Corn stover is a low-quality forage rich in lignocellulosic material. However, it is also a palatable and low-cost feedstuff for cattle. In this study, we hypothesized that feeding exclusively a corn stover-based diet (CSD), compared with one based on corn silage-oat hay, increases the abundance of the bacterial families and genera involved in cellulose and hemicellulose metabolism. For that purpose, we collected filtered ruminal fluid from six dry Holstein cows during two periods: Period 1, cows consumed a control diet based on corn silage and oat hay (CD), and in the Period 2, cows were fed CSD for 45 days. Ruminal fluid was collected through esophageal tubing. Ruminal microorganisms were identified by sequencing the 16S rRNA gene using the Illumina MiSeq platform. Compared with CD, feeding CSD for 45 days increased bacterial families and genera associated with higher neutral detergent fiber content and esterase and hemicellulolytic activities, such as Rikenellaceae, Prevotella, and Pseudobutyrivibrio. These results indicate that the liquid fraction of ruminal digesta contains a large number of microorganisms that help degrade lignocellulosic complexes, especially when diets with low-quality forages such as CSD are fed.