Optimized amino acid ratios in reduced crude protein diets sustain broiler performance, gut health, and resilience to necrotic enteritis.

This study investigated the effects of essential-to-total amino acid (E:T) ratios in reduced crude protein (CP) diets on broiler performance, gene expression, and cecal microbiota under necrotic enteritis (NE) challenge. A total of 756 mixed-sex 0-d-old Cobb 500 broilers were assigned to six treatments in a 2 × 3 factorial arrangement, with two challenge conditions (yes or no) and three dietary treatments: 17% reduced CP (RCP) with E:T ratio of 0.64 (RCP0.64), 17% RCP with E:T ratio of 0.68 (RCP0.68), and 19% normal CP as control (NCP) from d 9 to 18. Performance was measured from d 8 to 18 post-hatch, while jejunal, spleen, and cecal samples were collected on d 16 for gene expression and microbiota analysis. Results showed a significant interaction between diet and challenge for weight gain (WG; P = 0.004) and feed intake (FI; P = 0.007). Among unchallenged birds, those fed the RCP0.64 diet achieved WG and FI comparable to control-fed birds and were significantly higher than those fed the RCP0.68 diet (P < 0.001). Under the NE challenge, FI was significantly higher in the RCP0.64 group than in the control group (P < 0.001). Both RCP diets and NE challenge increased the feed conversion ratio (P < 0.001). Gene expression analysis showed a diet × challenge interaction for IgG (jejunum, P = 0.045) and IgM (spleen, P = 0.042). The NE challenge increased IgM expression only in RCP0.64-fed birds (P = 0.012). The main effects showed that the NE challenge upregulated nutrient transporter, immune, and apoptotic genes and downregulated TJP1 and GLUT2 (P < 0.001) in the jejunum. In the spleen, IL2 and IL6 expressions were significantly lower (P < 0.05) in RCP0.64-fed birds than in those fed a control diet. In RCP0.64-fed birds, microbiota differential analysis revealed higher Firmicutes in unchallenged birds but increased pathogenic Clostridium sensu stricto 1 in challenged birds. These findings demonstrate that the RCP0.64 diet can maintain WG and FI in healthy birds and partially preserve gut health during NE challenge by modulating immune responses (IgG and IgM) and microbial communities. This dietary strategy could be effective for reducing protein in sustainable poultry production.