Assessment of Rumen Processes by Selected-ion-Flow-Tube Mass Spectrometric Analysis of Rumen Gases

摘要

This work investigated the potential to use measure- ment of the concentration of certain gases in the rumen headspace to gain information about rumen processes and as a potential diagnostic tool. We used new equip- ment (selected-ion-flow-tube mass spectrometer) that allows rapid and precise analysis of 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 fistu- lae, at intervals after receiving their morning feed allo- cation (grass silage and concentrates). Hydrogen su1- fide, methyl sulfide, and dimethy1 sulfide, were the pre- dominant gases that were measured in the rumen headspace by this technique. The concentrations of these sulfur compounds declined over the interval after feeding, mirroring ammonia concentrations measured in rumen liquor, reflecting their common dependence on the fermentation of sulfur amino acids. Ammonia concentrations in rumen headspace gas varied in the opposite direction to the concentration of ammonia in rumen liquor and likely depend more on the pH of ru- men liquor. Consideration of the pKa of ammonia sug- gests that ammonia concentrations in rumen gas wil1 be very low below pH 6, representing a useful diagnostic for subaeute ruminal acidosis. Low concentrations of volatile fatty acids were detected in rumen gas. The molar proportions of volatile fatty acids were similar in gas and liquor samples, with rumen gas containing slightly less acetic acid and disproportionately more valeric and caproic acids.