Animal Science Department


Date of this Version



Professional Animal Scientist 2009 25:33-40


Copyright TBC American Registry of Professional Animal Scientists


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.