Agro-residues and aquatic biomass are potential sustainable ruminant feeds. However, their effects on rumen digestibility, fermentation, and greenhouse gas emissions remain insufficiently characterized. This study used two in vitro batch culture experiments to evaluate crushed almonds, peanut skins, and water hyacinth as feed additives (1–10%) and grass hay replacements (25–100%). Their chemical composition varied significantly. Crushed almonds are rich in crude protein (22.47%) and ether extract (38.38%). Peanut skins contained moderate protein (13.87%) and high fiber and lignin, while water hyacinth had the lowest protein (7.27%) and highest fiber and ash fractions. All ingredients had no effect on fermentation or methane production when used as additives, whereas grass hay replacement induced marked ingredient-specific responses. Crushed almonds showed the highest in vitro dry matter digestibility (IVDMD), increased propionate, and lowered the A:P ratio; however, methane production increased by 12%. Peanut skins sharply reduced IVDMD with increasing inclusion, resulting in the strongest methane mitigation (57% reduction). Water hyacinth displayed a modestly low IVDMD and moderate methane reduction (14%). In conclusion, methane production is closely associated with substrate degradability, and each ingredient exhibits distinct functional tendencies: crushed almonds enhanced energy availability and digestibility, peanut skins showed methane mitigation potential, and water hyacinth functioned primarily as a low-fermentability grass hay substitute.

The need for new feed ingredients that could reduce methane (CH₄) emissions from dairy cattle while maintaining rumen function is essential for sustainable milk production. This study aimed to evaluate the CH₄ mitigation potential of selected microalgae and macroalgae, along with an agro-industrial by-product, using two feeding strategies, and hypothesized that lipid- and polyphenol-rich materials would reduce CH₄ production in an inclusion-dependent manner. An in vitro batch culture study (24 h) was conducted to evaluate microalgae (Euglena gracilis and Aurantiochytrium spp.), macroalgae (Undaria pinnatifida), and an agro-industrial by-product (grape marc) either as feed additives (5%) or as a partial replacement of the concentrate mixture (30%, 50%, and 70%) in a basal diet consisting of 50% Klein grass hay and 50% concentrate mixture. As a feed additive, grape marc stands out for its potential to reduce CH₄ yield by about 43.3% without adversely affecting digestibility, pH, or total volatile fatty acid concentrations. When used as feed replacements, Euglena-, Aurantiochytrium-, and grape marc-based feeds reduced CH₄ yield at the highest replacement levels (50 and 70%); however, these effects were accompanied by decreased total gas production and volatile fatty acid concentrations, indicating reduced fermentation activity. Meanwhile, at a 30% replacement level, they showed promising efficiency as alternative feeds. Overall, CH₄ mitigation depends more strongly on inclusion strategy rather than feed type. Lipid-rich microalgae showed potential as concentrate replacements up to 30%, whereas grape marc was most effective as a feed additive for reducing CH₄ emissions.