The relevance of glycine and serine in poultry nutrition: a review.

1. Dietary glycine equivalents (Gly) for glycine and serine represent the first-limiting non-essential amino acid in poultry diets. Targeted adjustment of essential amino acids and Gly in diets can considerably decrease crude protein (CP) in poultry diets below the limit of CP reduction when only essential amino acids are adjusted. 2. The level to which CP can be reduced in diets adequate in Gly depends on the objective; which includes reducing dietary CP without affecting performance and increasing nitrogen utilisation efficiency. Dietary CP can be reduced to ~15-16% in diets for up to 21 d old broiler chicken without affecting growth performance compared to responses to diets with currently common CP concentrations by considering Gly in the diet formulation. Dietary CP can be further reduced to maximise nitrogen utilisation efficiency; however, this leads to reduced growth performance. 3. The dietary Gly requirement of poultry varies depending on other dietary constituents. In broiler chickens up to 21 days of age, the dietary Gly requirement is estimated to be between 11 and 20 g/kg. This estimate is influenced by the concentrations of Cys and the endogenous Gly precursors, threonine and choline. Urinary nitrogen excretion seems to be a major determinant of the response to dietary Gly, because it is needed for uric acid formation. 4. The variable requirement for dietary Gly means that its static recommendation in poultry diets would lead to high safety margins in Gly supply or the risk of Gly deficiency. Variable recommendations for dietary Gly concentrations would help to supply birds based on their specific requirements and could reduce nitrogen emissions originating from poultry farming.