Current climate policies represent a serious threat to the cattle industry, since cattle account for appr. 75% of emission from Danish agriculture of the powerful greenhouse gas, methane. It has been discovered that a tropical red macroalgae, Asparagopsis taxiformis, when fed in small quantities to cattle, virtually blocks the enteric methane emission. Northern macroalgae species (eg. Saccharina latissima, Ascophyllum nodosum, Alaria esculenta) induced a partial suppression of methane formation in an in vitro system simulating rumen fermentation. The bioactive compounds responsible for this are as yet unknown, but their use as anti-methanogenic feed additives for ruminants could have enormous market potential – nationally and globally. The project is funded by AU Forsknings Fond.