Alpine herbs affect rumen in vitro degradability and methane emissions

Grazing ruminants intake variable amounts of medicinal plants when fed on mountain pasture. These plants contain secondary metabolites (tannins, phenols, essential oils, saponins) that have shown an inhibitory effect on rumen methanogenesis. Few studies were conducted to evaluate the effect of Alpine herbs on rumen fermentations, and their potential use as feed to mitigate rumen methane emissions. This study aimed to evaluate the effect of 6 Alpine herbs on rumen fermentations and methane (CH4) emissions. Six Alpine herbs (Alchemilla vulgaris L., Sanguisorba officinalis L., Tanacetum vulgare L., Cicerbita alpina L. Wallr., Galium odoratum L. Scop., Sisymbrium officinale L. Scop.) and a grass hay (control) were incubated using an in vitro automated system in 3 consecutive runs, in 3 replicates and using as inoculum the rumen fluid of 3 dairy cows, for a total of 189 batch fermentations and 27 blanks. The kinetic of gas production (GP) was measured and, after 24h, samples of liquid and of gas were collected from each bottle. A mixed model was implemented in R to evaluate the fixed effects of 7 treatments on dry matter degradability (DMd), volatile fatty acid (VFA), GP at 24h (mL/g DM) and gas composition (H2, CO2 and CH4). CH4 was expressed as proportion of gas produced (% v/v), per g of DM incubated (mL/g DM) and per g of DMd, and was also computed from VFA profile. Orthogonal contrasts (P<0.05) were built for all the traits investigated, comparing the effect of each Alpine herb to the control test. The Alpine herbs showed a lower DMd than the grass hay (P<0.001) and produced a lower amount of VFA (P<0.001) and of GP (P<0.001) but, when data were expressed as g of DMd, no differences were observed among herbs and grass hay. All the tested herbs changed the VFA profile increasing the acetic acid proportions and reducing those of propionic and butyric acid (P<0.001), increasing the CH4 emission computed from VFA. Some herbs slightly reduced (P<0.01) the proportions (% v/v) and the amounts (mL/g DM) of CH4 emitted compared to grass hay. However, when data were expressed per g of DMd, no differences were observed between substrates. In conclusion, the tested Alpine herbs showed a lower degradability than the grass hay and it was related to the higher fibre and lignin content. These herbs also showed a lower emission of CH4 than predicted from the VFA profile suggesting a potential role of plant secondary metabolites in the modulation of fermentations.