In vitro rumen fermentation traits of Mentha piperita, and products of oil extraction

By-products obtained from essential oil extraction could be valued as ruminant feeds for their fibre and bioactive compounds content. This study aimed to evaluate the in vitro rumen fermentable characteristics of Mentha piperita (Mp, as dry forage), the residual product achieved from a supercritical CO2 extraction (residue: rMp), and the residual of extraction plus the extract (erMp, to reconstitute Mp). A sample of 1 gram for each of these 3 materials and a grass hay (control) were tested in 3 repeated incubations, using 3 different rumen fluids, for a total of 36 bottles and 9 blanks. DM degradability (DMd), gas production (GP) kinetic and composition, volatile fatty acid (VFA), and bacterial profile over 24h of fermentation were investigated. A linear mixed model was run to evaluate the fermentation characteristics of these materials. Orthogonal contrasts were used to compare the control vs Mp, the Mp vs rMp and the Mp vs erMp. The control and Mp in DMd did not differ for total VFA and total GP. However, Mp decreased n-butyric acid and slightly increased acetic acid proportions, increased the GP release between 3 and 9 h, and reduced CH4 (% v/v) emissions compared to the control. The fermentation parameters of rMp did not differ from those of Mp, except for an increase of GP between 15 and 24 h, and a slightly increase of CH4 (% v/v) at 24 h. Also, erMp did not affected the fermentative parameters compared to Mp, except for an increase of GP rate in the first 3 h of incubation, a slight increase of branched VFA, and an increase of CH4 proportions. The Mp caused a modification of the microbial profile in the fermentation fluid with a reduction of the diversity indexes compared to the control. The Mp degradability was similar to that of the grass hay, notwithstanding the signs of some changes in the fermentation pattern and in the rumen microbial population. The extraction of essential oil had only a limited effect on fermentative properties of mint suggesting a potential use of this residual by-product as ruminant’s feed.