This study investigated the effect of alternative forages for corn silage such as sugar cane and sugar cane bagasse included in a similar forage NDF level (fNDF) in diets with crude glycerin (80.64% of glycerol) on ruminal fermentation of Nellore feedlot steers. Nine ruminally cannulated Nellore steers (300.0 ± 30kg of BW and 18 ± 2 mo of age) were used in a 3x3 Latin Square experimental design with 3 treatments and 3 animals in 3 simultaneous triplicates to evaluate the effect of different sources of forage in diets with crude glycerin (80.64% of glycerol) on ruminal pH and ammonia-N concentration. Experimental periods were 15 d (14 d for adaptation and 1 d for ruminal sampling). The treatments were different sources of forage (fixed 15% of NDF from forage; fNDF): corn silage (CS), sugar cane (SC) and sugar cane bagasse (SB), in diets with 10% (DM) of crude glycerin. Ruminal contents were obtained at 0, 2, 4, 6, 8, 10, and 12 h after feeding. Data were analyzed as a triple Latin Square design and repeated measurements on time using the PROC MIXED procedure of SAS. The least squares means were generated and compared (P < 0.05) using Tukey’s test. The pH values of animals fed with SB (6.40) were greater compared with CS (6.08; P < 0.05), which did not differ from animals fed with SC (6.27; P > 0.05). Animals fed with SC presented lower values of NH₃-N (11.54; P < 0.05) compared with animals fed with CS (15.42) and SB (15.40). Sugar cane and sugar cane bagasse included in 15% of fNDF in diets with crude glycerin (10% DM) altered ruminal parameters, however, maintained adequate conditions for animal performance.