Countering Antimicrobial Resistance: Impacts of Natural Growth Promoters on Growth Performance, Immunity, Gut Microbiota and Morphology in Broiler Chicken: A Systematic Review

Abstract

Background and Objective: Antibiotic Growth Promoters (AGPs) have been widely used to improve broiler performance. However, their use is declining due to concerns about antimicrobial resistance (AMR) and increasing consumer demand for antibiotic-free products. This systematic review aimed to evaluate the effects of alternative growth promoters on growth performance, immune response, gut microbiota, and intestinal morphology in broiler chickens. Materials and Methods: Relevant studies published between 2015 and 2025 were collected from PubMed, ScienceDirect, Scopus, and Google Scholar. A total of 183 articles were identified, and 99 were included after a rigorous screening process. Data on growth performance, gut morphology, microbial modulation, and immune responses were synthesized qualitatively. Results: The findings showed improved growth performance, indicated by better Feed Conversion Ratio (FCR) and increased Body Weight Gain (BWG). Enhanced immune function was observed through higher serum antibody levels and increased weights of immune organs. Natural alternatives also promoted beneficial microbes while reducing pathogenic populations. Positive changes in gut morphology were noted, including increased villus height and a higher villus height to crypt depth ratio. Dietary inclusion was the primary method of administering these growth promoters. Conclusion: Alternative growth promoters have positive and often synergistic effects on growth performance, gut health, immune function, and intestinal microbiota in broiler chickens. However, existing challenges and knowledge gaps highlight the need for optimized formulations and standardized application protocols.