The optimal level of balanced protein intake for growing chicks is ~18%–23% of the diet; for growing poults and gallinaceous upland game birds, ~26%–30%; and for growing ducklings and goslings, ~20%–22%. If the protein and component amino acid content of the diet is below these levels, birds tend to grow more slowly. Even when a diet contains the recommended quantities of protein, optimal growth also requires sufficient quantities and proper balance of all the essential amino acids.
Few specific signs are associated with a deficiency of the various amino acids, except for a peculiar, cup-shaped appearance of the feathers in chickens with arginine deficiency and loss of pigment in some of the wing feathers in bronze turkeys with lysine deficiency. All deficiencies of essential amino acids result in retarded growth or reduced egg size or egg production. If a diet is deficient in protein or certain amino acids, the bird may initially consume more feed in an attempt to resolve the deficiency. After a few days, this transient increase in feed intake shifts to a situation of reduced feed intake. Consequently, there will be inferior feed efficiency, and the birds are invariably fatter as a consequence of overconsuming energy.
All commercial breeds of poultry have an amazing ability to consume energy to requirement regardless of dietary energy concentration, assuming they can physically eat enough feed in extreme situations. A deficiency of energy can therefore occur only if the diet is so low in energy concentration that the bird physically cannot eat a sufficient quantity of feed to normalize energy intake. With a deficiency of energy, the bird will grow slowly or stop ovulating. As sources of energy, protein and amino acids will be deaminated, and any lipids will undergo β-oxidation. The latter condition can lead to ketosis, which more commonly occurs in mammals, yet the classic signs are similar.