Assessment of archaeol as a molecular proxy for methane production in cattle

The objective of this study was to assess archaeol, a membrane lipid present in methanogenic Archaea, in cattle feces as a molecular proxy for methanogenesis in the rumen. Feces from 16 heifers either in early lactation [71d in milk (DIM)] or mid lactation (120DIM), consuming a diet consisting of 30/70 grass silage/concentrates [dry matter (DM) basis], were analyzed for archaeol. To prepare the feces for analysis, total lipids were obtained by Bligh-Dyer extraction, polar head groups were removed by acid methanolysis, an alcohol fraction was obtained by column chromatography, and finally, the alcohol fraction was trimethylsilylated before analysis by gas chromatography-mass spectrometry. Archaeol was quantified by comparison to an internal standard. A highly significant positive relationship was found between fecal archaeol concentration (mg/kg of DM) and methane output (g/kg of DM intake). A highly significant effect of stage of lactation on this relationship was observed. The significant relationship was surprising, given the lack of agreement between methane and total methanogens in previous studies using molecular biology techniques. Variation in the relationship between fecal archaeol concentrations and methane output could be attributed to differences in the methane-producing capability per cell and the selective retention of methanogens in the rumen. The effect of stage of lactation may have been due to differences in DM intake, affecting rumen passage rates and, thus, methanogen populations and activities.