Poultry litters with diverse physicochemical and microbial characteristics are associated with significant variation in the degradation of veterinary antimicrobials

The environmental impact of veterinary antimicrobials is assessed through an environmental risk assessment, which includes simulating the degradation of these compounds in manure to refine estimates of environmental exposure. However, manure degradation guidance only applies to liquid manures and current regulations require one source of manure per animal type to be used for these studies. Poultry litter, characterised by a significantly lower water content, is relatively understudied, and the impact of variability in the physicochemical and microbial properties of poultry litter on antimicrobial degradation is poorly understood. Current guidance on the number of litter samples required may not accurately represent the inherent variability in antimicrobial degradation and ultimately the environmental risk posed by these compounds. A series of experiments has been undertaken exploring the impact of poultry litter from various farms with different production strategies on the degradation of five antimicrobials: monensin, salinomycin, sulfadiazine, tiamulin and trimethoprim. Litters from different farms varied significantly in physicochemical properties (p < 0.05) and microbial diversity (p < 0.05). Measured degradation rates varied significantly (p < 0.05) across all farms for all tested antimicrobials, notably for salinomycin, with DT50 values (time taken for the compound concentration to reduce by half) ranging from 0.6 to over 10,000 days. These results indicate the need for further research to understand the influence of specific chemical and biological properties of manure on degradation processes. These findings suggest that multiple litters with varying characteristics are needed for manure degradation studies to deliver a robust and realistic environmental risk assessment.