Biological responses of beef steers to steroidal implants and zilpaterol hydrochloride

British × Continental steers (n = 168; 7 pens/treatment; initial BW = 362 kg) were used to evaluate the effect of dose/payout pattern of trenbolone acetate (TBA) and estradiol-17β(E₂) and feeding of zilpaterol hydrochloride (ZH) on serum urea-N (SUN), NEFA, IGF-I, and E₂ concentrations and LM mRNA expression of the estrogen (ER), androgen (ANR), IGF-I (IGF-IR), β₁-adrenergic (β₁-AR), and β₂-adrenergic (β₂-AR) receptors and IGF-I. A randomized complete block design was used with a 3 × 2 factorial arrangement of treatments. Main effects were implant (no implant [NI], Revalor-S [REV-S; 120 mg TBA + 24 mg E₂], and Revalor-XS [REV-X; 200 mg TBA + 40 mg E₂]) and ZH (0 or 8.3 mg/kg of DM for 20 d with a 3-d withdrawal). Steers were fed for 153 or 174 d. Blood was collected (2 steers/pen) at d -1, 2, 6, 13, 27, 55, 83, 111, and 131 relative to implanting; LM biopsies (1 steer/pen) were collected at d -1, 27, 55, and 111. Blood and LM samples were collected at d -1, 11, and 19 relative to ZH feeding. A greater dose of TBA + E₂ in combination with ZH increased ADG and HCW in an additive manner, suggesting a different mechanism of action for ZH and steroidal implants. Implanting decreased (P < 0.05) SUN from d 2 through 131. Feeding ZH decreased (P < 0.05) SUN. Serum NEFA concentrations were not affected by implants (P = 0.44). There was a day × ZH interaction (P = 0.06) for NEFA; ZH steers had increased (P < 0.01) NEFA concentrations at d 11 of ZH feeding. Serum E₂ was greater (P < 0.05) for implanted steers by d 27. Serum trenbolone-17βwas greater (P < 0.05) for implanted steers by d 2 followed by a typical biphasic release rate, with a secondary peak at d 111 for REV-X (P < 0.05) implanted steers. Implanting did not affect mRNA expression of the ANR or ER, but the IGF-IR and the β₁-AR and β₂-AR were less (P < 0.05) for REV-S than NI at d 55 and β₂-AR mRNA was less (P < 0.05) for REV-S than for REV-X. Expression of the IGF-IR and the β₁-AR at d 111 was greater (P < 0.05) for REV-X than for REV-S and NI at d 111, and the β₂-AR was less (P < 0.05) for REV-S than for REV-X. Feeding ZH did not affect mRNA expression of the β₁-AR and β₂-AR. Both implanting and feeding ZH decreased SUN, but a greater dose of TBA + E₂ did not result in further decreases. In addition, feeding ZH increased serum NEFA concentrations. Metabolic changes resulting from implanting and feeding ZH may aid in explaining steer performance and carcass responses to these growth promotants.