In-vitro experiments were conducted to determine the impact of tannins, saponins, and DirectFed-Microbials (DFM) on rumen fermentation characteristics. Rumen fluid was collected 2h post feeding from three cannulated crossbred (Angus x Hereford) feedlot steers (450±12.2 kg), adjusted to finishing diet. Rumen fluid from all steers were filtered, combined, and mixed with buffer. Vaccine bottles containing ground finishing diet and appropriate treatments were incubated with rumen fluid mixture (39 ?C). Treatments consisted of 1) Control; 2) Commercial Tannin (TCH); 3) Hydrolysable Tannin (HT); 4) saponin; 5) TCH+DFM; 6) HT+DFM; and 7) Saponin+DFM. At 12 h post incubation, HT+DFM and Saponin+DFM had lesser (P<0.05) molar proportions of acetic acid when compared to all other treatments. Saponin+DFM had greater (P<0.05) molar proportions of propionic acid and a lesser (P<0.05) acetate: propionate ratio when compared to treatments not containing DFM. Dry matter digestibility and pH were greater (P<0.05) for saponin, TCH+DFM, HT+DFM and saponin+DFM, whereas pH was greater in treatments containing DFM compared to the controls. Percent CO2 was lesser (P<0.05) in saponin and THC+DFM in vitro vessels compared to controls. At 18 h post fermentation total VFA concentrations, molar proportions of butyric and valeric acid, and gas cap N2 percentage were greater (P<0.05) in in vitro vessels containing HT+DFM compared to controls. The percentage of CO2 was lesser in in vitro vessels containing HT compared to control and in vitro vessels containing saponin, HT+DFM and Saponin+DFM. At 12 h, alpha and beta diversity of the microbial community between treatments differed (P<0.05). Across all samplesat 18 h of fermentation the genera Sharpea and Dialister were present, however Saponin+DFM and TCH+DFM treatments demonstrated the greatest relative abundance. While Sharpea contribute to rumen biohydrogenation, the presence of genus Dialister indicates association with energy harvesting bacteria and suggests that the inclusion of several of these feed additives allowed shifts of certain bacteria to occur.