The aim of this study was to evaluate 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 microbiology. 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. Experimental periods were 15 d (14 d for adaptation and 1 d for ruminal sampling). Ruminal samples were taken 3 h after feeding. Protozoa population were preserved in formalin and quantified in Sedgewick-Rafter chamber. The technique to quantify total bacteria was qPCR. Data were analyzed using R software (version 3.1.1) as a triple Latin Square design with 3 treatments and 3 animals in 3 simultaneous triplicates. The fixed effects were treatments and Latin Square, and random effects were time, animal and error. The statistical test used was Tukey, and the significance was P < 0.05. The protozoa population was not influenced by the different sources of forage with crude glycerin, except Dasytricha and Isotricha. The concentration of Dasytricha population was increased in animals fed with sugar cane which not differed from animals fed with corn silage diet (P > 0.05). Additionally, Isotricha population concentration increased (P < 0.05) in animals fed with sugar cane. The population of fibrolytic bacteria (Ruminococus flavefaciens, Ruminococcus albus and Fibrobacter succinogenes) were similar (P > 0.05) among diets. On the other hand, the population of Selenomonas ruminantium increased (P < 0.01) in animals fed with corn silage. Corn silage and sugar cane included in 15% of fNDF in diets with crude glycerin (10% DM) altered ruminal microorganisms populations.