OK< good question. I didn't think you were arguing - you want to get in the weeds, welcome to my jungle!
I'll link sources. CHICKS have higher nutritional needs than adult chickens do. and the two amino acids they most need are Methionine and Lysine. When people look at a bag of feed, checking the protein level, they are indirectly assuming that higher protein equals higher level of those two most critical amino acids. Lots of reasons. One, its easy. Two, until recently, amino acid levels were hard to measure directly, with methionine measurement only becoming commerically practical in the last few decades. Three, in America, protein for animal feeds is (relatively) cheap. In the EU, on the other hand, with much less farmable area relative to population (and much of it not suited to grain crops, besides), they've made use of much lower protein feeds, supplimented by increased levels of synthetic amino acids - dl-Methione and L-Lysine. They have been at the cutting edge of that research for a while.
That leads to my last comment about the science and the studies. For essentially everything (except calcium - over which people continue to argue) except fat, the difference between the desired level of a thing, and an unsafe level of a thing is typically somewhere on the scale of an order of magnitude (10x) - so a little bit too much of a trace ingredient generally isn't a problem. Too little is always a problem, though the severity varies, and chickens (all living beings, really) have ways they can compensate (for some). i.e. a chicken short on Cysteine can make it internally, by converting Methionine - but it can't turn Cysteine BACK into Methionine - so Met shows up an a feed label, Cys does not. Tryptophan can be converted into seratonin, melatonin, and with enough B6 and Iron, a chicken can even turn it into Niacin! But anyhow, the studies are now mostly focused on maximizing production, seeking the minimums, no longer focused on finding maximums. Additionally, they are largely concerned with COMMERCIAL management practices - short lifespans, not the typical backyard experience.
Some amino acid needs references - these are the most handy ones I keep in my click list.
This is the old NRCS list for broilers, high yield broilers, turkeys, and LAYING hens - mostly based on the 70s research, with a few updates from the early 2000s. Its limited in part because Met couldn't be directly measured in the 70s.
This is
from UGA's Extension, and has some updated data, but largely agrees - it has some pullet data, too.
This is
an incredibly useful meta-study, with reference to lots of modern studies, mostly being done outside the US now. China, India, etc all VERY concerned with feeding their populations.
Speaking of India (this is much more basic, its intended for their population to use, but again, similar conclusions)
UVM - Broilers (page 33 - from the manual for raising Ross 308 Broilers)
etc.
I can also link (though I don't keep them favorited) various studies showing improved resistance to bacterial, parasytic, and even (limited) viral challenges in birds with improved key amino acid intakes. Protein (you asked above) is comprised of a bunch of amino acids, in different proportions. MOST of the amino acids we can completely ignore - chickens can produce needed levels on their own. Some (as alluded to above) can be made from others. But adequate levels of Methionine, Lysine, Threonine, and Tryptophan they MUST get by eating it. So those are the ones we focus on.
Hope that helps.
