Nutritional studies involving ruminants have traditionally relied on relatively short transition or wash-out periods between dietary treatments, typically two to four weeks. However, little is known about adequate adaptation periods required to reach stabilization of the rumen microbiome that could provide more accurate results from nutritional studies in ruminants. This study determined the rumen bacterial communities and rumen environment parameters over ten weeks following transition from a forage-based to concentrate-based diet. Several α-diversity metrics, including observed OTUs and Simpson's Evenness fluctuated throughout the trial, but were typically either greatest (observed OTUs) or lowest (Simpson's) at week 5 of the trial contrasted from weeks 1 and 10 (P < 0.05). At week 4, several orders associated with the shift to the final bacterial community composition, including Pasteurellales, Aeromonadales, and Bacteroidales. At week 5, rumen pH was correlated with α-diversity (P = 0.005) and predictive of the rumen microbiome signature at week 10 (R2 = 0.48; P = 0.04). Rumen microbiome stability did not occur until approximately 9 weeks following adaptation to the diet and was associated with changes in specific bacterial populations and rumen environment. The results of this study suggest that adaptation and wash-out periods must be re-evaluated in order to accommodate necessary rumen microbiome acclimation.