Folks on this forum will try to tell you all kinds of things that WILL happen to your chickens if you do this or that with feed, but they don't know any of that with any certainty...they were just told by someone else or read it in a book or such.
There have been numerous and extensive studies on poultry feed over the last 100 years and new studies continue to roll out... There is a lot of factual data that has been gathered about chicken nutrition and what is bad and good for them and what they require to perform... The poultry industry has spend billions on nutritional studies for poultry, so they can optimize their bottom line, no reason to not take advantage of their findings...
With the exception of layer feed the only real difference between the other types of feeds is the protein content, and chickens bodies have no problem ridding themselves of excess protein within reason, thus the little extra protein won't harm them and can also be beneficial for show birds where feather quality is desired, meat birds that are growing at an exponential rate, or highly active birds where they burn more calories daily as all those things require sufficient protein to achieve... With that said your average chicken that is in a confined area and doesn't do much while being restricted to a commercial feed generally only needs a 16% protein diet, but 18% or even 20% won't hurt but it could be wasteful if they don't need it resulting in it just being pooped out... If the birds are feds 'treats' regularly or allowed to range and eat greens, or are very active a food with a little higher protein content might benefit as it will help compensate for the lower protein snacks they are getting or provide additional energy...
Layer food has extra calcium added, this is really only beneficial to laying hens that need the extra calcium for egg production... Unlike protein that the chicken body can easily pass, chickens don't have the ability to pass excess calcium very well, thus it can build up in their body and cause health issues... There have been multiple studies showing excess calcium negative effects on young growing birds and lesser studies showing it's effects on roosters... You might not see these effects and maybe it won't effect your bird as it's not an absolute thing or exact science, but there are numerous studies that show negative effects of excess calcium in young birds and male birds... The reason there are limited studies in regards to roosters, is that the poultry industry has little use for roosters beyond fast growing broilers that are butchered at an early age, they have little interest in the long term effects of calcium in roosters as it's not part of their business model to keep rooster for years...
Of course there will always be people that use anecdotal evidence to discredit scientific study like "My grandpa smoked until he was 95 and didn't have cancer, thus those studies are wrong, smoking is not harmful to your health and it doesn't cause cancer..." and those that simply choose to ignore the studies...
I'm personally not one to place anecdotal evidence above scientific study, nor one to ignore the studies until there are other studies that show otherwise...
