Multi-drug residues and antimicrobial resistance patterns in waste milk from dairy farms in Central California

Abstract

In the dairy industry, most antimicrobial drugs used to treat lactating cows result in the production of non-saleable milk due to the presence of drug residues above the tolerable concentration established by the U.S. Food and Drug Administration (FDA). To mitigate the economic loss due to waste milk, 33% of dairy farms in the U.S. feed this milk to pre-weaned calves. Though economically sound, this practice may favor increased selection of antimicrobial-resistant bacteria. Currently, there are no reports evaluating the presence and concentration of drug residues in waste milk on California dairies. The aims of this cross-sectional study were to: 1) identify and measure concentrations of antimicrobial residues of waste milk samples from dairy farms in Central California, 2) identify management practices associated with the occurrence of specific antimicrobial residues in waste milk, and 3) characterize the antimicrobial resistance patterns of E. coli cultured from waste milk samples.