Seven red Canadian seaweeds (Callophyllis flabellulata, Graciliariopsis verrucosa, Mastocarpus papillatus, Mazzaella splendens, Mazzaella japonica, Palmaria mollis, and Prionitis lanceolata) and a positive control (Asparagopsis taxiformis) were selected to evaluate their chemical and elemental composition and their effects on in vitro fermentation and methane (CH₄) production in an alfalfa hay or barley straw diet. The in vitro batch culture was conducted as a completely randomised design with a control (alfalfa hay or barley straw) and seven increasing concentrations of seaweed. Chemical and elemental composition varied greatly across seaweed genera. Increasing supplementation of A. taxiformis linearly decreased (P < 0.001) dry matter disappearance (DMD) and gas production (GP; mL, mL/g DMD) with CH₄ production eliminated (P < 0.001) at 1.0% inclusion of A. taxiformis in both diets. Inclusion of Mastocarpus papillatus, Mazzaella japonica, Mazzaella splendens, Palmaria mollis, and Prionitis lanceolata increased (P ≤ 0.05) GP (mL/g DMD) at 0.5% and 1.0% in alfalfa diets. Graciliariopsis verrucosa linearly decreased (P = 0.01) CH₄ production (mL/g DMD) in the straw diet, but no doses were different compared to the control. In conclusion, the Canadian red seaweeds examined in this study did not exhibit anti-methanogenic potential when incubated with alfalfa hay or barley straw.