The influence of substituting dietary peptide-bound with free amino acids on nitrogen metabolism and acid-base balance of broiler chickens depends on asparagine and glutamine supply

Reducing dietary crude protein (CP) concentration while maintaining adequate amino acid (AA) supply by free AA inclusion can contribute to attenuate the negative environmental effects of animal farming. This study investigated upper limits of dietary free AA inclusions without undesirable effects including the dependence on asparagine (Asn) and glutamine (Gln) supply. Ten broilers were allocated to 63 metabolism units each and offered 9 experimental diets from day (d) 7-21 (n 7). One diet (167 g CP/kg) contained 80 g soy protein isolate (SPI)/kg. In the other diets, 25, 50, 75, and 100 % of the digestible AA from SPI were substituted with free AA. Digestible Asn+aspartic acid (Asp) and Gln+glutamic acid (Glu) were substituted with Asp/Glu or 50/50 mixes of Asp/Asn and Glu/Gln, respectively. Total excreta were collected from d 11-14 and d 18-21. Growth and nitrogen accretion were unaffected by 25 and 50 % substitution without and with free Asn/Gln, respectively, but decreased at higher substitution (P ≤ 0.024). Circulating concentrations of Asp, Glu, and Gln were unaffected by treatment, while Asn decreased at substitution higher than 50 % when Asn/Gln were not provided (P ≤ 0.005). Blood gas analysis on d 21 indicated a compensated metabolic acidosis at substitution higher than 50 and 75 % without and with free Asn/Gln, respectively (P ≤ 0.017). Results suggest that adding Asn/Gln increased an upper limit for proportion of dietary free AA from 10 to 19% of dietary CP and enabled higher free AA inclusion without affecting the acid-base balance. © The Author(s), 2023. Published by Cambridge University Press on behalf of The Nutrition Society.