I can. 15% is too low.*
Along time ago, in a galaxy far far away.... OK, here in America, mostly back in the 70s, commercial egg laying operations (always a very tight margin business) were interested in feeding their birds as cheaply as possible, without losing more value in production than they saved with cheap feed. A LOT of studies were done. Not modern, small scale studies with 20 birds in each of 4 cohorts, but big studies with lots of birds. Obviously, they focused on certain breeds, which were then used for commercial egg production, mostly leghorns. They were also interested in then (and now) typical management practice - layers become dog food/other products after 18 months +/-, when egg production drops in association with the first adult molt. (why feed something whose egg production has dropped significantly, and whose future egg production will be less per day, on average, than that first productive year? - answer when the loss of production from a second year layer is less than the cost of raising a bird from hatching to onset of lay - a rarity.)
Fast forward to the (then) end result. The studies found that for a bird of those breeds, under commercial management practices, whose purpose was egglaying needed about 4x more calcium than a non-laying bird, and could be maintained at adequate health on a 16% protein feed ration. Commercial "Layer Feed" was born, purpose mixed as cheaply as possible for that use. Below 16%, egg quality (contents) dropped, egg production dropped, overall bird health dropped - individually not a lot, but measurably, and at greater economic cost to the operation than increasing protein quantity in the feed.
Note that these studies had nothing to say about the raising of "meaties", of "dual purpose" birds, of birds which are intended to be kept for most (or all) of their natural lifespans, etc. Merely the bare minimum for (typically) laying leghorns, a relatively lean breed.
Fast forward to the present. Between the 70s when small family farms were being displaced by large scale battery hen egg producers and now, science has come a long way. So have genetics and chicken breedings. Modern egg laying birds can put those justly famous (in their time) leghorns of the 70s to shame. There are some gains in feed efficiency, yes, but a good argument can be made that modern breeds, less lean than their heritage counterparts, benefit from an even higher protein diet, even the egg layers. Dual purpose birds have always needed more protein, to help put on mass, which later becomes table weight. Higher protein feed also benefits birds in early development, in stress environments, and with feather replacement during molting. Numerous studies show egg quality (contents) benefiting from higher protein feed, as well as (very slight) improvement in quality of lay on a diminishing basis at 17%, 18%, 19%... through 22-24% protein feeds.
Cost of that additonal protein rises faster than benefits, so you still don't see wholesale adoption of higher protein feeds by the commercial producers. Additonally, higher protein feed results in higher Nitrogen chicken waste, which is also of concern for environmental reasons at commercial scale
For backyard owners, OTOH, buying a few 50# feed bags a year, the cost difference is quite marginal (typically $1.50 to $2.00 per bag) and easily pays for itself in improved health, table weight, and condition. Additionally, most of us aren't raising leghorns pullets - those studies were never meant to speak to the protein needs of the new "Rangers", Orpingtons, Brahma, your cute little Silkies, or any other thing that doesn't have a breast like a razor and a tight layer of relatively small feathers. Nor do we suffer the consequences of higher nitrogen levels in the waste, we aren't shoveling tons of the stuff off property, and we mostly don't raise our birds in large buildings were concentrated ammonia levels can require respirators and protective boots and gloves while checking on our birds...
For that reason, the overwhelming majority of posters here on BYC (of which I am one), recommend feeding an 18-20% protein "All Flock", "Flock Raiser" or similar commercially complete feed, with free choice oyster shell on the side to support the calcium needs of your layers for the typical owner of the typical backyard flock, which is often small, typically contains multiple breeds, often multiple ages, sometimes mixed gender. You may count me among the posters making that recommendation (though, full disclosure, I don't folow it myself - but neither my flock size nor my management practices are typical for reasons which would make thispost much longer).
* Our friends across the pond, in the EU, have routinely made use of lower protein feeds than what we do here in the US. 14% and 15% feeds are not uncommon there. As it turns out, there are some modern studies showing that a 14% or 15% feed *can* be adequate to a laying birds needs ***IF*** certain limiting amino acids are supplimented in the feed. Unfortunately, feed labels do not show the guaranteed analysis amino acid levels in the typical feed, and worse, modern studies show great disagreement in what AAs are needed, in what quantities, to best support the needs of various breeds.
Hope that helps???
