Comparative effectiveness of somatotropin and anabolic steroids in feedlot steers

Crossbred steers (n = 252, BW = 379 +/- 28 kg) were allotted to 42 pens in a 2 x 3 factorial arrangement of treatments: control or steroid implant (STR; estradiol benzoate+progesterone [three lighter blocks reimplanted on d 84] and trenbolone acetate [reimplanted on d 63]), and either 0, 80, or 160 mg/wk of recombinant bovine somatotropin (bST). Steers were adapted to the finishing diet (12% roughage equivalent, 13% CP) before the start of the experiment and fed for 84 or 119 d. Blood samples were taken on d 0, 14, 28, 56, and 84 for plasma urea N (PUN), serum somatotropin (ST), plasma insulin-like growth factor I (IGF-I), and plasma amino acid assay. Few interactions were noted (P.1). Gain was increased by both treatments: 1.30 vs 1.66 kg/d for control vs. STR (P.001) and 1.44, 1.49, and 1.51 kg/d (linear, P = .07) for 0, 80, and 160 mg of bST/wk, respectively. Gain efficiency was also improved: 169 vs 205 g/kg (P.001) and 177, 189, and 195 g/kg (linear, P.001), respectively. Average PUN was decreased (P.001) 29% by STR and decreased 17 and 29% by 80 and 160 mg of bST/wk, respectively (linear, P.001). Somatotropin decreased mean serum ST compared with controls; STR increased ST 36% compared with controls. Average plasma IGF-I was increased (P.001) 12% by STR and 13 and 19% (linear, P.001) by 80 and 160 mg of bST/wk, respectively. Both STR and bST influenced (P.05) plasma amino acid profiles. Indicators of carcass fatness were decreased linearly (P.05) by bST; STR implant tended to decrease carcass fatness and increase longissimus muscle area, which was related to carcass weight. The anabolic effects of STR and bST were found to be additive and possibly independent in feedlot steers.