Epidemiology of Antimicrobial Residues and Phenotypic Resistance of Bacterial Isolates from Waste Milk on California Dairies

Waste milk fed to dairy calves often contains low levels of antimicrobial drug residues, particularly ceftiofur. In this study of 40 samples from eight Californian dairies, such residues were detected and linked to increased antimicrobial resistance (AMR) in bacteria isolated from the milk. Notably, florfenicol residues were associated with higher resistance in coliforms, and ceftiofur with resistance in Streptococcus species. Overall, the findings indicate that feeding waste milk may contribute to the development and spread of AMR in calf gut microbiota, underscoring the importance of improved antimicrobial stewardship in dairy farming.