I'm not ready to condemn a prepared ration simply because the chickens have found other things to eat. They are creatures of habit like most other critters - and are also keen opportunists. This in no way condemns prepared feeds. In fact, it places a greater burden on us to ensure we KNOW what they are eating out there, as they roam around.
So what I'm curious to know is this: Why eliminate grains?
To my way of thinking, grains are a part of the birds natural diet. They provide much needed energy in the form of carbohydrates. Along with seeds, nuts, bugs, carrion, fruit, mice, well - you name it - grains are an intrinsic part of the diet.
A hundred years ago, when a chickeneer might have to prepare his own feed, every recipe from the period included grain of some kind. They had this stuff figured out.
What do we know that they did not?
The Jungle Fowl don't eat grains, although to be sure they will eat various kinds of seeds and nuts and fruits and all kinds of things, when they can find them. The reason that chickens have historically been fed grains in the olden times alluded to was because grains were the staple crop that farmers already happened to be growing, not because grains are a natural part of the chicken diet. The earliest domesticated chickens had to adapt to eating domestic grain. And BTW you can be sure that they weren't fed any more of it than the farmers though absolutely necessary to keep them, because grain was, first and foremost, an important and expensive people food (all grain had to be grown and harvested without machines running on cheap oil, and without government subsidies). Flocks were also smaller as a result, and were culled seasonally so that only the breeders were kept through the winter (the peak grain-feeding season).
The reason grains found their way into all these recipes in early America and in much of Eurasia was because that was what was available as a concentrated, storable source of calories for omnivorous livestock like pigs and chickens, and for people. So in east Asia they fed rice, in Europe, wheat or rye, in America, corn. In many parts of the world (Oceania comes to mind), farmers have always grown other staple crops for calories. In Polynesia for example grains were unknown--instead, people ate taro, cassava, sweetpotatoes, breadfruit, and yams. If true domestication of chickens had occured in Polynesia "back in the day," they would have adjusted to eating sweetpotatoes or something else to supplement their foraging instead of grain. Chickens don't NEED to eat grain any more than people do, and there's nothing especially natural about it, it's just a case of convenience--and one that the early domestic chickens had to adapt to (meaning it wasn't "natural" at the time). Fortunately chickens, like people, and most omnivores by definition, are very adaptable creatures. Other nutritious sources of calories can fit the bill just as well. In the past, small farms (for whom growing grain is impractical on a small scale) have also grown mangel beets, squash, and potatoes (and I'm sure other things) to supplement omnivorous livestocks' feeding, and this would be, in the big scheme of things, no more or less natural than feeding grain--if that is what makes sense, that's what farmers will do, being on the whole pragmatic folks.
That said, I think the whole "grass-fed" chicken concept is kinda silly. As other people said, chickens are undeniably omnivores that eat an omnivorous diet. While they may nibble on some tender grass shoots now and then, unlike herbivores, they lack the ability to reap any significant sustainance from grass, just as humans do (if you don't believe me, YOU try living indefinitely on nothing but grass). It's one thing to talk about "pastured poultry" or free-roaming flocks, but "grass-fed poultry" is a stupid and misleading term that just confuses things more and contributes to a general public ignorance about livestock and farming ecology that serves no one. There, I said it--sorry if that sounds harsh, but I've gotta call a spade a spade here...