Efficacy of metaphylactic tilmicosin for controlling bovine respiratory disease in high-risk northern feeder calves

The objectives of this study were to: (1) evaluate the effect of metaphylactic tilmicosin on the health and performance of high-risk northern calves, and (2) determine the efficacy of tilmicosin for treatment of bovine respiratory disease (BRD) following its use in an arrival metaphylaxis programme. 400 crossbred steer calves weighing an average of 273 kg each were purchased at livestock auctions in South Dakota and Montana, and shipped to a commercial research feedlot in northern Colorado. At processing, calves were randomly assigned to one of two treatment groups: (1) non-medicated controls or (2) metaphylaxis with tilmicosin. Cattle assigned to the metaphylaxis group were administered tilmicosin once at label dose. Control cattle received no antibiotic at processing. Metaphylactic use of tilmicosin at processing reduced BRD morbidity compared to controls (25.5 vs, 56.5%, P<0.01). Average number of days to first pull were greater in the metaphylaxis group (13.9 vs. 9.6 d, P<0.01). BRD mortality and chronic rates did not differ between treatments. When calculated on a "deads out" basis, calves receiving tilmicosin at processing gained more weight during the first 28 d (1.66 vs. 1.41 kg/d, P<0.01), and 102 d on feed (DOF) (1.75 vs. 1.67 kg/d, P<0.01) compared with control calves. At harvest (191 DOF), daily gain tended to be greater (1.55 vs. 1.52 kg/d, P=0.10) for calves in the metaphylactic tilmicosin treatment group. There were no differences in feed intake or feed conversion between groups. Hot carcass weight tended to be greater for calves in the metaphylactic treatment group (359.8 vs. 353.5 kg, P=0.06). Other carcass traits did not differ. When tilmicosin was used as first treatment for calves with BRD, response rates were identical for both treatment groups (86%). No differences were observed between treatment failure (6.5 vs. 4.6%, P=0.68), first relapse (7.5 vs. 9.4%, P=0.71), second relapse (4.4 vs. 2.7%, P=0.72) or new episode (11.9 vs. 10.8%, P=0.85) rates for calves in the metaphylaxis and control groups, respectively. In this study, metaphylactic treatment with tilmicosin at processing decreased morbidity, and increased days to first treatment for those calves that subsequently required BRD therapy. Efficacy of tilmicosin as the first line therapy for BRD following its use in an on-arrival metaphylaxis programme was not compromised based on the results of this study.