Temperate seaweeds Himanthalia elongata and Fucus vesiculosus significantly reduce rumen methane emissions in vitro due to their high phlorotannin content

Abstract BACKGROUND Global food insecurity and the fact that food production contributes around 30% of greenhouse gas (GHG) emissions is a major planetary challenge. Ruminant products are widely consumed since they are macro‐ and micronutrient dense; however, ruminants produce enteric methane (CH 4 ), a potent GHG. Feeding seaweeds, such as Asparagopsis spp., to ruminants reduces enteric CH 4 emissions. This study investigates the CH 4 mitigation potential of seaweeds, including Alaria esculenta (AE), Ascophyllum nodosum (AN), Asparagopsis taxiformis (AT; positive control), Chondrus crispus (CC), Fucus vesiculosus (FV), Himanthalia elongata (HE) and two seaweed‐derived extracts – Himanthalia elongata (XHE), and Chondrus crispus (XCC) – on rumen fermentation (CH 4 , ammonia (NH 3 ), volatile fatty acids (VFA) and pH) at three timepoints (4, 24 and 48 h). The contents of volatile organic compounds (VOC) and phlorotannin (PT) of the seaweeds were also investigated for insight into the mode of action. RESULTS As expected, AT consistently reduced CH 4 production in comparison to the respective negative grass silage controls (~93.3%) and other tested seaweeds ( P < 0.05) at all timepoints. At 4 h AN, FV, XCC and XHE elicited a reduction in CH 4 of 2.0%, 3.0%, 40.9% and 31.1%, respectively, over the negative controls. XHE was the only tested seaweed to reduce CH 4 production (4.9%) at the 24 h timepoint. At 48 h FV, CC, HE, XCC and XHE showed reductions of 14.4%, 2.9%, 1.9%, 2.8% and 42.8%, respectively, over the negative controls. CONCLUSION As a consequence of their high PT content, XHE and FV show promise for GHG mitigation in ruminants, thereby aiding ruminant food security. © 2025 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.