Optimizing Reduced Protein Diets for an Efficient and Sustainable Layer Production

Dietary protein is essential for poultry performance, influencing egg production, growth, immunity, and physiological functions. However, the high cost and environmental impact of soybean meal has encouraged the development of reduced crude protein (CP) diets supplemented with crystalline amino acids (AAs), coupled with the reduced cost and improved availability of synthetic AA. This strategy enables precise AA balancing, improves nitrogen utilization, and reduces nitrogen excretion and ammonia emissions. Among these strategies, supplementation with arginine (Arg), an indispensable AA in chickens, is particularly important due to its critical roles in protein synthesis, reproductive function, and eggshell formation. In addition, functional Arg precursors, such as guanidinoacetic acid (GAA) and citrulline (Cit), can further enhance Arg availability while supporting energy metabolism and hormonal regulation. Despite these benefits, excessive protein reduction without adequate AA balance can impair laying performance, particularly in aged hens. Furthermore, nutrient digestibility remains a key limitation in reduced protein (RP) diets, as non-starch polysaccharides present in cereal-based feeds can reduce nutrient availability. The inclusion of exogenous enzymes, such as xylanase and β-glucanase, can improve digestibility, while phytase supplementation further enhances nutrient utilization by increasing phosphorus availability and mitigating the anti-nutritional effects of phytate; however, responses are often inconsistent. Another critical factor is maintaining an optimal energy-to-protein ratio, as imbalances may lead to increased fat deposition and reduced productive performance. While supplementation of key limiting AAs (lysine, methionine, and threonine) supports protein reduction, other AAs, such as Arg, isoleucine, and valine, may become limiting in RP diets. Hence, this review explores developments toward RP diets by highlighting three key strategies: enzyme supplementation to enhance nutrient digestibility, inclusion of functional Arg sources to improve performance and bone quality, and optimization of the energy–protein ratio in RP diets through precise AA formulation, with the overall aim to improve the sustainability of the poultry industry.