The purpose of this project is to investigate to what extent substances in algae inhibit microbial methane formation in cows. Several substances, so-called bioactive substances, in red, green and brown algae, have a significant methane reducing effect. These are promising results but in order to reach a commercial product with efficient methane-reducing effects, knowledge of the effect on the cows' microflora by these substances is needed. This study will use algae substances, which in already funded projects have been identified to give effects when fed to cows, and further investigate the impact on these on the rumen microbial community. In addition we aim to investigate the possibility of optimizing the substances in algae through a more controlled cultivation. Reduced methane production will strengthen the ompetitiveness of the dairy and also beef industry.