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). 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.