AimsRecent studies have demonstrated RAMP((R)), a complete starter feed, to have beneficial effects for animal performance. However, how RAMP may elicit such responses is unknown. To understand if RAMP adaptation results in changes in the rumen bacterial community that can potentially affect animal performance, we investigated the dynamics of rumen bacterial community composition in corn-adapted and RAMP-adapted cattle. Methods and ResultsDuring gradual acclimation of the rumen bacterial communities, we compared the bacterial community dynamics in corn and RAMP-adapted using 16S rRNA gene amplicon sequencing. Significant shifts in bacterial populations across diets were identified. The shift in corn-adapted animals occurred between adaptation step3 and step4, whereas in RAMP-adapted cattle, the shift occurred between step2 and step3. As the adaptation program progressed, the abundance of OTUs associated with family Prevotellaceae and S24-7 changed in corn-adapted animals. In RAMP-adapted animals, OTUs belonging to family Ruminococcaceae and Lachnospiraceae changed in abundance. ConclusionsRumen bacteria can be acclimated faster to high concentrate diets, such as RAMP, than traditional adaptation programs and the speed of bacterial community acclimation depends on substrate composition. Significance and Impact of the StudyThese findings may have implications for beef producers to reduce feedlot costs, as less time adapting animals would result in lower feed costs. However, animal feeding behavior patterns and other factors must be considered.