Poultry do not have a protein requirement per se. Rather, they require each of the nutritionally essential amino acids in the correct levels and balance, as well as sufficient amino nitrogen (nitrogen arising from amino acids) to synthesize the nonessential amino acids. Traditionally, formulating diets to contain sufficient levels of the first 3 or 4 limiting essential amino acids, as well as a minimum level of dietary crude protein, was typically appropriate to meet the needs of poultry using typical ingredients. However, modern poultry have increased growth rates and body composition (meat-type birds) and egg production (egg-laying birds). Additionally, in many regions, there is a desire to reduce dietary crude protein levels to reduce excretion of nitrogen by the birds, and subsequently environmental nutrient pollution.
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 each of the essential amino acids plus sufficient amino nitrogen for synthesis of the nonessential amino acids.
Few specific signs are associated with a deficiency of the various individual 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. A deficiency of any of the essential amino acids results in retarded growth in growing birds or reduced egg size or egg production in egg-laying birds. If a diet is deficient in crude protein or specific amino acids, the bird may initially consume more feed in an attempt to resolve the deficiency. However, after a few days, this transient increase in feed intake shifts to a situation of reduced feed intake. Consequently, there will be reduced feed efficiency, and the birds become fatter as a consequence of overconsuming energy relative to protein.
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. A deficiency of energy can therefore occur only if the diet is so low in energy concentration that the bird cannot physically compensate by increasing feed consumption to normalize energy intake. With a deficiency of energy, the bird will grow slowly or stop ovulating. With a dietary energy deficiency, amino acids will be deaminated and the carbon skeletons oxidized, and lipids will undergo beta-oxidation. The latter condition can lead to ketosis, which more commonly occurs in mammals, yet the classic signs are similar.