Optimizing low crude protein diet: Evaluating Glycine as a conditionally essential Amino Acid

Global interest has emerged in implementing low crude protein (LCP) diets for poultry. The necessity of their development stems from low-cost formulation, maintaining egg quality, and environmental impacts, including health and welfare concerns for current egg production in the world. Researchers have conducted several studies to provide biochemical insights into the effect of LCP on laying hen diets, but practical limitations of LCP diets still exist. Supplementing laying hen feed with crystalline essential amino acids (AAs) like methionine, lysine, and threonine is standard industry practice. It has enabled a reduction of dietary crude protein (CP) to the standard level. However, research considering nonessential AAs like glycine (Gly) in LCP diets is still limited. Therefore, this review aims to enhance the understanding of LCP diets for laying hens, focusing on Gly. However, dietary Gly equivalents (Gly_equi) for Gly represented the first-limiting nonessential amino acid in poultry diets. Targeted adjustment of essential AAs and Gly_equi can considerably decrease CP in poultry diets. Gly is generally considered a nonessential AA in standard CP poultry diets. However, it becomes conditionally essential AA under dietary LCP situations. The variable requirement for dietary Gly_equi suggested that its static recommendation in poultry diets provides high safety margins of Gly in the LCP of laying hen diets. These variable recommendations for dietary Gly_equi in LCP diets would help optimize egg production and albumin quality and reduce nitrogen (N) emissions in poultry farms. The current review suggests that Gly could be a conditionally essential AA for laying hens fed with LCP diets.