You are asking a good question, but no one has provided the necesary grounding for you to phrase it well.
Excess protein is excreted in some form of nitrogen, often as concentrated urates and increased ammonia. Potentially Bad smell. Many Organic feeds are also corn free, soy free, animal protein free, vegan, or any number of other words which appeal to a segment of the population and make the feed appear to be super premium, seemingly justifying the higher price. Because there is so much variation, I hesitate to make broad pronouncements, except to say that they all tend to be low in Methionine, which is the most critical Amino Acid in a young birds life, and many (but certainly not all) are low in Lysine, the second most critical amino acid in a young birds life.
Proteins are made up of amino acids, the exact ratio of which varies considerably. Most plant sources are "incomplete" proteins, meaing they lack adequate amounts of one or more amino acids that animals, such as ourselves or our chickens, need. For poultry purposes, the four most critical Amino Acids, in descending order of importance, are Methionine, Lysine, Threonine, Tryptophan.
Until relatively recently, AA levels could not be quickly, easily, cheaply measured - particularly Methionine - so total protein levels were used as a stand in. The assumption being that if you got enough protein in the daily diet, you would reach your minimum levels of the amino acids you most needed - particularly here in the US, where its relatively cheap to do so because our feed crops are abundant. In the EU, wherethey have much less land available (and largely less suitable lands) for feed crops, they have made due with much lower levels of protein supplimented with synthetic amino acids in the past two decades or more. Large scale US producers are starting to follow that trend, to reduce costs associated with that excess nitrogen in their operations, but that hasn't trickled down thru the feed supply for us, and there's a contrary movement here looking to eliminate all synthetic amino acids supplimentation.
Our birds' needs do vary as they age, and also based on breed and gender - but its poorly studied outside of commercial broilers and commercial layers. The good news is that the studies are useful for making some broad assumptions about our home-based, largely non-commercial flocks, that the variations are largely not great between studies, and that feeding an excess is (apart from price - protein is relatively expensive) generally not harmful, certainly at the levels we are likely to find in the typical feed bag.
So. You can do perfectly well with a surface understanding and some general advice - which will put you well above the knowledge of most from the 60s, 70s, 80s.
I usually recommend that the typical backyard owner, of the typical backyard flock, with typical backyard management practices feed an All Flock/Flock Raiser* type crumble to all their birds, all their lives, with free choice grit, oyster shell, and fresh clean water available at all times in seperate dishes. Without regard to age, gender, onset of of lay, condition of molt, etc.
*Essentially an 18-20% protein, 3.5% fat+/-, 1.5% calcium +/-, 3.5% fiber +/- feed, preferably with about .6% Phosphorus, at least .35% Methionine, and .7% Lysine.
Because its easy for you, and its good nutrition for them.
All of my adult birds - chickens and ducks - get exactly the same feed, which happens to be 20% protein. I feed my youngsters a higher protein mix for their first 6-8 weeks (basically, until they join the adult flock) as I have facilites to separate the flocks. My reasons for that are complicated, its not something I recommend for everyone. But All Flock, all the time, for all the birds of all the genders is a safe and easy way to raise chickens, ducks, geese. Doing something different has limited benefits in limited conditions, and should only be considered after weighing all practical options.
My flock is in my Sig, below. You can see the results of me feed regimen
here.
Or you can get down into the weeds, and play at the levels of individual amino acid requirements based on age, gender, type (to the extent that it has been studied, or can be reasonably inferred from study), understanding that just because you know a thing does not mean that you wll be able to buy the optimum thing off the shelf at the local feed store.
Example
Here. You may find that useful. Apologies for tone of that post, I was responding to another who held themselves out as some sort of authority, yet their claims appeared overblown, and their evidence "lacking". I don't find that to benefit others who come to BYC looking for solid information, and honestly, it was very early this AM, I've not had enough sleep, I was decaffeinated and *annoyed*. Also, I don't play well with others. Explanation, not excuse.
If you want to get to that level, the first two sources I linked will help, and I'd be happy (to the extent of my limited knowledge - I'm not an expert, merely well read) to help with any further questions you might have.
Except of course immediately after a cecal poop
