This study comprised four in vitro experiments, two batch culture and two Rumen Simulation Technique (RUSITEC), to evaluate the potential of biochar to mitigate enteric methane (CH4) in beef cattle diets. Biochar products used in the study were coconut (CP001 and CP014) or pine (CP002, CP015, CP016, CP023, CP024), differing in their physical and chemical composition. In the batch culture, they were evaluated at different levels of inclusion (Exp. 1: 4.5, 13.5 and 22.5 %; Exp. 2: 2.3 and 4.5% diet DM) and particle size (Exp. 2: < 0.5, 0.5-2.0, > 2.0 mm) to determine effects on DM disappearance (DMD), total gas and CH4 production and ruminal fermentation parameters (pH, volatile fatty acids (VFA), and ammonia nitrogen (NH3-N)) when added to a barley silage-based diet. In Exp. 1, level of biochar inclusion linearly (P < 0.01) decreased DMD but had no effect (P > 0.05) in Exp. 2. In both experiments, total gas production and CH4 were not affected (P > 0.05) by biochar treatment nor level of inclusion. Rumen fermentation parameters were also not affected by treatment (P > 0.05) or level of inclusion (P > 0.05) in either experiment. Additionally, particle size had no effect on any measured parameters (P > 0.05). Subsequently, two RUSITEC experiments evaluated: 1) three pine-based biochars (CP016, CP023, CP028), and 2) three spruce-based biochars treated post-pyrolysis with salt (ZnCl2) or acids (HCl/HNO3 or H2SO4), respectively. In both experiments, biochar was included in a barley silage-based diet at 2 % of diet DM. Biochar did not affect (P > 0.05) nutrient disappearance parameters (DM, OM, CP, NDF, ADF or starch disappearance), total gas or CH4 production in either experiment (P > 0.05). Rumen fermentation parameters (P > 0.05), total protozoa counts (P > 0.05) and microbial protein synthesis were also unaffected (P > 0.05). Lastly, alpha and beta diversity and rumen microbiota families were unaffected by biochar (P > 0.05), except for family Rikinellaceae. In conclusion, biochar did not offer the potential to mitigate enteric CH4 emissions nor improve rumen fermentation parameters in a barley silage-based diet in either batch culture or RUSITEC.