The objectives of this study were to assess the magnitude of differences among barley silages with different in vitro neutral detergent fiber digestibility (ivNDFD) in comparison with corn silage in (1) predicted carbohydrate digestibility, (2) rumen fermentation characteristics, and (3) microbial protein synthesis using rumen simulation technique (RUSITEC). The experiment was carried out in a completely randomized block design with four treatments. The four whole-plant silages utilized in this study were CS-TMR = corn silage (30 h ivNDFD = 32%), HNDFD-TMR = barley silage with high ivNDFD (30 h ivNDFD = 37%), INDFD-TMR = barley silage with intermediate ivNDFD (30 h ivNDFD = 28%), and LNDFD-TMR = barley silage with low ivNDFD (30 h ivNDFD = 26%). Results from RUSITEC showed that nutrient disappearance, rumen fermentation characteristics, and microbial protein synthesis did not differ among diets that contained different varieties of barley silage (P > 0.1). However, CS-TMR tended to have a higher microbial protein yield than all barley silage diets (P = 0.06). These results show higher ivNDFD of barley silage may not necessarily correspond with greater impact on rumen fermentation and microbial protein synthesis. However, feeding the corn silage had higher microbial protein synthesis in the RUSITEC and might enhance the dairy cattle performance compared with barley silage.