The majority of beef cattle production in northern Australia is extensively grazed on unimproved pastures. In the Northern Territory cows which are surplus to breeding requirements (culled cows) are sold and the revenue generated can have a large influence on the profitability of the beef enterprise. Shifting healthy but underweight culled cows to graze on floodplain pastures to improve their growth performance and carcase characteristics has been identified as having the potential to increase the profitability of culled cows. The effect of shifting cows grazing native pastures to grazing floodplains on the rumen microbiome was investigated in 41 cows sourced from either commercial (COM) properties (32 cows) or Beatrice Hill Research Station (BHRS) (9 cows). Rumen fluid samples obtained from animals at induction to the floodplain (Day 0), and again at Days 34 and 137 were used for microbial diversity profiling (barcoded V3-V4 16S rRNA gene amplicon). Sequencing was done using the Illumina MiSeq platform and the sequence data was analysed using the QIIME 1.9 software package. Four alpha diversity measures (Chao1, Shannon, Observed species and Phylogenetic diversity whole tree) were significantly different between COM and BHRS cows at Day 0 with reduced diversity in COM cows. However by Day 34 the measures showed the number of observed microbial species for the COM cows had reached similar levels to the BHRF cows. Following a further 103 days on the floodplain, the extent of microbial diversity remained similar across the herd. The taxonomic composition of the rumen microbiomes was significantly different by Day 137 to the composition at Day 0 indicating that diet is one of the primary drivers in determining the relative taxonomic composition of the rumen, even in a relatively uncontrolled, extensively grazed feeding system.