Yes you can over do the protein. But, It's protein quality that matters not just quantity. A protein must be complete (supply all the essential amino acids) and most plant proteins are not. Plant proteins tend to be deficient in some of these, for example chicken feeds I have purchased have to be supplemented with lysine. So there's not much benefit feeding a high protein feed, for example 24% made up from soy, you will have happier healthier birds feeding a lower protein (say 15-18% depending on the stage of growth and sex ) using a meat source. But it seems to be impossible these days to get feed that isn't "vegetable" based (ie corn and soy mostly). I have a very good older book called Practical Poultry Keeping. In there the protein requirements are stated to be 20 percent for chicks, 15% for growers, and 16-18% for layers or breeders. But the protein in the feed comes not only from wheat, barley and oats, but also from grass meal, fish meal and dry skim milk- not from soy like today. Today's feed IMO is basically like trying to stay healthy on crackers and a vitamin.
Plant proteins are completely inadequate for cats, who are obligate carnivores. They cannot live on a completely plant based diet.
This book said that if the protein quality/quantity is inadequate, chickens will eat too much trying to get enough protein, eating far too much carbohydrate in the process and becoming overweight- with the negative consequences of that. Chickens cannot make chicken, feathers, and eggs from carbohydrates- they make it from protein.
That's excellent to know! Thank you so much! These are the analysis labels from the 2 pkgs I ferment, any feed back?