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 imp lant (STR; estradiol benzoate + progesterone [three lighter blocks rei mplanted on d 84] and trenbolone acetate [reimplanted on d 63]), and e ither 0, 80, or 160 mg/wk of recombinant bovine somatotropin (bST). St eers 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 insulinlike growth factor I (IGF-I), and plasma amino acid assay. Few interactions were noted (P > .1). Gai n was increased by both treatments: 1.30 vs 1.66 kg/d for control vs S TR (P < .001) and 1.44, 1.49, and 1.51 kg/d (linear, P = .07) for 0, 8 0, and 160 mg of bST/wk, respectively. Gain efficiency was also improv ed: 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 (lin ear, P < .001). Somatotropin decreased mean serum ST compared with con trols; STR increased ST 36% compared with controls. Average plasma IGF -I was increased (P < .001) 12% by STR and 13 and 19% (linear, P < .00 1) by 80 and 160 mg of bST/wk, respectively. Both STR and bST influenc ed (P < .05) plasma amino acid profiles. Indicators of carcass fatness were decreased linearly (P < .05) by bST; STR implant tended to decre ase carcass fatness and increase longissimus muscle area, which was re lated to carcass weight. The anabolic effects of STR and bST were foun d to be additive and possibly independent in feedlot steers.