Effects of implant status and breed type on feedlot performance, carcass characteristics, sera metabolites, and immunohistochemical responses in finishing steers

The number of beef × dairy animals entering feedlots has increased, but the response of beef × dairy cattle to growth-promoting implants has not been well characterized. The objective of this study was to evaluate the effects of breed type and implant administration on live performance, carcass characteristics, sera metabolites, and immunohistochemical (IHC) outcomes. Forty-eight steers (average body weight [BW] = 417 ± 22 kg) were sorted by breed into groups of predominantly Angus (B), black-hided beef × primarily Holstein (B × D), or Holstein (D), and half of the steers within each breed type were administered a steroidal implant. Data were analyzed as a 3 × 2 factorial, including repeated measures for sera metabolite and IHC outcomes. Main effects were breed (B, B × D, or D) and implant status (no implant [CON] or Revalor-XS [200 mg trenbolone acetate + 40 mg estradiol-17β] on day 0 [IMP]). Interaction of main effects to determine breed-specific responses to implants were also evaluated. Steers were fed to a target final shrunk BW of 658 kg within breed type. Blood, longissimus thoracis biopsies, and BW were collected on days 0, 28, 70, 98, and 126; BW was also collected prior to harvest. Implanted steers had greater average daily gain (ADG), interim BW, and final BW (P ≤ 0.04). Overall ADG tended to be greatest (P = 0.07) in B × D steers. The IMP treatment, and B × D and D steers had increased (P < 0.01) mean sera concentrations of non-esterified fatty acids and insulin-like growth factor-I (IGF-I), compared to CON and B steers. The IMP carcasses had greater hot carcass weight (HCW), dressing percentage, and REA (P ≤ 0.03) and decreased KPH (P < 0.01). By analyzation of IHC, myosin heavy chain (MHC) Type I fibers were the largest and most abundant in D (P ≤ 0.03). Increased abundance of estrogen receptors and greatest G protein-coupled receptor-1 score was observed for D, while B × D were intermediate, and B had the fewest estrogenic receptors (P ≤ 0.02). Regardless of breed type, IMP steers had increased estradiol-17β and trenbolone-17β concentrations (P < 0.01) with no effect on MHC fiber type and minimal effects on satellite cell outcomes. These results indicated B × D and IMP treatments increased muscling and decreased internal fat deposition compared to D and CON, respectively. Dairy-influenced breed types exhibited increased estrogenic receptor abundance. Additionally, few breed × implant interactions suggest B, B × D, and D steers responded similarly to growth-promoting implants.