There are variations in how well dairy heifers grow and develop while grazing pasture. The quality and amount of forage that they consume has been shown to account for approximately fifty percent of those variations. We hypothesized that the composition of the rumen microbiome would account for a significant portion of the remaining variation. To test this hypothesis, 288 dairy heifers from four different "breeds" were used. Holsteins, Jerseys, Holstein/Jersey crossbreds, and ProCROSS heifers were grazed on two different pasture types. Our two treatments were grass monoculture pastures and grass interseeded with birdsfoot trefoil. There were also 48 heifers as a control group that consumed a total mixed ration in a feedlot. Rumen fluid was collected at the start of the trial and then after each of three 35 d grazing rotations through the pastures. The rumen fluid was analyzed for ammonia and volatile fatty acid concentrations, and the rumen microbiome was characterized. Overall breed did not have an effect on ruminal ammonia, or any of the three primary VFA (acetate, propionate, or butyrate). However, treatment did have an effect on ammonia, acetate, propionate, and butyrate concentrations in the rumen. There were differences in alpha diversity between some breeds with Holstein/Jersey cross and ProCross having greater alpha diversities than Holsteins and Jerseys. Breed did not have an effect on beta diversity. Both pasture treatments were different for alpha and beta diversities when compared to the TMR control group, but there were no differences between the two pasture types. Using growth data, heifers were ranked from worst to best. The microbiome profiles from the best 20% were compared to the worst 20%. Differences were found for both alpha and beta diversities between groups. There were also differences found for specific phylum, genus, and species between the top 20% and the bottom 20% of heifers, based on growth. It was therefore concluded that the rumen microbiome does have a significant effect on dairy heifer growth/performance and these findings provide an opportunity for dairies to increase the efficiency of raising dairy heifers through the rumen microbiome.