Many layer feeds are formulated at 15% protein. I sometimes wonder whether adhering to a minimum is good for birds?
Too much protein can force hens to lay too often, and that can be bad for their long term health. If fed too high a protein mix while young, their combs can overdevelop. But to my mind it could also be that these symptoms relate as much to the protein source (e.g. meat meal) as to the presence of extra protein at all. Without having full access to nutritional information databases (as I would have if I were a scientist), these are questions, not conclusions.
In sheep, adding protein (in the form of lupins and lucerne with a little barley) to their grass diet vastly improves worm resistance. Agricultural theory seems to suggest this is entirely to do with feeding dry matter uncontaminated with worm larvae (therefore reducing the amount of contaminated grass ingested). Having kept sheep without the use of commercial wormers (except when attempting to save a sheep that hadn't been fed extra protein or minerals) on wet, warm summer coastal grass, I think there's a fair bit more going on. It does seem as though extra protein may strengthen the gut's ability to fend off even the nasty barber's pole worm (which causes anaemia and can easily kill its host). Without wishing to generalise from a handful of sheep, these experiences still make me wonder whether 15% protein is high enough to keep chickens healthy. Interestingly, my 1940s chicken book suggested that worms were not a big issue in poultry, and the author was somewhat skeptical about the introduction of worming chemicals. (I've often said before, I wonder whether wormers themselves may predispose the gut to new worms?)
There are strong incentives for feed producers to aim at the lowest possible protein level: protein is among the more expensive of feed inclusions. But perhaps at minimum levels, chickens can't fend off worms very well. It's possible the use of minimum protein helps us stay addicted to agricultural chemicals.
These are questions, and I don't mean to make a claim either way. Whatever the case, 15% is too low for growing birds or for table birds. 20% is closer to the mark. 21-23% probably wouldn't hurt. I'll make sure I watch my chickens for signs of worms, and I'm not going to let animals waste away or get ill if they clearly require chemical worming. But I certainly won't be jumping for the chemical bottle if other methods prove to be useful in a preventive sense.