Two studies were conducted to evaluate the effect of sources of glycerol on in vitro fermentation, and dry matter and neutral detergent fiber digestibilities of corn silage. In the first trial, rumen content was collected from 2 ruminally cannulated male sheep (68.5 kg BW) fed diet composed of 40% corn silage and 60% concentrate (corn, soybean hulls, soybean meal, urea, and minerals). Substrates tested were (1) corn silage, (2) corn silage + 20% crude glycerin (86% glycerol), (3) corn silage + dry glycerin (68% glycerol), and (4) corn silage + pure glycerin (99% glycerol). Substrates (200 mg) and buffered rumen fluid (20 mL McDougall’s buffer and 10 mL rumen fluid) were placed into 60-mL bottles (n = 20), purged with helium gas and sealed. Gas production, pH, ammonia nitrogen (NH₃-N) and DM disappearance were evaluated after 24 h of incubation at 39°C. In the second trial, Ankom Daisy^II fermenter was used to evaluate in vitro digestibility of DM and NDF. Filter bags (n = 25) were filled with substrates, and incubated for 48 h (39°C) into vessels containing a solution composed of 400 mL rumen fluid, 1330 mL buffer A and 266 mL buffer B. After 48-h incubation, 40 mL of 6 N HCl and 8 g of pepsin was added to each digestion vessel, and incubated for another 24 h. Substrates and residues were evaluated for DM and NDF content. All the procedures were replicated for both trials. There was no effect of treatments on 24-h pH (6.1) and NH₃-N (38.4 mg/dL), nor for production of total gas (51.2 mL), CH₄ (8.9 mL), and CO₂ (39.7 mL). When DM disappearance. When the dry matter disappearance was taken into account, corn silage produced more gas (total gas) and more CO₂ than glycerol treatments (P = 0.01 and P = 0.003, respectively), and no difference was observed among treatments for CH₄, and among glycerol treatments for CO₂. Glycerol, regardless the source, increased IVDMD of corn silage (P < 0.0001), and no changes were observed among glycerol treatments (71.7%). IVDMD was unaffected by treatments (41.7%). All sources of glycerol (crude, dry or pure) do not alter neither rumen fermentation nor in vitro digestibility, when inoculated at 20% with corn silage, thus any of these sources are suitable to improve IVDMD of corn silage.