The effects of ractopamine-hydrogen chloride (Optaflexx) on performance, carcass characteristics, and meat quality of finishing feedlot heifers.

Two experiments were conducted at the Kansas State University Beef Cattle Research Center to determine the effects of ractopamine-HCl (Optaflexx) on growth performance, carcass characteristics, and meat quality of finishing feedlot heifers. In Exp. 1, heifers implanted with Revalor-H (n = 302, initial BW = 479 kg) were fed steam-flaked corn diets with 0 (control) or 200 mg of ractopamine-HCl (OPT) per heifer daily for 28 d before slaughter. Average daily gain and DMI were not different between treatments (P > 0.17); however, OPT cattle tended to have a greater G:F (P = 0.06). Treatments did not differ with respect to final BW, HCW, dressing percentage, USDA yield grade, USDA quality grade, marbling score, LM area, KPH, Warner-Bratzler shear force, weight loss during cooking, or L*, a*, or b* colorimetric values during a 7-d retail display or purge loss from loin steaks during retail display (P > 0.19). In Exp. 2, nonimplanted crossbred heifers (n = 281, BW = 451 +/- 2 kg) were fed finishing diets based on steam-flaked corn. A control diet (no ractopamine) was compared with diets providing 200 mg of OPT per heifer daily for periods of 28 or 42 d (200 x 28 and 200 x 42, respectively), 300 mg/d for 28 d (300 x 28), and a step-up regimen consisting of 14 d at 100 mg, followed by 14 d at 200 mg, and the final 14 d at 300 mg of OPT (step-up). Feeding OPT had no effect on carcass weight gain among treatments (P = 0.18). The efficiency of carcass gain was 34 and 35% greater (P = 0.06) for the 200 x 42 and step-up groups compared with control, respectively. Feeding OPT at 300 mg for 28 d reduced DMI compared with the control, 200 x 28, and 200 x 42 (P < 0.05) groups. Administration of OPT had no effect on marbling score, yield grade, LM area, KPH, or percentages of carcasses grading USDA Choice (P > 0.10). Feeding ractopamine-HCl (Optaflexx) to finishing heifers generally improved the efficiency of carcass gain with minimal effect on carcass characteristics. These effects were most pronounced in heifers fed ractopamine for 42 d.