The effect of using steam-flaked and extruded corn on in vitro fermentation of total mixed rations (TMR) was studied. Six dietary treatments were used: ration 1 (control) - corn grain at 20% in a TMR, ration 2 – stem-flaked corn at 20% in a TMR, ration 3 – extruded corn at 20% in a TMR, ration 4 – corn grain at 10% + steam-flaked corn at 10% in a TMR, ration 5 – corn grain at 10% + extruded corn at 10% in a TMR and ration 6 – steam flaked corn at 10% + extruded corn at 10% in a TMR. The TMR used in the in vitro evaluation was made up of the corn and or processed corn (20%) alfalfa hay (40%), beet sugar pulp (20%), soybean meal (10%) and barley grain (20%). A 200 mg sample of each ration was weighed and placed in 100 ml glass syringes. Each sample was then inoculated with artificial saliva and filtered rumen liquor (2:1 ratio), then incubated at 39°C in a ventilated oven. Gas production (GP) was recorded at 2, 4, 6, 8, 12, 24, 48, 72 and 96 hours. At 96 hours, 10 ml of supernatant from each sample was taken for the determination of volatile fatty acids (VFA) concentration. The total in vitro gas and VFA produced were corrected for the blank. Production of acetate, propionate, valerate and isobutyrate was different (P<0.05) between treatments because the organic matter digestibility (OMD) was different as well and in treatment 2 the OMD was 80.41%. There was a significant difference between treatment 1 and other treatments including processed corn (P<0.05) for propionate concentration, while control treatment had the highest concentration of propionate (26.32 mol/100mol). Acetate to propionate ratio was significantly different between treatments (P<0.05). Moreover, treatment 1 had the lowest acetate to propionate ratio (1.71) in comparison to treatment 3, 5 and 6. Based on the results of this study; steam flaking and or extruding corn can potentially alter the VFA production in the rumen. Using processed corn in place of corn grain in TMR might alter rumen fermentation and probably increase the digestibility of feeds in dairy cows.