Effects of preshipping vs. arrival medication with tilmicosin phosphate and feeding chlortetracycline on health and performance of newly received beef cattle.

Our objective was to determine the effects of preshipping (PRE) vs. arrival (ARR) medication with tilmicosin phosphate (MIC; Exp. 1 and 2) and feeding chlortetracycline (CTC; 22 mg/kg of BW from d 5 to 9; Exp. 2) on health and performance of beef calves received in the feedlot. Ninety-six steers (Exp. 1; pay weight 236 kg) and 240 (Exp. 2; average pay weight 188 kg) steer and bull calves were used. For Exp. 1, treatments included no MIC (CON), PRE, and ARR. For Exp. 2, treatments were arranged in a 3x2 factorial. Treatments included CON, PRE, and ARR, either with CTC or without CTC. For Exp. 2, serum concentrations of immunoglobulin (Ig)G and alpha-1-acid glycoprotein (AGP) were determined on samples collected on d 0, 5, 10, and 28 and d 0, 5, and 10, respectively. No MIC x CTC interactions were observed. No differences were noted among MIC or CTC treatments in any of the experiments for ADG, daily DMI, or gain:feed ratio for the overall receiving periods. For Exp. 1, percentage of steers treated for bovine respiratory disease (BRD) was decreased (P<.05) for MIC-treated animals vs CON (71.9, 45.2, and 46.9 for CON, PRE, and ARR, respectively), and the week that calves were treated for BRD differed (P<.10) among treatments. For Exp. 2, the number of calves treated for BRD was decreased (P<.01) for MIC-treated steers vs CON and decreased (P<.05) for ARR vs. PRE (40.0, 18.7, and 7.5% for CON, PRE, and ARR, respectively). Averaged across days, serum IgG was decreased (P<.05) for MIC-treated steers vs. CON, with no differences noted among treatments for AGP. Results suggest that preshipping medication programs are no more effective than arrival medication programs using tilmicosin phosphate.