The most natural feed is not to feed them at all -- let them scratch and forage for 100% of their calories.
You can pull that off if your
chicken paddock is a piece of tropical jungle with lots of leaf litter like the habitat that the wild jungle fowl evolved in. The further away from that you are, the more you have to supplement. Also, if you want a high level of egg production, you have to make sure that they are getting all the right nutrients. But giving
chickens special formulations of corn and soy is only a recent development. A hundred years ago, before the advertising campaigns of the feed manufacturers,
chickens were the farm's garbage patrol. Anything and everything that was unfit for consumption by other animals, the chickens got last crack at it. And why not? Chickens can find nutrition is the unlikeliest of places, like a 3-day old cow plop that is teeming with
maggots.
When I first got chickens, I too thought that I needed to buy them food. And I ought to buy them "good" food. I now realize that I had been listening to too much advertising. I now feed my chickens on what gets thrown out at the grocery store, what gets thrown out of the refrigerator, what gets weeded out of the garden, what gets caught in slug traps, and what gets sucked up in the
lawn mower bag. I haven't noticed a drop in egg production from the days when I used to buy food, if anything they lay better.
There is an enormous amount of waste in the way food makes its way from field to the consumer's dining table; by tapping into that waste stream, you need never buy feed for your chickens again.