Alpine pasture herbs redirected hydrogen towards alternative sinks, inhibiting methane production: in vitro study

The impact of six alpine herbs (Alchemilla vulgaris L., Sanguisorba officinalis L., Tanacetum vulgare L., Cicerbita alpina (L.) Wallr., Galium odoratum (L.) Scop., and Sisymbrium officinale (L.) Scop.), was compared with grass hay on rumen degradability and fermentation parameters. The seven plants were fermented using an automatic in vitro system to evaluate the kinetics of gas production (GP), degraded dry matter (dDM) and fermentation end products [volatile fatty acids (VFA), carbon dioxide, methane, and hydrogen]. Gas and methane productions were also computed from VFA using specific stoichiometric relationships. The partitioning factor (PF: ratio between dDM and GP) was calculated as an index of microbial growth. Compared to grass hay, the alpine herbs exhibited lower degradability (on average −12.8%) due to their high lignified fibre content. The alpine herbs also increase the PF (+9.1%), suggesting a reduction in microbial growth efficiency, and altered the VFA profile by increasing the proportion of acetic acid (+9.9%) at the expense of propionic (−11.9%) and n-butyric acids (−19.4%). Stoichiometric relationships typically associate these variations with an increase in methane proportion. However, this was not observed; in fact, Sanguisorba officinalis L. (−15.0%) and Galium odoratum L. Scop. (-13.9%) reduced methane production. The discrepancy between the measured and expected methane production indicates that part of the hydrogen, not used for methane synthesis, was redirected to alternative sinks such as reductive acetogenesis. This change in the fermentation profile appears to be modulated by bioactive compounds present in the medicinal herbs, which are potentially found in grazing pastures.