Effects of steroidal implantation and ractopamine-HCl on nitrogen retention, blood metabolites and skeletal muscle gene expression in Holstein steers

Summary Six Holstein steers (231 ± 17 kg) housed in metabolism crates were used in a randomized complete block design with three blocks of two steers based on previous serum insulin‐like growth factor (IGF)‐I concentrations. One of the two steers in each block was implanted with 120 mg trenbolone acetate and 24 mg oestradiol‐17β on day 0. None of the steers was fed ractopamine‐HCl in the initial 28 days, and then all steers were fed 200 mg of ractopamine‐HCl per steer daily from day 28 until the end of the trial. Steers were fed a corn‐based diet (62% rolled corn, 20% expeller soya bean meal and 15% alfalfa hay) twice daily with an average dry matter intake of 4.8 kg/day. Blood and M. longissimus biopsy samples were collected prior to implantation and on days 14, 28, 42 and 56. There was an implant × ractopamine interaction for retained nitrogen (p