Effects of antimicrobial metaphylaxis using no antimicrobial, tilmicosin, or tildipirosin and 2 different days on feed on the health and growth performance of lightweight beef steer calves originating from Mexico

No antimicrobial, tilmicosin, or tildipirosin were evaluated for the control of bovine respiratory disease (BRD), and additional days on feed (DOF) were evaluated for effects on growth performance and carcass characteristics of Mexican-origin steers. A split-plot design was used with 2 DOF treatments nested within 3 metaphylaxis treatments. Steers originating from Mexico (n = 4,086; 213 ± 31.2 kg) were randomly assigned to on-arrival metaphylaxis treatments: 1) no antimicrobial (CON), 2) 13.2 mg/kg tilmicosin (MIC), or 3) 4 mg/kg tildipirosin (ZUP). Ten pen replications per metaphylaxis treatment were used (30 pens). Approximately 30 to 45 d before projected slaughter date, steers were randomly assigned within each of the 30 pens to 1 of 2 DOF (sort groups) treatments shipped for slaughter 28 d apart: 1) early slaughter averaging 50 d (range: 46 to 63 d) after the sort or 2) late slaughter averaging 78 d (range: 74 to 91 d) after the sort, resulting in 60 pens. Metaphylaxis with MIC (5.56%) or ZUP (3.58%) reduced (P < 0.01) BRD morbidity compared with CON (12.08%), and ZUP reduced (P = 0.01) BRD morbidity compared with MIC. Metaphylaxis reduced (P ≤ 0.02) BRD mortality compared with CON, though mortality was not different between ZUP and MIC (P = 0.70; 1.09 or 0.95%, respectively, compared with 2.19% for CON). Although ADG and efficiency were reduced in the late-slaughtered steers, carcass weight increased (P < 0.001; 390 or 405 kg, respectively). Late-slaughtered steers had an increase in marbling score and empty body fat (P < 0.001). Metaphylaxis improved animal health compared with no metaphylaxis, and tildipirosin improved morbidity but not mortality compared with tilmicosin. Live growth slowed but live and carcass weight increased with additional DOF. Carcass characteristics indicated increased marbling and fatness occurred with additional DOF.