A dual-flow continuous culture fermentor system was used to assess the effect of supplementing 2 levels (5 or 10% of diet DM) of starchy (barley: BAR) or fibrous (beet pulp: BP) carbohydrate (CHO) to an orchardgrass diet on nutrient digestibility, VFA production, bacterial protein synthesis, and methane output. Treatments were randomly assigned to fermentors in a 4 × 4 Latin square design with a 2 × 2 factorial arrangement using 7 d for diet adaptation and 3 d for sample collection. Treatments included: 1) 57 g DM herbage + 3 g DM BAR; 2) 54 g DM herbage + 6 g DM BAR; 3) 57 g DM herbage + 3 g DM BP; 4) 54 g DM herbage + 6 g DM BP. Feeding and pH sampling occurred at 0730, 1030, 1400 and 1900 h. Gas samples for methane analysis were collected at 0725, 0900, 1000, 1355, 1530, and 1630 h. Effluent samples were analyzed for OM, CP, NDF, nutrient digestibilities, estimation of bacterial protein synthesis, ammonia-N and VFA. Data were analyzed using the MIXED procedure of SAS with period and treatment as fixed effects and fermentor as random. Orthogonal contrasts were tested for CHO type and level. No significant interactions were detected. Apparent and true OM digestibilities were not affected (P > 0.10) by CHO source (72.4 and 81.9%, respectively). True CP digestibility was greater (P < 0.05) for BP (75.3%) than BAR (52.5%) diets. Apparent NDF digestibility was lower (P < 0.05) for BP (79.5%) than BAR (85.1%) diets. Barley diets produced lower (P < 0.05) molar proportions of acetate (43.5 vs. 49.4 mol/100 mol, respectively), lower concentrations of total VFA (67.2 vs. 72.2 mmol/L, respectively) and tended (P = 0.08) to have greater mean pH (6.75 vs. 6.72) compared with BP diets. Methane production was not affected (P > 0.10) by CHO source. The 10% supplement produced greater (P < 0.05) concentrations of methane (35 vs. 27 mmol/d) and tended (P = 0.07) to increase apparent DM digestibility. Diet had no effect on bacterial efficiency or ammonia-N. Supplementation of an herbage-based diet with BP improved CP digestibility compared with barley but did not affect OM digestibility, methane production, or microbial efficiency.