Impact of dietary synbiotics on growth performance, gut morphology, and immune function, and welfare indicators of broiler chickens with or without early life antibiotic supplementation via drinking water

Abstract The widespread use of antibiotics in broiler production has contributed to antimicrobial resistance, necessitating the development of sustainable alternatives. Synbiotics, combining probiotics and prebiotics in a synergistic formulation, have emerged as promising candidates to replace antibiotics. This study evaluated the efficacy of dietary synbiotics as alternatives to early-life antibiotic supplementation in broiler chickens, examining their effects on growth performance, gut morphology, and welfare. A total of 80 mixed-sex Cobb 500 broiler chickens were allocated to a 2 × 2 factorial design investigating synbiotic supplementation ( Bacillus subtilis and dried fermented Saccharomyces cerevisiae extract) and enrofloxacin treatment (days 0-5) administered via drinking water. During the initial growth period (days 0-7), synbiotic supplementation reduced body weight gain compared to controls. However, synbiotic × antibiotic interactions were observed during days 7-14 and 14-21, where synbiotics without antibiotics produced the highest weight gain in the second week, while controls without either treatment achieved superior performance in the third week. Feed intake was increased by synbiotic supplementation during days 7-14, while antibiotic treatment consistently elevated feed consumption throughout multiple periods and overall trial duration. Feed conversion efficiency was initially impaired by synbiotics (days 0-7) but improved during days 14-21. Antibiotic supplementation resulted in a poorer overall feed conversion ratio. Gut morphological analysis revealed significant synbiotic × antibiotic interactions for duodenal length and empty weights of duodenum and caeca, with synbiotics enhancing these parameters only when combined with antibiotic treatment. Both synbiotic and antibiotic supplementation independently reduced ileal content weight, while antibiotic treatment specifically decreased ileal weight and caecal length. Immune function evaluation through heterophil-to-lymphocyte ratios demonstrated a tendency for interaction, where synbiotics reduced this ratio in the absence of antibiotics but showed no effect when combined with antibiotic treatment. Neither treatment affected spleen or bursa weights, indicating minimal impact on immune organ development. These findings suggest that while dietary synbiotics show promise as alternatives to antibiotics in broiler production, their efficacy varies depending on concurrent antibiotic exposure and growth phase. Synbiotics demonstrated beneficial effects on gut morphology and stress markers, particularly when administered without concurrent antibiotic treatment, supporting their potential as functional alternatives in antimicrobial-free poultry production.