FYI: "Feeding cows seaweed cuts 99% of greenhouse gas emissions from their burps, research finds" https://www.independent.co.uk/environment/cows-seaweed-metha...
That is... not exactly confidence inspiring. It may very well be a statistical fluke and the reality is much less reassuring.
Not saying we shouldn't research in that direction, but this looks way too early to be confident in it.
[1] https://animalscience.ucdavis.edu/news/research-led-ermias-k...
https://link.springer.com/article/10.1007/s10811-016-0830-7
It would be a cheap food supplement since the methane inhibition required only about 2% seaweed in cattle food, which in turn implies a bromoform feed content of only about 35 parts per million. Though no doubt some farmers would prefer to be able to say that they add seaweed rather than a particular chemical to their cattle feed.