Ruminally and duodenally cannulated steers (n = 7; 363 kg) were utilized in a 7 × 4 Latin rectangle to determine the effects of crude glycerin (GLY) on total-tract digestibility, rate and extent of fiber digestibility an rumen fermentation parameters. Crude glycerin replaced soybean hulls (SH) at 0, 4, 8, or 12% of diet DM. Basal diets consisted of 50% wheat straw (WS), SH, 4% supplement, and soybean meal, to maintain a consistent CP level. Fecal samples, rumen and duodenal fluid were collected 1 h before feeding and at 2, 5, and 8 h post feeding. In situ bags containing ground WS or SH were incubated in the rumen for 0, 6, 12, 16, 24, 48, and 96 h to determine NDF digestion rates. Both DM intake and OM intake, decreased quadratically (P = 0.04) as GLY increased in the diet, with lowest DM intake and OM intake at 4% GLY inclusion. As GLY inclusion increased from 0 to 12% diet DM, NDF intake linearly decreased (P < 0.01) (5.86 and 5.27kg, respectively). Glycerin inclusion level had no effect on total-tract DM or OM digestibility (P ≥ 0.66). As GLY inclusion increased from 0 to 12%, total-tract NDF digestibility decreased linearly (P = 0.02) from 64.78% to 53.76%, respectively. The inclusion of GLY had no effect on in situ rate of NDF digestibility (P ≥ 0.27) for WS or SH. Molar acetate proportion decreased linearly (P < 0.01) as time post-feeding progressed as well as GLY increased. Molar propionate concentrations increased (P < 0.01) as GLY inclusion increased and time to feeding increased from −1 to 8 h. Butyrate concentration increased (P < 0.01) at 2, 5, and 8 h post feeding for 8 and 12% GLY, while concentrations were unchanged for 0 and 4% GLY. Subsequently, acetate to propionate ratio decreased (P < 0.01) as GLY level increased from 0 to 12% and time from feeding increased from −1 to 8 h. Although the inclusion of GLY in forage-based diets has a negative impact on NDF digestibility, a decrease in acetate to propionate ratio could result in improved G:F.