Effects of Dietary Metabolizable Energy and Crude Protein Levels on the Nutrient Metabolism, Gut Development and Microbiota Composition in Jingyuan Chicken

The effects of varying dietary metabolizable energy (ME) and crude protein (CP) levels, along with their interactive effects, on the apparent nutrient metabolism, development of digestive organs, intestinal morphology, and microbiota composition in Jingyuan chickens during the growing phase were evaluated. A total of 540 seven-week-old male Jingyuan chickens were randomly assigned to nine groups, with six replicates per group and 10 chickens per replicate. The trial lasted for 11 weeks. A 3 × 3 factorial design was adopted, comprising three levels of ME, namely, low (11.28 MJ/kg, LE group), medium (11.70 MJ/kg, ME group), and high (12.12 MJ/kg, HE group) and three levels of CP, namely, low (14.00%, LP group), medium (15.50%, MP group), and high (17.00%, HP group). The levels of ME and CP, along with their interactions, had significant effects on the average daily gain (ADG), average daily feed intake, feed conversion ratio (FCR), apparent metabolizable rate of CP, gizzard weight, duodenal lengths, jejunal villus height (VH), crypt depth (CD), and muscle layer thickness (MLT) (p < 0.05). The combination of medium level ME (11.70 MJ/kg) and medium level CP (15.50%) (MEMP group) exhibited the best performance, with the highest ADG and the lowest FCR (p < 0.05). Moreover, this group showed significantly higher duodenal length, jejunal CD, VH/CD and MLT compared with the other groups (p < 0.05). Dietary ME and CP levels greatly influenced cecal microbiota composition. Chickens in the MEMP group exhibited an increased abundance of Erysipelotrichaceae, Syntrophomonadaceae, Akkermansia, and Clostridia_vadinBB60_group, and there was an obvious decrease in the relative abundance of Desulfobacterota (p < 0.05). This study demonstrated that dietary ME and CP levels, along with their interactions, could significantly influence the growth performance, apparent nutrient metabolism, and intestinal development of Jingyuan chickens during the growing phase. Dietary ME and CP levels modulated the cecal microbiota composition, potentially inhibiting the abundance of harmful bacteria Desulfobacterota, while enriching the abundance of beneficial bacteria, thereby enhancing gut development and nutrient absorption. The combination of medium-level ME and CP (11.70 MJ/kg ME, 15.50% CP) demonstrated the most favorable outcomes in our study.