Interactive effects of dietary total sulfur amino acids and crude protein on growth performance in broiler chickens: Meta-analysis and experimental validation

This study conducted the meta-analysis of interactive effects of dietary total sulfur amino acids (TSAA) and crude protein (CP) on average daily feed intake (ADFI), average daily gain (ADG), and feed conversion ratio (FCR) in broiler chickens, aiming to estimate phase-dependent optimal digestible TSAA levels across dietary CP levels. Moreover, the optimal digestible TSAA levels with different CP levels, which were identified in the meta-analysis, were further validated using a broiler feeding trial. In the meta-analysis, a total of 37 studies published between 2007 and 2024 were included. An initial full quadratic model with CP and digestible TSAA showed limited significance of model coefficients with little biological interpretation across growth phases. Therefore, a simplified quadratic model was employed, incorporating the product of digestible TSAA and CP (TSAA × CP) as a single interactive term with quadratic effects. The results indicated significant coefficients (P < 0.05) for ADG and FCR in the starter phase, ADFI and FCR in the grower phase, FCR in the finisher phase, and ADG and FCR in the overall growth phase. Nonlinear interactions for FCR were consistent across all phases. Model performance evaluations also indicated that FCR was the most reliable variable for predicted responses, exhibiting the least prediction errors and between-study heterogeneity. Model simulations across fixed CP levels from 16.0% to 24.0% revealed that optimal digestible TSAA levels for ADFI, ADG, and FCR increased as dietary CP decreased, with an increment of approximately 0.02 to 0.07 percentage points in digestible TSAA for each 0.5-percentage point reduction in dietary CP. Experimental validation trial further supported these predictions, demonstrating that diets formulated with the model-derived optimal digestible TSAA levels decreased FCR (P < 0.05) compared with diets following their commercial recommendations, particularly under reducing CP levels. In conclusion, this meta-analysis suggested that optimal digestible TSAA levels required to maximize growth performance in broiler chickens consistently increased with reducing CP levels in diets across all growth phases.