CHALLENGES AND LIMITATIONS OF PROBIOTIC USE

Probiotics can improve digestive health, modulate immunity, and enhance pathogen resistance in poultry, but their application faces multiple challenges [275]. Efficacy often varies depending on the strain, dosage, administration form, bird age, and breed, making standardized recommendations difficult [38]. Reproducibility across studies is limited by inconsistent experimental designs, endpoints, and reporting standards, highlighting the need for precision-based strategies tailored to flock genetics, baseline microbiota, and farm conditions [276].

Field applications introduce further variability. While controlled trials often show clear benefits, commercial farm outcomes are influenced by management practices, biosecurity, feed quality, environmental stress, and pathogen load [277]. This heterogeneity complicates generalization of results, emphasizing the importance of multi-site trials under standardized protocols [278].

Safety and regulatory considerations are critical. Certain probiotic strains may harbor transferable ARGs, requiring genomic screening to prevent potential horizontal gene transfer and AMR dissemination [279]. Strain authentication, virulence exclusion, and compliance with GRAS/QPS frameworks are necessary, while regulatory requirements vary across jurisdictions, affecting registration, labeling, and guidance for farmers [280]. Rare opportunistic infections have been reported in specific Enterococcus strains, underscoring the need for continuous post-market surveillance [281].

Stability and quality control are additional limitations. Non-spore-forming strains are sensitive to heat, oxygen, and acidic conditions, reducing viable cell counts and efficacy [282]. Batch-to-batch consistency, accurate CFU labeling, and shelf life management are essential, particularly under intensive production systems and variable storage conditions [283]. Delivery methods, including in-feed, in-water, and early-life _in _ovo administration, also face technical and operational challenges that require robust field validation [284].

Economic and practical constraints influence adoption. Cost-effectiveness depends on product price, inclusion rate, baseline farm performance, and disease pressure, while scalability, feed processing, and infrastructure limitations may hinder implementation. Farmer education and clear communication of scientific evidence are crucial for adoption and appropriate use [285].

Finally, future research should focus on standardized immune biomarkers, precision dosing, longitudinal ecological impacts on gut microbiota and AMR, and emerging delivery strategies. Integrating early-life interventions with microbiome-informed flock management may improve consistency and support the sustainable use of probiotics in post-AGP poultry production systems.