Effects of ractopamine and trenbolone acetate implants with or without estradiol on growth performance, carcass characteristics, adipogenic enzyme activity, and blood metabolites in feedlot steers and heifers

Two experiments were conducted to evaluate effects of ractopamine (RAC) and steroidal implant treatments on performance, carcass traits, blood metabolites, and lipogenic enzyme activity in feedlot cattle. In Exp. 1, yearling steers (n = 486; initial BW = 305 kg) were used in a 3 × 3 factorial arrangement of RAC doses of 0 (R0), 100 (R100), or 200 (R200) mg·steer(-1)·d(-1) fed for 28 d and implant regimens (implant-reimplant) of no implant-no reimplant (NI-NI), 120 mg of trenbolone acetate (TBA) and 24 mg of estradiol-17β (E17B)-no implant (RS-NI), or 80 mg of TBA and 16 mg of E17B followed by 120 mg of TBA and 24 mg of E17B (RI-RS). Except for KPH and skeletal maturity score, no RAC × implant interactions were noted (P > 0.10). Carcasses from R200 were 6.3 kg (P = 0.042) heavier than those from R0. Marbling, calculated empty body fat (EBF), and USDA quality grade did not differ (P > 0.10) among RAC treatments. The RI-RS steers had 12.6 kg (P = 0.001) and 41.1 kg (P 0.10) in EBF, marbling score was decreased for RI-RS (P 0.10) were detected. The RAC-supplemented heifers had greater HCW (P 0.10), TE (P = 0.021) and TO (P = 0.039) had fewer Choice carcasses than NI. Heifers with implants had decreased cortisol and increased IGF-1 and NEFA (P < 0.10) compared with NI heifers. An implant × RAC interaction was detected (P = 0.001) for serum urea nitrogen (SUN), with TE and RAC-supplemented heifers having decreased SUN. These data suggest that the effects of implant and RAC on growth and carcass traits are independent and that USDA quality grade and marbling score can differ significantly among carcasses with similar calculated EBF values.