Sampling procedure for the measurement of dissolved hydrogen and volatile fatty acids in the rumen of dairy cows

Dissolved hydrogen (dH2) influences the pathways of VFA production and is a precursor of methane formation in the rumen. Measurements of dH2 in rumen fluid taken at the same time as measuring other rumen fermentation end products would improve our quantitative understanding of the role of dH2 as a controller of rumen fermentation. Sample collections though a rumen cannula and using oral stomach tubing were compared for measurements of dissolved gases and fermentation end products in the rumen fluid of 4 ruminally cannulated dairy cows fed a total mixed ration of corn silage and concentrate. Rumen fluid was collected at 0, 2.5, and 6 h after morning feeding through the cannula from cranial dorsal rumen, cranial ventral rumen, central rumen, caudal dorsal rumen, and caudal ventral rumen and in parallel by oral stomach tubing at 2 insertion depths of 180 cm (sampling the central rumen) and 200 cm (sampling the caudal dorsal rumen). The cranial dorsal rumen had the greatest pH and smallest VFA concentration among 5 sites sampled. Samples collected by oral stomach tubing had greater (P < 0.001) rumen pH and less (P < 0.001) dissolved methane (dCH4) and lower VFA concentration than that collected through rumen cannula. The dH2 concentrations were positively correlated (r > 0.8) in rumen samples collected by the 2 sampling techniques, with a concordance correlation coefficient larger than 0.8 and scale shift being about 0.1 away from unity. The variations in the measurement of dH2, dCH4, pH, and VFA in samples collected by oral stomach tubing are most likely the result of saliva contamination. The time of sampling relative to feeding had significant influence (P < 0.01) on dissolved gases and fermentation end products, with the greatest concentrations of dH2, dCH4, and VFA measured 2.5 h after morning feeding. The dH2 was correlated positively (r > 0.58) with dCH4 and negatively (r < -0.65) with the estimated net H2 production relative to the amount of VFA produced. This indicated that greater dH2 enhanced rumen CH4 production and also led to fermentation pathways that produce less H2, such as enhanced propionate and butyrate production. In summary, oral stomach tubing could be a feasible method to measure ruminal dH2 in intact animals, but caution should be taken to minimize saliva contamination. Measurements made using both techniques yield similar conclusions for the effects of dH2 on fermentation pathways and CH4 generation.