Assessment of Antimicrobial Exposure on Genericandspp. Concentration, Prevalence, and Resistance to Antimicrobials in Beef Cattle Raised with or Without Antimicrobials.

Abstract

The aim was to longitudinally evaluate the association between antimicrobial exposure and resistance occurrence within genericandspp. populations in feedlot beef cattle raised conventionally or raised without antimicrobials. Angus steers (= 180) were sorted into 1 of 2 treatments over 2 consecutive years (108 in yr 1 and 72 in yr 2): steers raised without antimicrobials (NAT) and conventionally raised steers exposed to antimicrobials (CONV). Pens within treatment were adjacent and separated by five empty pens from the other treatment. Monensin and tylosin were included in CONV steer diets. On d 123, CONV steers received a metaphylactic antimicrobial. Longitudinal diet (= 6/year) and fecal (= 5/year) sampling timepoints were collected to determineandspp. concentration, prevalence, and resistance patterns. Dietaryspp. concentrations, and erythromycin (8ERY; 128ERY), tetracycline (TET), trimethoprim-sulfamethoxazole (COT), and cefotaxime (CTX) resistantconcentrations and prevalence were greater in NAT diets than CONV diets (< 0.02). Fecalconcentrations tended to be greater in NAT steers than CON steers (= 0.07). Fecal TETconcentrations were greater in CONV steers than NAT steers (= 0.03). Fecal COTand CTXprevalence was greater for CONV steers at the beginning of the finishing phase while greater for NAT steers at the end of the finishing phase (< 0.01). Fecalspp. concentrations did not differ between treatments (= 0.11). Concentrations of 8ERYand 128ERYspp. were greater in CONV steers on d 64, 130, and 168 than NAT steers (< 0.05). Overall, antimicrobial resistantspp. andwere detected regardless of antimicrobial exposure.