Form-specific selenium supplementation in poultry: linking bioefficacy to mechanisms and precision applications.

Selenium (Se) supplementation in poultry is shifting from deficiency prevention to precision use aimed at measurable improvements in resilience, product quality, and biofortification. Evidence increasingly indicates that, at comparable total dietary Se, bioefficacy varies primarily with chemical form, exposure definition, and retention kinetics. Inorganic salts can correct deficiency and support basal selenoprotein activity, but they provide limited reserve formation. By contrast, selenomethionine (SeMet)-type inputs, including SeMet and OH-SeMet, can build protein-bound Se pools that stabilize Se supply during prolonged stress and are associated with measurable outcomes such as improved feed conversion under heat stress and more predictable egg or tissue enrichment. Biotransformation-derived and engineered systems may provide additional advantages in specific contexts, particularly for mucosal delivery or barrier-related outcomes, but their effects remain strongly product- and formulation-dependent. Mechanistically, source-specific effects reflect differences in Se delivery to selenoprotein synthesis and in the stability of redox and inflammatory control under challenge. A central implication is that matched nominal Se doses do not necessarily represent biologically equivalent exposure, and maximal Se deposition does not necessarily coincide with maximal functional benefit. Thus, deposition targets should be distinguished from functional outcomes such as performance, product quality, and physiological resilience. This review integrates form-specific bioefficacy with mechanistic and application-level evidence and proposes a goal-based precision framework to align Se source and dose with defined production or physiological objectives. Future progress will depend on rigorous source characterization, clearer endpoint alignment, and layered monitoring of exposure, status, and function to enable more predictive and actionable Se use in poultry systems.