You have a very good question, and I wish this was covered better. In their FAQ they explain that the actual fibrolytic microbes that digest cellulose in the r

You have a very good question, and I wish this was covered better. In their FAQ they explain that the actual fibrolytic microbes that digest cellulose in the rumen, produce CO2, H2 and other compounds but not methane. And then later the methanogenic microbes consume these and produce methane. I get the impression that the methanogenic bacteria are not needed in digesting cellulose, that they just "steal" part of the products from the earlier fibrolytic steps. And the chemical, bromoform, from seaweed only blocks this methanogenic metabolic pathway. "How do cow stomachs digest feed, and why might they emit so much methane?" https://research.csiro.au/futurefeed/faq/ https://research.csiro.au/futurefeed/faq/ https://en.wikipedia.org/wiki/Fibrolytic_bacterium https://en.wikipedia.org/wiki/Fibrolytic_bacterium