Timing of an antioxidant-acidifier feed additive modulates growth performance and cecal microbiota in broiler chickens during enrofloxacin exposure.

Nutritional interventions targeting oxidative stress and gut ecology may help sustain broiler performance during antimicrobial perturbation. This study evaluated the timing-dependent effects of an antioxidant-acidifier feed additive, alone or combined with enrofloxacin, on growth performance and cecal microbiota in broilers. A total of 1,200 Ross 308 male broilers were assigned to six treatments (4 pens/treatment; 50 birds/pen) and reared to 42 d: control, enrofloxacin (d15-19), additive for 4 weeks (A4; d15-42), A4+enrofloxacin, additive for 6 weeks (A6; d1-42), and A6+enrofloxacin. The additive was included at 4% of the diet, providing fumaric acid, heat-stable vitamin C, and all-rac-α-tocopheryl acetate. Performance traits were recorded by phase and overall. Cecal contents were collected from 2 birds/pen on d14, d20, and d42 and analyzed by shotgun metagenomic sequencing; microbiota analyses were conducted on pen-level aggregated profiles. Growth performance showed modest, timing-dependent numerical differences: A4 had numerically higher finisher and overall average daily gain, whereas A4+enrofloxacin was associated with lower overall gain than enrofloxacin alone. Cecal community structure was driven primarily by sampling time point. No detectable enrofloxacin-associated separation was observed at d20, whereas a trend toward separation was evident at d42. By d42, only limited genus-level differences were detected, with the strongest signal observed for Phocaeicola in the targeted differential abundance analysis. Overall, additive timing influenced growth responses and cecal microbiota trajectories during enrofloxacin exposure, supporting the importance of supplementation timing when nutritional strategies are used to improve robustness under antibiotic perturbation.