Dose-dependent effects of locally produced microbial phytase on caecal microbiota, hepatic insulin-like growth factor 1 and growth hormone receptor expression, carcass traits, and immune responses in broiler chickens

Background and Aim: Microbial phytase is extensively used in poultry nutrition to improve phosphorus utilization and reduce the anti-nutritional effects of phytic acid. Beyond nutrient digestibility, phytase may influence intestinal microbiota, immune responses, and growth-related molecular pathways in broiler chickens. However, limited information is available regarding the integrated effects of locally produced microbial phytase on caecal microbial populations, hepatic growth-related gene expression, carcass characteristics, and humoral immunity under the same experimental conditions. Therefore, this study evaluated the dose-dependent effects of dietary microbial phytase supplementation on caecal microbiota, hepatic expression of insulin-like growth factor 1 (IGF-1) and growth hormone receptor (GHR), plasma immunoglobulin concentrations, and carcass characteristics in broiler chickens. Materials and Methods: A total of 576 day-old Cobb 500 broiler chicks were randomly assigned to six dietary treatments under a completely randomized design. T1 received a basal diet without phytase supplementation, whereas T2–T6 received basal diets supplemented with microbial phytase at 200, 300, 400, 500, and 600 FTU/kg, respectively. Each treatment consisted of eight replicates with 12 birds per replicate. The experiment lasted for 35 days, covering starter, grower, and finisher phases. Caecal microbial populations were determined using selective culture techniques. Plasma immunoglobulin A and immunoglobulin G concentrations were quantified using enzyme-linked immunosorbent assay kits. Hepatic IGF-1 and GHR mRNA expression levels were analyzed using real-time polymerase chain reaction and quantified using the 2⁻ΔΔCt method with Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH) as the reference gene. Carcass characteristics were evaluated at the end of the feeding trial. Data were analyzed using the general linear model procedure at p < 0.05. Results: Dietary microbial phytase supplementation significantly increased beneficial caecal bacteria, including Lactobacillus spp. and Streptococcus spp., while reducing Escherichia coli populations compared with the control group (p < 0.05). Birds receiving 600 FTU/kg phytase exhibited the greatest beneficial microbial populations and the lowest E. coli counts. Plasma immunoglobulin A and immunoglobulin G concentrations increased significantly in phytase-supplemented birds, particularly during the finisher phase (p < 0.05). Hepatic IGF-1 and GHR mRNA expression levels were significantly upregulated in a dose-dependent manner. Carcass weight and carcass yield also improved significantly following phytase supplementation, with superior responses observed at 600 FTU/kg phytase supplementation. Significant linear responses were detected for most microbial, immunological, molecular, and carcass-related parameters. Conclusion: Locally produced microbial phytase demonstrated multifunctional physiological benefits beyond phosphorus release by improving gut microbial balance, humoral immunity, hepatic growth-related gene expression, and carcass performance in broiler chickens. Supplementation at 600 FTU/kg yielded the greatest overall biological responses and may represent an effective nutritional strategy to improve poultry productivity and intestinal health under tropical production conditions. Keywords: broiler chickens, carcass characteristics, caecal microbiota, growth hormone receptor, hepatic gene expression, immune response, insulin-like growth factor 1, microbial phytase.