METH’ALGUES project aims at evaluating the anti-methanogenic capacity of algae available on the French coasts with in vitro and in vivo trials. From a review of the literature, the species Ascophyllum nodosum, Chondrus crispus, Fucus vesiculosus, Ulva sp. and Arthrospira sp. were identified as candidate to reduce CH4 emissions thanks to their bromoform and polyphenol concentrations. In vitro fermentation tests on rumen juice were carried out on the identified species to select those with the highest methanogenic power and the lowest impact on ruminal fermentations. F. vesicolosus and C. chrispus were selected to proceed the in vivo trials. Indeed, F. vesiculosus decreased by almost 100% the production of CH4 but had an impact on fermentations. C. crispus reduced CH4 production and contain bromoforme. Based on these results, an in vivo experiment was carried out with C. crispus and F. vesiculosus as well as a mix of additives (Roullier Group), compared to a control diet, on the performance of dairy cows and enteric methane emissions. The experiment was conducted in a Latin square design with 20 mid-lactation Holstein cows during four 3-week periods. The basal diet consisted of maize silage, dehydrated alfalfa and concentrates. Enteric methane emissions were recorded with a Greenfeed system. Individual DM intake (25.6±0.3 kg DM/cow/day) and milk production (34.6±0.8 kg/cow/day) did not differ between treatments, suggesting that there were no palatability problems with the addition of algae. There was also no difference in enteric CH4 emission between treatments (533±4.5 gCH4/cow/day). However, the inclusion rate was lower than expected due to higher-than-expected DM intake levels. As a complement to the first in vivo experiment, a second experiment is ongoing testing higher algae inclusion rates for longer periods with two groups of 28 Holstein cows and will end in April 2023.