At first I was confused... thanks for pointing out that this study is totally impractical.

Cattle, like most ruminants, rely on the fermentation of cellulose for most of their energy needs. Any feed additive that shuts down this process will result in malnourished cattle, and is unlikely to see broad adoption in the industry. Luckily for the PEI farmer, the seaweed he used didn't have this effect.

That's covered in the paper. The in vitro test was on bacterial samples that do that very fermentation and measured the output of nutrients (fatty acids) that can be absorbed by the animals.

I.e. shutting down the methanogenesis does not shut down all fermentation, it just takes other pathways. It is not an essential component.

Wow I guess this is why I'm not a biochemist; I wouldn't have thought that was possible. This might be something to look for at the feed store in another decade or so...