The rumen microbial community in cattle associated with inclusion of dietary nitrate and high starch diets
Background: Methane represents 16% of total anthropogenic greenhouse gas emissions. It has been estimated that ruminant livestock produce ca. 29% of this methane. There is good evidence that inclusion of nitrate in the diet can reduce methane emissions from cattle but responses are variable and diet-dependant. Here we investigated changes in the rumen microbial community to inclusion of nitrate in the diet when medium and high concentrate basal diets were fed. Results: Charolais or Luing beef cattle  were fed high (92%) or medium (48%) concentrate diets with or without 2.2% nitrate (all on a dry matter (DM) basis). 16S rRNA gene abundances indicated that although within basal diet there were fed differences in the microbiome in the response to nitrate, there was an association between overall community structure (as measured by principal components) and methane emissions for the medium but not the high concentrate diet. A novel finding was the high abundance of a specific OTU, OTU 1 belonging to the family Succinivibrionaceae in the high concentrate diet. As all cattle were fed the same medium concentrate diet prior to the experiment, this abundance of OTU 1 was interpreted as an adaptation to high starch feeding for the high-concentrate diet. Of particular interest was that there were two different sub-populations present in the high-concentrate diet. Again, the most notable difference was the substantially greater abundance of OTU 1 in a sub-population which was associated with lower methane emissions. This sub-population with high OTU 1 was also associated with superior animal performance and contained substantially greater proportions of cattle of the Charolais breed than of the Luing breed.