Prospects of feed additive incorporation in laying hen diets: a narrative review of principal biological effects and recent developments.

Selected feed additives (probiotics, prebiotics, synbiotics, postbiotics, phytogenics, feed enzymes, and organic acids) are reviewed for reported biological responses, and some recent developments when incorporated into laying hen diets. Several feed enzymes (phytase, carbohydrase, protease, and multi-enzymes) have been adopted to improve the nutritive quality of feedstuffs by mitigating inherent digestive function inefficiencies, complementing endogenous enzyme activity, and cleaving anti-nutritional factors abundant in vegetable-based poultry diets. Phytase use is targeted at hydrolyzing phytate to liberate phosphorus and possibly other encapsulated nutrients, with widely reported environmental and economic benefits. Proteases often improve the hydrolysis of amino acids and protein complexes to improve dietary protein digestibility and utilization, potentially restoring performance losses and maintaining the egg quality of hens fed low-protein diets whose CP level has been further reduced. The digestibility-enhancing effects of fiber-degrading carbohydrases are associated with the reduction of intestinal viscosity and improved energy utilization through depolymerization of soluble non-starch polysaccharides. Considering that nutrients exist in a complex matrix involving starch and non-starch polysaccharides, protein, lipids, minerals, and vitamins, laying hens could also benefit from potential additive and synergistic effects accrued from adopting defined feed enzyme combinations. The incorporation of gut-health-promoting feed additives (pre, pro, syn- and postbiotics, phytogenics, organic acids) optimizes feed nutrient utilization by inducing immuno-stimulatory, antimicrobial, and antioxidant activities, modulating gut immune function, and microbial balance and population. Hen responses to feed additives are context-dependent and highly variable due to various factors, including rearing system, age, breed, health status, environmental factors, feed composition and quality, and management. However, on balance, feed additive products with proven efficacy and financial value are recommended for inclusion in laying hen diets; and could potentiate optimized performance and egg quality, reduced feed costs, improved animal welfare and skeletal health, and reduced environmental stress due to nutrient excretion, thereby improving the economic and environmental sustainability of hen egg production. Strategic application of feed additive combinations could potentiate additive and synergistic responses.