RECOMBINANT BOVINE SOMATOTROPIN IMPROVES GROWTH-PERFORMANCE IN FINISHING BEEF STEERS

The objectives of this study were to determine the efficacy of various doses of rbST on ADG and feed efficiency (FE) and to describe carcass composition changes in finishing beef steers. In Exp. 1, 96 crossbred beef steers (393 kg) received daily i.m. injections of buffer or 33, 100, or 300-mu-g/kg of BW of rbST (0ST, 33ST, 100ST, 300ST). In Exp. 2, 200 crossbred beef steers (417 kg) received daily i.m. injections of buffer or 8.25, 16.5, 33, or 66-mu-g/kg of BW of rbST (0ST, 8.25ST, 16.5ST, 33ST, 66ST). Treatments were administered until steer BW per pen averaged 540 kg in Exp. 1 and 560 kg in Exp. 2. An 86% concentrate: 14% roughage diet was fed once daily (CP: 16.5% in Exp. 1, 20.2% in Exp. 2). In Exp. 1, growth performance of steers receiving rbST was dose-dependent; ADG changed linearly (P = .01), DMI decreased linearly (P = .03), and FE changed quadratically (P < .03). The 33ST steers responded with improved ADG and FE, 100ST with improved FE, and 300ST with lower ADG and poorer FE, compared with 0ST. In Exp. 2, the ADG response was quadratic (P = .01), DMI decreased linearly (P = .003), and FE improved quadratically (P = .004) with increasing dose of rbST. Steers receiving 16.5ST and 33ST responded with improved ADG and FE, whereas steers receiving 8.25ST and 66ST responded with improved FE but not ADG relative to 0ST steers. In Exp. 1 and 2, rbST administration altered carcass composition by increasing carcass protein and decreasing carcass fat. Our results indicate that a daily dose between 16.5 and 33-mu-g/kg of BW was required to optimize both ADG and FE in finishing beef steers; however, the dose response for changes in carcass chemical composition had not attained a plateau even at 300-mu-g/kg of BW.