Antimicrobials are included in finishing cattle diets for growth promotion, feed
efficiency, and protection against liver abscesses. The inclusion of in-feed antimicrobials at or
below therapeutic concentrations may provide a selective pressure for antimicrobial resistant
microorganisms. Additionally, heavy metals such as copper and zinc may be included in cattle
diets because of growth-promoting effects. Heavy metal resistance genes are on transferable
plasmids that also contain antimicrobial resistance genes. The objectives of this research were to
1) determine the prevalence of food-borne pathogens, Salmonella and E. coli O157, in cattle fed
diets with or without monensin and tylosin and 0 or 25% wet corn distiller's grains (WDGS), 2)
determine the prevalence of food-borne pathogens in cattle fed elevated concentrations of copper
and zinc 3) evaluate the effect of antimicrobials on antimicrobial susceptibility of food-borne
pathogens and commensal fecal bacteria, and 4) determine a possible association between in-feed
antimicrobials and the concentration of antimicrobial resistance genes in the feces of cattle.
Inclusion of 25% WDGS was associated with a higher prevalence of E. coli O157 on one of two
sample collection days; however, there was no association between the use of monensin and
tylosin, or copper and zinc on the prevalence of food-borne pathogens. Including monensin and
tylosin in cattle diets was associated with an increased resistance of enterococci to macrolides,
but was not related to concentration of the common macrolide resistance gene, ermB. In cattle
fed diets with copper and/or zinc, no differences were observed in antimicrobial susceptibility or
the concentration of antimicrobial resistance genes. In conclusion, results indicate that including
growth-promoting antimicrobials in cattle diets at below therapeutic concentrations only
limitedly impacted antimicrobial susceptibility and concentration of fecal antimicrobial
resistance genes; however, this research encompassed only a select number of microorganisms.
The positive association between WDGS and E. coli O157 prevalence in cattle has important
implications for food safety, and warrants further investigation.
