Optimal glycine allowance levels in low-protein diets and the dynamic requirement model for broilers

Abstract

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 days 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^0.75 + 8.381 × ADG (R2 = 0.998, P < 0.001). Feeding LP134, LP140, and LP145 diets significantly improved nitrogen deposition rates in broilers (P < 0.05). Low-protein diets significantly upregulated mRNA levels of b0,+AT, EAAT3, and SGLT1 genes in the duodenum (P < 0.05). In conclusion, appropriate glycine supplementation in low-protein diets can enhance growth performance, and nitrogen deposition efficiency, and regulate the expression of intestinal amino acid and glucose transporters. The optimal ratio of SID glycine + serine to lysine in low-protein diets for broilers aged 29 to 42 days is 137%.