Dose-dependent effects of dietary quercetin on cecal microbiota, hematological responses, and production efficiency in Arbor Acres broiler chickens.

Background and Aim: The gastrointestinal microbiome plays a key role in nutrient absorption, immune regulation, and growth performance in broiler chickens. As restrictions on antibiotic growth promoters increase, phytogenic compounds like quercetin (QC) have gained attention as potential alternatives. Although QC is recognized for its antioxidant and immunomodulatory effects, its dose-dependent influence on gut microbiota composition and systemic immune parameters remains not fully understood. This study aimed to assess the effects of graded dietary QC supplementation on cecal microbiome structure, hematological profiles, and production performance in Arbor Acres (AA) broiler chickens to identify an optimal and safe inclusion level. Materials and Methods: A total of 180 seven-day-old AA broiler chickens were randomly divided into four groups (n = 45 per group; three replicates). Birds received either a basal diet (BD, control) and the BD supplemented with QC at 5 mg/kg (QC1), 10 mg/kg (QC2), or 15 mg/kg (QC3) of feed daily for 35 days. Growth performance indicators, including body weight gain, feed conversion ratio, livability, and the European Production Efficiency Factor (EPEF), were recorded. Hematological parameters were analyzed using an automated veterinary hematology analyzer. Cecal microbiota composition was examined through high-throughput 16S Ribosomal ribonucleic acid (rRNA) gene sequencing, followed by alpha- and beta-diversity analyses and differential abundance testing. Results: Dietary QC significantly affected broiler performance, immune status, and gut microbiota composition in a dose-dependent way. The QC1 group achieved the highest final body weight, average daily gain, and EPEF, with an 11.6% increase in production efficiency compared to the control. Hematological analysis showed increased total leukocyte and lymphocyte counts, along with decreased neutrophil, monocyte, eosinophil, and basophil levels, reflecting immunomodulatory and anti-inflammatory effects. Microbiome analysis indicated that Bacillota and Bacteroidota were dominant across all groups. QC at 5 mg/kg boosted beneficial, butyrate-producing genera, especially Faecalibacterium, while preserving microbial balance. Conversely, higher doses (10-15 mg/kg) led to a notable rise in Campylobacterota, suggesting a possible shift toward dysbiosis. Alpha-diversity measures were not significantly affected, but beta-diversity analysis confirmed distinct changes in microbial communities among the treatment groups. Conclusion: Dietary QC has a clear dose-dependent effect on the gut microbiota-immune-performance axis in broiler chickens. Supplementation at 5 mg/kg of feed is the optimal level, improving growth performance, feed efficiency, immune balance, and beneficial microbial populations without increasing pathogenic taxa. Higher supplementation levels may disturb microbial balance and raise the levels of potentially harmful bacteria. These findings support QC as a promising phytogenic alternative to antibiotic growth promoters and provide a scientific basis for its rational use in sustainable, antibiotic-free poultry production systems.