Good question! How much is too much? Last spring we had an outbreak of coccidiocis & the hatchery suggested lowering our protein to no more than 16-17%, but everything I’ve seen on message boards seem to suggest 20% is a better goal.
High production birds output a lot of protein in their eggs, but they're also bred to process protein more efficiently. Heritage birds actually need more protein than high-production birds, which is counter-intuitive, I know.
I've heard that too much animal protein causes kidney failure. On further research, that seems to be mostly an urban myth resulting from one study on a diet done in mammals (human beings.)
I've also heard that too much protein makes the animals eat less (they're getting all of their nutrients) and they don't build up any reserves or mass associated with carbohydrates. Not sure how true that is, or how relevant it is to an animal being fed a complete feed.
The only problem that I know for certain to be associated with too much protein is that it makes poop stink.
EDT: bottom line is I generally feed about two percent above the recommended minimum (18% to production layers, 20% to chicks) to allow for the fact that the nutrients degrade as the feed ages. After that, I don't worry about it. I haven't seen any problems in my flock.
Good information without checking I would have bought 24% protein thinking higher is better. Basically I'm raising chicks in the chickens layers so I will stick to 20% starter and transferred to regular 18% feed after 6 weeks