Interactive effects of dietary total sulfur amino acids and crude protein on productive performance in laying hens: A meta-analysis

Dietary total sulfur amino acids (TSAA) play essential roles in egg protein synthesis and various metabolic functions in laying hens, underscoring the importance of accurate estimation of TSAA requirements, particularly when formulating low-CP diets. A previous meta-analysis in broiler chickens revealed that optimal TSAA levels increase as dietary CP levels decrease; however, quantitative information regarding this interactive effect across different phases of the prolonged laying cycle remains limited in laying hens. Therefore, the current meta-analysis aimed to determine the interactive relationship between dietary CP and digestible TSAA levels for optimizing productive performance in laying hens across laying phases. A total of 28 studies published between 2010 and 2026 were analyzed using a mixed model with study as a random effect and observations weighted by the number of replicates. Productive performance variables included hen-d egg production (HEP), egg weight (EW), egg mass (EM), average daily feed intake (ADFI), and feed conversion ratio (FCR). The quadratic model with the product of digestible TSAA and total CP (TSAA × CP) as a single predictor, which was developed in our previous meta-analysis for broiler chickens, was fitted separately in different laying phases. Models were conservatively retained only when the intercept and regression coefficients were all significant ( P < 0.05). Accordingly, models were retained for ADFI and FCR in phase 1 (early-phase), for HEP, EM, and ADFI in phase 2 (mid-phase), and for all variables in the overall dataset. However, no significant models were identified in phase 3 (late-phase), possibly due to the limited number of observations and between-study heterogeneity. Results indicated that as dietary CP concentrations decreased from 18.0% to 15.0%, optimal digestible TSAA concentrations increased from 0.75% to 0.90% for FCR in phase 1, EM in phase 2, as well as HEP, EM, and FCR in the overall dataset. In conclusion, this meta-analysis demonstrated that the optimal digestible TSAA concentrations required to maximize productive performance in laying hens increased as dietary CP concentrations decreased, particularly during early- and mid-laying phases.