Effects of Dietary Standardized Ileal Digestible Crude Protein Levels on Growth Performance, Meat Quality, and Cecal Microbiota of Arbor Acres Broilers

Crude protein (CP) is commonly used to characterize dietary nitrogen supply, but it does not account for ingredient-to-ingredient differences in protein digestibility and the importance of non-essential amino acids; thus, dietary standardized ileal digestible (SID) CP may provide a more precise basis for protein nutrition. Four independent experiments were conducted to investigate the effects of different SID CP levels on male Arbor Acres broilers at different growth stages (Exp. 1, 0–10 d; Exp. 2, 10–21 d; Exp. 3, 21–32 d; Exp. 4, 32–42 d). In each experiment, 175 one-d-old chicks were allotted to 5 graded SID CP levels with 7 replicates (5 broilers/replicate); SID CP level ranged from 17.5–21.5% during 0–21 d and 16.5–21.0% during 21–42 d, with other nutrients meeting NRC (1994) recommendations. Data were analyzed using general linear model (GLM) with each cage as the experimental unit, and dose responses were evaluated using orthogonal polynomial contrasts; the optimal SID CP level estimates were derived by regression when responses were significant. Increasing SID CP improved ADG and feed efficiency during 0–21 d with linear responses ( P <0.01). During 21–42 d, ADG responded quadratically to SID CP ( P <0.05), with optimal SID CP estimates of 19.00% (21–32 d; R² = 0.92) and 19.40% (32–42 d; R² = 0.99). Nutrient utilization (dry matter and crude protein) during 21–32 d exhibited significant linear and quadratic responses to SID CP ( P <0.05 to P <0.01). Serum non-esterified fatty acids and pyruvic acid increased as SID CP increased ( P <0.01), whereas most other serum indices were unaffected. Carcass traits and key meat quality parameters were generally not compromised across SID CP levels. Cecal microbiota analysis revealed no significant differences in overall community diversity among treatment groups; however, SID CP altered the relative abundances of several specific genera and the composition of the microbial community. Specifically, as the SID CP level decreased from 21% to 18.75%, the abundance of Ligilactobacillus increased, and when the SID CP level was further reduced to 16.5%, the abundance of Ruminococcus decreased. Overall, SID CP levels of 19.0–19.4% optimize broiler of 21–42 d growth and nutrient utilization; higher SID CP offers no extra benefit but raises certain serum metabolites, whereas lower SID CP alters cecal microbiota without harming meat quality.