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Abstract To investigate the effects of antimicrobial exposure, Angus steers [n = 288; initial body weight (BW) = 314 and 300 kg) were sorted into 1 of 2 treatments over 2 consecutive years: steers raised without antimicrobials (NAT) and conventionally raised steers exposed to antimicrobials (CONV). Pens within treatment were adjacent and separated via empty pens from the other treatment. Before cattle arrival, pens were renovated, and soil was replaced from an area not subjected to livestock production. Monensin and tylosin were included in CONV steer diets targeting 250 and 85 mg/steer daily, respectively. On d 123, CONV steers received a metaphylactic antimicrobial; NAT steers received a sterile saline injection. From d 0 to 168, diet and rectal fecal samples were collected longitudinally at 5 timepoints to determine E. coli and Enterococcus spp. concentration, prevalence, and resistance patterns. The MIXED procedure of SAS was used with fixed effects of treatment, day, and treatment × day interaction, and random effect of pen nested within a treatment × year interaction. Feed E. coli and Enterococcus spp. concentrations were greater in NAT diets than CONV diets (P 0.01). Erythromycin (8ERYR; 128ERYR), tetracycline (TETR), trimethoprim-sulfamethoxazole (COTR), and cefotaxime (CTXR) resistant E. coli concentrations and prevalence were greater in NAT diets than CONV diets (P 0.02). Fecal E. coli concentrations tended to be greater in NAT steers than CON steers (P = 0.07). A treatment × day tendency was detected for 8ERYRE. coli concentrations (P = 0.06), with NAT steers tending to have greater concentrations on d 64 and 130. Conversely, CONV steers had greater 128ERYRE. coli concentrations on d 130 and 168 than NAT steers (P 0.01). Fecal TETRE. coli concentrations were greater in CONV steers than NAT steers (P = 0.03). Fecal COTR and CTXRE. coli prevalence had a treatment × day interaction (P 0.01), with CONV steers having greater prevalence at the beginning of the finishing phase (d 0). Conversely, NAT steers had greater COTR (d 123 and 130) and CTXR (d 130 and 168) E. coli prevalence towards the end of the finishing phase than CONV steers. Fecal Enterococcus spp. concentrations did not differ between treatments (P = 0.11). A treatment × day interaction was detected for 8ERYR and 128ERYREnterococcus spp. concentrations (P 0.05), with CONV steers having greater concentrations on d 64, 130, and 168 than NAT steers. Overall, these data suggest that antimicrobial exposure does not inherently increase E. coli and Enterococcus spp. concentrations. Treatment differences observed for ERYREnterococcus spp. concentrations are likely a result of tylosin exposure whereas differences in 128ERYR and TETRE. coli concentrations are likely an effect of metaphylactic administration. USDA is an equal opportunity employer.
Dornbach et al. (Sun,) studied this question.