Ractopamine hydrochloride and estradiol-trenbolone acetate implants alter live performance and carcass components of heifers during the finishing phase 1 1 Supported in part by funding from Elanco Animal Health, Greenfield, Indiana, and the Gordon W. Davis Regent's Chair in Meat and Muscle Biology Endowment at Texas Tech University, Lubbock

The objectives were to evaluate the interaction of ractopamine hydrochloride (Optaflexx, OPT) and terminal implant (TI) on growth performance, carcass characteristics, and meat quality of finishing beef heifers. A 3 × 2 factorial randomized complete block design was used with 2 levels of OPT and 3 durations of TI. British × Continental heifers (n = 216; 341.6 kg) were blocked by BW and randomly allotted to 54 pens (9 pens per treatment; 6 pens per block; 4 heifers per pen). The main treatment effects were implant [TE-200 with Tylan (tylosin phosphate; 200mg of trenbolone acetate + 24mg of estradiol-17[beta]) administered 140, 100, or 60 d from slaughter] and OPT (0 or 200mg per head per day). No implant × OPT interactions were detected, so only main effects are reported. Average daily gain (0.14kg difference), predicted carcass ADG (0.24kg difference), and HCW (5.6kg difference) were increased (P < 0.05) by OPT, but DMI was not affected. Supplementation with OPT decreased (P < 0.05) Prime and Choice carcasses by 16.5%. No differences (P > 0.10) in Warner-Bratzler shear force were detected at d 3, 7, and 21 aging postmortem, although the shear force values of the OPT steaks at d 14 were higher (0.45 kg; P < 0.05). Results from this experiment demonstrated that OPT, when fed to heifers, increased ADG, calculated carcass ADG, and HCW with minimal effect on carcass quality. Furthermore, this experiment indicated that the duration of the TI window did not affect overall performance, final BW, or carcass quality.