This study investigated the effects of adding a blend of essential oils, tannins, bioflavonoids (ANAVRIN®, VetosEurope, Lugano, Switzerland), monensin, and their combination on the daily gain, feed conversion, and rumen fermentation of finishing steers fed a total mixed ration (TMR). Thirty finishing steers (441 kg body weight, 34.3 months old) consuming a TMR were blocked by BW and randomly assigned to three treatments according to the addition of (1) EOTB (ANAVRIN® at 0.35 g/100 kg body weight), (2) MON (monensin at 0.033 g/kg dry matter), and (3) EOTB + MON (at the same dose as in (1) and (2). A 60-day feeding trial was conducted, including a 19-day adaptation period. Data collected included daily dry matter intake (DMI), average daily gain (ADG), feed conversion ratio (FCR), and rumen fermentation parameters (pH, NH3-N, and volatile fatty acid (VFA) concentrations). EOTB resulted in higher DMI compared to EOTB + MON, while no significant differences were observed in ADG or FCR. The ruminal pH and NH3-N concentrations were similar among treatments, but there were interactions among treatment × hour for VFAs. The EOTB + MON led to a more stable VFA concentration in rumen suggesting possible complementary supplementation effects. However, more studies are needed to confirm this and to go deeper into the underlying mechanism of action.