Dietary crude protein level and arginine analogue supplementation affect growth performance, nutrient digestibility, ileal morphology, and serum metabolites in broilers

This study evaluated the effects of dietary crude protein (CP) level and arginine analogue supplementation on growth performance, nutrient utilization, intestinal morphology, and serum metabolites in broiler chickens during the finisher phase. A total of 256 male Ross 308 broilers were assigned to a 2 × 2 factorial design consisting of two dietary CP levels and two arginine analogue supplementation levels. From day 22 to day 35, birds were fed either a normal-CP diet containing 19% CP or a low-CP diet containing 17% CP, with or without arginine analogue supplementation. Each treatment included eight replicate cages with four birds per cage. Growth performance, apparent ileal digestibility (AID) of nutrients and amino acids, ileal morphology, and serum biochemical parameters were assessed on day 35. Broilers fed the normal-CP diet showed greater body weight on day 35, greater average daily gain, and a lower feed conversion ratio (FCR) during days 22–35 than those fed the low-CP diet (p < 0.05). Arginine analogue supplementation reduced FCR (p < 0.05) regardless of dietary CP level. Additionally, supplementation increased (p < 0.01) the AID of CP, arginine, and methionine, and improved ileal morphology (p < 0.05) by increasing villus height and crypt depth. Birds fed the low-CP diet exhibited higher (p < 0.05) AID values for several amino acids, including arginine, histidine, isoleucine, leucine, lysine, phenylalanine, valine, alanine, glycine, proline, and tyrosine. However, the low-CP diet also elevated (p < 0.05) serum alanine aminotransferase activity and decreased (p < 0.05) serum calcium concentration. Conversely, arginine analogue supplementation decreased (p < 0.05) serum triglyceride concentration. Overall, reducing dietary CP from 19% to 17% negatively affected growth performance, despite maintaining comparable levels of lysine, methionine plus cysteine, and threonine. However, arginine analogue supplementation improved feed efficiency, nutrient digestibility, and intestinal morphology. These findings suggest that arginine analogue supplementation may serve as an effective nutritional strategy to improve nutrient utilization and intestinal health in broilers fed reduced-CP diets, although additional amino acid optimization may be required to fully maintain growth performance.