Assessment of rumen processes by selected-lon-flow-tube mass spectrometricanalysis of rumen gases

This work investigated the potential to use measurement of the concentratio n of certain gases in the rumen headspace to gain information about rumen p rocesses and as a potential diagnostic tool. We used new equipment (selecte d-ion-flow-tube mass spectrometer) that allows rapid and precise analysis o f many of the gases present in a sample. Samples of rumen headspace gas and corresponding samples of rumen liquor were taken from three lactating cows , prepared with rumen fistulae, at intervals after receiving their morning feed allocation (grass silage and concentrates). Hydrogen sulfide, methyl s ulfide, and dimethyl sulfide, were the predominant gases that were measured in the rumen headspace by this technique. The concentrations of these sulf ur compounds declined over the interval after feeding, mirroring ammonia co ncentrations measured in rumen liquor, reflecting their common dependence o n the fermentation of sulfur amino acids. Ammonia concentrations in rumen h eadspace gas varied in the opposite direction to the concentration of ammon ia in rumen liquor and likely depend more on the pH of rumen liquor. Consid eration of the pK(a) of ammonia suggests that ammonia concentrations in rum en gas will be very low below pH 6, representing a useful diagnostic for su bacute ruminal acidosis. Low concentrations of volatile fatty acids were de tected in rumen gas. The molar proportions of volatile fatty acids were sim ilar in gas and liquor samples, with rumen gas containing slightly less ace tic acid and disproportionately more valeric and caproic acids.