The issue, in short, is that we humans have bred chickens to be very dependent upon us, needing a much better diet than they would likely secure in the wild. In return, they grow faster, produce larger eggs, more frequently, than thewy did 100 years ago, 50 years ago, even 20 years ago. But the research from the 80s and the 2000s provides some recommends for optimal nutritional needs of various breeds.
HERE is the old NRCS recommends for broilers (it contains only ADULT recommends for laying hens). Pay close attention to the Methionine and Lysine recommendations.
More recently, UGA published this. (scroll down - same focus, Methionine and Lysine) This includes numbers for pullets, and updates the laying hens chart - broilers was left mostly unchanged.
and HERE is a very up to date study of studies. Published in 2021, it lists most of the recent research, and identifies the studies upon which the old NRCS recommend were made.
Why am I not looking at protein levels? Because protein levels are made up of amino acids. MOST amino acids, a chicken can make for itself. A few amino acids, a chicken can make from other amino acids (though those reactions aren't always reversible). A few amino acids a chicken can ONLY obtain from its diet.
Of those critical amino acids (often called "limiting", because their absence limits the chicken's ability to use the rest of them), the four most important are Methionine, Lysine, Threonine, and Tryptophan. Of those four, the two most important are Methionine and Lysine, both of which appear on a guaranteed nutritional label here in the US. Threonine is very important in the formation of membranes, but doesn't appear. Neither does Tryptophan - but in a grain based diet, its very hard to "miss" those target numbers, particularly if you get the first two right.
Now the thing about protein is that not all protein is created equal. Every protein has a different proportion of amino acids making it up. That's why vegans engage in "protein complimentation", combining a food high in some amino acids with a food high in others so that, in combination, they get a "complete" protein. Part of why "red beans and rice" or "black beans and yellow rice" or even "wedding soup" are so satisfying, though rice on its own is not, nor are beans on their own, or pasta on its own.
and unfortunately, plant based sources high in methionine are few and far between.
There are some foods (grains, particularly) which have relatively high amounts of methionine as a percentage of total protein, but extremely low levels of protein over all, so a lot of a very little is... not much. [part of why that serving of black beans and yellow rice has so much rice, and so few beans]
In order to get enough Met into a growing chicken's diet to meet its optimum target number, you would either have to feed it more grains than it would eat (and have problems with excess "energy" (that is, carbs...), or lots of seeds (with their high fat content - remember, chickens don't deposit intramuscular fat - its placed on the keel, along the thigh, and packed into the abdominal cavity where it puts pressure on the heart and other internal organs and results in sudden chicken death due to either fatty liver disease or heart failure, most commonly, while also making them more subject to heat stress).
I could go on thru the readily available ingredients one by one. ALL of them have problems, its a very careful balancing act to make a complete feed.
Many, perhaps most, non organic feeds here in the US get their Met levels from a combination of synthetic Met (appears as dl-Methionine, total quantity allowed limited by law), and a Met dense source. Either an animal-based byproduct, i.e. fish meal, shrimp meal, crab meal, porcine blood meal, all of which can contribute accumulations of undesired things (like heavy metals) present in their environment; or a highly processed vegetable source. Soybean meal, Cottonseed meal, low fiber Peanut meal are all good sources. and what makes them "meal"??? Usually, the oils are extracted via chemical solvents, greatly reducing the fat content, then the resulting mass is dried, further concentrating protein, before its ground to powder.
Few of those by products (though more than previously) carry the "Organic" label, and many seeking organics to avoid highly processed feeds reject them anyways.
That's the short form. SO when most people go to feed their chicken "Organic", often in conjunction with other buzz words, in the ignorant belief that they are offering the best diet for their birds, what they are most often doing is offering unenriched white bread as a "complete" ration.
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