Effects of Rearing Practices that Restrict Antimicrobial Exposure in Feedlot Cattle on Antimicrobial Resistance in Enteric Bacteria

Abstract

The primary objective of this research was to evaluate differences in antimicrobial resistance among enteric bacteria recovered from feedlot cattle being raised without exposure to antimicrobial drugs (AMDs) and those reared using conventional practices. Forty-seven pens of feedlot cattle (6,347 total animals) produced with restricted antimicrobial drug exposure were purposefully selected for enrollment as were 48 pens of cattle (5,273 total animals) contemporaneously being fed for production of conventional beef products. Fecal samples were collected from the floors of pens throughout the feeding period and cultured to recover non-type-specific Escherichia coli (NTSEC) and Salmonella enterica. Isolates were evaluated for susceptibility to a panel of AMDs. Resistance among NTSEC isolates was most common to tetracycline, streptomycin and sulfamethoxazole, and slightly higher rates of resistance among NTSEC isolates were recovered from conventionally reared cattle. Recorded therapeuticAMD exposures had no detectable impact on the prevalence of resistance among NTSEC. While there were detectable temporal trends among NTSEC for resistance to tetracycline, naladixic acid, chloramphenicol and cephalothin, the direction of trends differed among drugs and were not associated with different rearing practices. Salmonella enterica was recovered rarely (0.73%), but at similar rates in cattle in both management groups. These findings suggest that conventional feedlot production methods (including parenteral and in-feed use of AMDs) do not predictably or uniformly increase the prevalence of antimicrobial resistance among fecal NTSEC when compared to rearing methods used to produce branded "natural" beef products (i.e., beef products with a label certification that source cattle were not exposed to antimicrobial drugs).