A feeding trial using 121 crossbred steers was conducted to evaluate the utilization of untreated mature switchgrass as a roughage source in feedlot diets. Steers were fed either cornstalks (STALK) or switchgrass (SWITCH) as roughage at 14.2% of diet DM. Performance and carcass characteristics were evaluated relative to roughage source. Cattle fed SWITCH had lesser marbling scores but backfat, HCW, KPH, ribeye area, and yield grade did not differ between treatments. Cattle fed SWITCH had lesser DMI than did STALK cattle but carcass-adjusted ADG and G:F did not differ between treatments. Untreated switchgrass may replace cornstalks at low inclusions in finishing diets, and digestibility of this low quality roughage may be improved through alkaline treatment.