Effects of Ractopamine (Optaflexx) Fed in Combination with Melengestrol Acetate on Feedlot Heifer Performance1

Two commercial feedlot experiments were conducted to determine the effects of feeding melengestrol acetate (MGA) or MGA plus ractopamine (MGA+OPT) on the performance and carcass characteristics of finishing heifers. In Nebraska (Exp. 1), 1,807 heifers (337.3 ± 20.0 kg) and in Texas (Exp. 2), 1,964 heifers (331.5 ± 6.1 kg) were fed 0.4 mg of MGA daily. For heifers fed MGA+OPT, 200 mg of ractopamine was fed daily the last 29 (Exp. 2) or 36 d (Exp. 1). Live and carcass-adjusted performance data were collected. On a carcass-adjusted basis, G:F for the entire feeding period was improved (P < 0.01) by 1.7 and 3.7% in Exp. 1 and 2, respectively, for heifers fed MGA+OPT compared with MGA. For the last 29 to 36 d, G:F was increased (P < 0.02) by 8.1% (Exp. 1) or 27.2% (Exp. 2) on a carcass-adjusted basis for heifers fed MGA+OPT compared with MGA. Fat thickness, USDA YG, marbling score, LM area, and percentage USDA Choice were not different (P > 0.47) between treatments in Exp. 1. Carcasses from heifers fed MGA+OPT had decreased marbling scores (P = 0.01) and greater LM area (P = 0.01) than carcasses from heifers fed MGA in Exp. 2. In Exp. 1, in which G:F was improved by 8.1%, no effect on QG was observed. In Exp. 2, in which G:F was improved by 27.2%, QG decreased. Based on these results, feeding MGA+OPT increased ADG and improved G:F, with variable effects on carcass characteristics.