The objectives of this study were to investigate the effects of 2 readily fermentable carbohydrate (RFC) sources in a urea-based liquid supplement on ruminal fermentation characteristics. Four ruminally cannulated Angus cows were fed 11.8 kg of bluegrass straw (BGS), 0.78 kg chopped triticale (TRIT), and 0.56 kg supplement on a DM basis. The experiment was performed in a Latin square design with repeated measures. Treatments were alfalfa hay (CON), a molasses-urea liquid supplement (MOL), a glycerol-urea liquid supplement (GLY) and a mixture of MOL and GLY (50/50). Ruminal fluid samples were collected at 0, 2, 4, 8, 12, 16, 20, and 24 h after feeding to analyze pH, VFA, and ruminal ammonia (NH3-N) concentration. Ruminally incubated samples of BGS and TRIT were removed at 0, 6, 12, 24, 48, and 72 h after feeding to assess digestibility. Ruminal fluid was collected for bacterial speciation. Disappearance of BGS DM, OM and CP were unaffected (P < 0.05). Disappearance of NDF (48.2, 48.5, 50.3 versus 44.2 ± 2.0, respectively) and ADF (47.1, 47.5, 49.0 versus 43.9 ± 3.4 respectively) in BGS increased 4–6% with MOL, GLY, and 50/50 (P = 0.03) compared with CON. There was no treatment effect for digestibility of TRIT (P < 0.05). Total VFA concentration, VFA ratios, and ruminal pH were unaffected (P < 0.05), but MOL, GLY, and 50/50 increased NH3-N concentration (P = 0.007). Average CH4 emissions (192.13 ± 25.6 g/d) and methane yield (g CH4/g gross energy intake; 5.2–5.5%) were unaffected (P < 0.05). Microbial species varied over time (P < 0.01) but not with RFC source. The improved fiber degradation in BGS indicates a GLY-based supplement is as effective as the standard MOL-based liquid supplement for wintering cows.