Partridge Wyandotte here are some of my first chickens, been eating home mix for four years...