Metabolomics reveals unhealthy alterations in rumen metabolism with increased proportion of cereal grain in the diet of dairy cows

This study presents the first application of metabolomics to evaluate changes in rumen metabolites of dairy cows fed increasing proportions of barley grain (i.e., 0, 15, 30, and 45% of diet dry matter). 1H-NMR spectroscopy was used to analyze rumen fluid samples representing 4 different diets. Results showed that for cows fed 30 and 45% grain, increases were observed in the concentration of rumen methylamine as well as glucose, alanine, maltose, propionate, uracil, valerate, xanthine, ethanol, and phenylacetate. These studies also revealed lower rumen 3-phenylpropionate in cows fed greater amounts of cereal grain. Furthermore, ANOVA tests showed noteworthy increases in rumen concentrations of N-nitrosodimethylamine, dimethylamine, lysine, leucine, phenylacetylglycine, nicotinate, glycerol, fumarate, butyrate, and valine with an enriched grain diet. Using principal component analysis it was also found that each of the 4 diets could be distinguished on the basis of the measured rumen metabolites. The two closest clusters corresponded to the 0 and 15% grain diets, whereas the 45% barley grain diet was significantly separated from the other clusters. Unhealthly levels of a number of potentially toxic metabolites were found in the rumen of cattle fed 30 and 45% grain diets. These results may have a number of implications regarding the influence of grain on the overall health of dairy cows.