Optimal Glycine Allowance Levels in Low-Protein Diets and the Dynamic Requirement Model for Broilers

This study aimed to investigate the effects of different glycine levels in low-protein diets on the growth, nitrogen deposition, and expression of intestinal amino acid and glucose transporters in broilers from 29 to 42 d of age, in order to determine the optimal glycine supplementation level. A total of 240 male broilers at 29 days old were randomly assigned to 5 groups: the control group with a crude protein level of 20%, and experimental groups with low-protein diets (LP130) containing 18% crude protein, supplemented with glycine to achieve standardized ileal digestible (SID) glycine + serine to lysine ratios of 134% (LP134), 140% (LP140), and 145% (LP145). The results showed that the LP134 group had similar growth performance and slaughter performance compared to the control group (P > 0.05), whereas other low-protein diet groups had significantly lower growth performance (P < 0.05). Regression analysis determined that the optimal ratio for SID glycine + serine to lysine was 137%. A dynamic model for glycine + serine requirements was established through binary regression analysis: y = 599.051 × BW