Dietary glutamine supplementation improves growth performance, intestinal morphology, and immunity function associated with cecal microbiota alterations in broilers

Broilers face challenges in improving intestinal health and growth performance. Glutamine (Gln), a functional amino acid, exerts beneficial effects in promoting intestinal development and immunity. This study investigated the effects of graded levels of dietary Gln supplementation on the growth performance, intestinal morphology, immune indices, and cecal microbiota of broilers. Three hundred one-day-old Arbor Acres broilers (51.67±1.42 g) were assigned to five groups in a completely random design, with six replicates each group and 10 broilers in each replicate. The control group was fed the basal diet (Con), while the experimental groups were fed the basal diet containing 4 g/kg (Gln1), 6 g/kg (Gln2), 8 g/kg (Gln3), and 10 g/kg Gln (Gln4), respectively. The results showed that dietary Gln supplementation had quadratic effect on ADG, F/G, and final BW of broilers (P < 0.01), with higher average daily gain (ADG), final body weight (BW) in Gln2 group (P < 0.05). Dietary Gln supplementation quadratically improved serum immune indices of broilers (P < 0.01), with higher IgA, IgG, and IgM levels in Gln1 and Gln2 groups (P < 0.05). The Gln1 and Gln2 groups increased Bursa of Fabricius index at both d21 and d42 (P < 0.05). There was a quadratic effect of Gln on intestinal development (P < 0.01). Gln1 and Gln2 groups had higher duodenal VH and V/C, and the Gln2 group increased jejunal VH and V/C, and ileal VH at 21d (P < 0.05). Duodenal V/C improved in the Gln1 and Gln2 groups, and duodenal CD reduced in Gln1 group at 42d (P < 0.05). Gln1 and Gln2 groups increased cecal α-diversity (ACE, Chao1) at d21 (P < 0.05). PCoA revealed distinct cecal microbiota profiles across groups, with Gln-supplemented groups reducing pathogenic Escherichia-Shigella and Mucispirillum, while enriching beneficial genera (e.g., Ruminococcaceae_UCG-014, Barnesiella, and Butyricimonas) (P < 0.05). Collectively, dietary Gln-supplemented at 4-6 g/kg optimally enhances growth, immunity, and intestinal development in broilers, which is related to modulate cecal microbiota toward a beneficial composition.