The optimal level of balanced protein intake for growing chicks is ~18–23% of the diet; for growing poults and galinaceous 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, satisfactory 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 consume more feed in an attempt to resolve the deficiency. Consequently there will be inferior feed efficiency and the birds are invariably fatter as a consequence of overconsuming energy.

A deficiency of energy can only occur if the diet is so low in energy concentration that the bird physically cannot eat a sufficient quantity of feed to normalize energy intake. All birds have an amazing ability to consume energy to requirement regardless of dietary energy concentration, assuming that they can physically eat enough feed in extreme situations. 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.

Last full review/revision March 2012 by Alex J. Bermudez, DVM, MS, DACPV; Mahmoud El-Begearmi, PhD; Steven Leeson, PhD; Kirk C. Klasing, BS, MS, PhD