How close are modern broilers to their physiological ceiling? Have genetic, nutritional, and precision technologies reached biological limits? A systems biology perspective through physiological ceiling theory.

This review examines the hypothesis that modern broiler chickens may be approaching a multidimensional physiological limit to further productivity improvement and evaluates the extent to which current evidence supports this proposition. Over the past century, genetic selection, nutritional optimization, health management, and technological innovations have dramatically enhanced growth rate, feed conversion efficiency, and carcass yield. However, these gains have increasingly been accompanied by physiological trade-offs, including skeletal abnormalities, muscle myopathies, metabolic disorders, reduced thermal tolerance, and welfare challenges. Rather than assuming the existence of a definitive biological ceiling, this review introduces the concept of a "broiler productivity ceiling" as a systems-based conceptual framework for interpreting the interaction between productivity gains and emerging biological constraints. Evidence relating to gastrointestinal capacity, mitochondrial bioenergetics, skeletal integrity, cardiovascular and respiratory function, immune competence, microbiome dynamics, and environmental adaptability is critically evaluated to assess whether these systems may act as limiting factors to future performance improvement. The review further examines the roles of metabolic heat production, nutrient utilization efficiency, oxygen delivery capacity, and muscle hypertrophy-associated disorders as potential indicators of increasing physiological strain. Advances in omics technologies, precision nutrition, microbiome modulation, and digital livestock systems are discussed as tools for identifying, monitoring, and potentially alleviating biological constraints. Overall, current evidence suggests the emergence of important physiological trade-offs and diminishing returns in some productivity traits but remains insufficient to conclusively demonstrate that an absolute biological ceiling has been reached. Future progress will likely depend on enhancing resilience, robustness, and system-wide biological efficiency rather than pursuing growth acceleration alone.