Feeding subtherapeutic antimicrobials to low-risk cattle does not confer consistent performance benefits

Two-hundred and forty steers were obtained from the same ranch in each of 2 yr. Growth performance, health status and carcass characteristics from steers fed subtherapeutic antimicrobials were compared with those of control steers without antimicrobial metaphylaxis. Experimental groups included Control, not fed antimicrobials; CTCS-700, fed 350 mg head−1 d−1 chlortetracycline (CTC) and 350 mg head−1 d−1 sulfamethazine; CTC-11, fed 11 mg kg−1 CTC; TYL, fed 11 mg kg−1 tylosin phosphate; and CTC-350, fed 350 mg head−1 d−1 CTC. Steers were housed in pens of 10 steers and fed antimicrobials during both backgrounding and finishing periods. The incidence of bovine respiratory disease, pinkeye and bloat did not differ among experimental groups, although the incidence of foot rot was lower in Control steers (P<0.05) than in steers receiving TYL. In both years of the study, overall performance (backgrounding+finishing) was not improved by subtherapeutic antimicrobials. Similarly, carcass characteristics and the incidence and severity of liver abscesses were not improved compared with Control steers by feeding subtherapeutic antimicrobials. The results of the present study demonstrate that low-risk steers may be managed in small pens with equivalent growth performance, carcass characteristics and health status to steers fed subtherapeutic antimicrobials.