Relevance of nonessential amino acids in low crude protein diets for broiler chickens – an updated review

Abstract Reducing crude protein in amino acid-adequate diets for broiler chickens is effective in reducing nitrogenous emissions and competition for resources between the food and feed sectors. This review provides a comprehensive analysis of the literature on the relevance of nonessential amino acids in low protein diets for broiler chickens. Glycine and serine, owing to their interconvertibility summarised as glycine equivalents (Gly equi ), limit growth when dietary crude protein is reduced below 19% in up to 3-week-old birds. Considering essential amino acids and the variable Gly equi requirements enables the reduction of dietary crude protein to ∼16% without compromising growth. Variation in Gly equi requirements likely occurs predominantly from the varying amounts of uric acid formed. Other influences seem to exert lower impacts on dietary Gly equi requirements. Asparagine or glutamine is probably the growth-limiting amino acid when crude protein is reduced below 16%. Alternatively, nonspecific amino-nitrogen may be lacking in such diets. The current potential to reduce dietary crude protein when using free essential and nonessential amino acids enables to increase the efficiency of nitrogen utilisation to a value above 80%. This coincides with reduced uric acid synthesis and energy expenditure for nitrogen excretion. The lower nitrogen excretion via the urine results in a lower energy expenditure. Hence, dietary energy may prospectively be reduced once the energy-sparing effect is quantified, thereby further reducing the competition for resources between food and feed.