DOSE-RESPONSE EFFECTS OF RECOMBINANT BOVINE SOMATOTROPIN IMPLANTS ON FEEDLOT PERFORMANCE IN STEERS

One hundred twenty crossbred beef steers averaging 377 kg were used in a 2 x 4 factorial experiment to determine the dose-response effects of recombinant bovine somatotropin [rbST] implants on growth performance and carcass characteristics. The implant dosages were 0 [sham], 40, 80, or 160 mg/wk administered s.c. in the tailhead region on a weekly basis throughout the experiment. The steers were fed a high-concentrate diet, which contained either a degradable [DP; soybean meal] or an escape [EP; corn gluten and blood meal] protein source. No dietary protein effect or dietary protein x rbST level interactions were detected. Recombinant bST decreased both DMI [P < .10] and feed/gain [P < .05] in a linear dose-dependent manner. Dosage of rbST did not significantly affect [P > .10] ADG or final weight of the steers. Recombinant bST decreased backfat depth [P < .10], marbling score [P < .05], and quality grade [P < . 10] and increased yield grade [P < .10] in a linear dose-dependent manner. Soft tissue composition of the 9-10-11 th rib section was altered [P < .01] by rbST administration in a linear dose-dependent manner. The percentage of protein in the rib section was increased by 9.4 % and fat was decreased by 11.8 % at the 160 mg/wk rbST level compared with the sham-implanted steers. Recombinant bST did not affect [P > .10] dressing percentage, hot carcass weight, kidney, pelvic, and heart fat, or longissimus muscle area. Serum insulin-like growth factor I [IGF-I] concentrations in the steers exhibited a linear response to dosage of rbST [P < .01]. These data indicate that rbST is an efficacious method of improving feedlot performance and partitioning nutrient deposition in feedlot steers.