Plant secondary metabolites are known to mitigate enteric methane emissions. They can be found in biodiverse pasture, rich in fobs, which have been shown to reduce enteric methane (CH4) production in vitro. We set up an in vivo experiment to measure methane (CH4) emission, dairy performances and digestive parameters of 28 grazing dairy cows, allocated to a low plant diversity level plot (LD, 17 species, H-index = 209) for a 3 weeks pre-experimental period. Then, cows were split for a 6 weeks experiment in two balanced groups: one stayed on LD plot managed under rotational grazing; the other continuously grazed a plot with a high plant diversity level (HD, 58 species, H-index = 323). Milk yield was 32% lower for HD cows while protein content was greater for LD cows (+ 1.5 g/kg). CH4 emission for HD cows was lower (- 14%) compared to LD and CH4 intensity was higher for HD cows (+5.4 g/kg MY). First results regarding volatile fatty acid (VFA) in the rumen showed a higher proportion of acetate in HD cows (73,1% vs 70,4% of total VFA). Phenological stage of LD regressed during the trial because LD cows grazed on regrowth and HD plot passed flowering stage during the last three weeks. More data are needed to understand the underlying physiological phenomenon, but we can hypothesise that a higher level of plant secondary compound could explain the reduction of methane emission of HD cows, despite the increase of herbage fiber content associated to its maturity and the probable lower level of intake. This trial should then bring new insight following in vitro fermentation trial showing a reduction of gas emission for biodiverse mountainous herbage.