Biological responses of beef steers to steroidal implants and zilpaterol hydrochloride1

British x 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 (Esub2) and feeding of zilpaterol hydrochloride (ZH) on serum urea-N (SUN), NEFA, IGF-I, and Esub2 concentrations and LM mRNA expression of the estrogen (ER), androgen (ANR), IGF-I (IGF-IR), βsub1-adrenergic (βsub1-AR), and βsub2-adrenergic (βsub2-AR) receptors and IGF-I. A randomized complete block design was used with a 3 x 2 factorial arrangement of treatments. Main effects were implant (no implant [NI], Revalor-S [REV-S; 120 mg TBA + 24 mg Esub2], and Revalor-XS [REV-X; 200 mg TBA + 40 mg Esub2]) 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 + Esub2 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 x ZH interaction (P = 0.06) for NEFA; ZH steers had increased (P < 0.01) NEFA concentrations at d 11 of ZH feeding. Serum Esub2 was greater (P < 0.05) for implanted steers by d 27. Serum trenbolone-17β was 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 βsub1-AR and βsub2-AR were less (P < 0.05) for REV-S than NI at d 55 and βsub2-AR mRNA was less (P < 0.05) for REV-S than for REV-X. Expression of the IGF-IR and the βsub1-AR at d 111 was greater (P < 0.05) for REV-X than for REV-S and NI at d 111, and the βsub2-AR was less (P < 0.05) for REV-S than for REV-X. Feeding ZH did not affect mRNA expression of the βsub1-AR and βsub2-AR. Both implanting and feeding ZH decreased SUN, but a greater dose of TBA + Esub2 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