From all I have seen (e.f. [1])the seaweed idea was still in small scale trials as of 2018, please provide some evidence for your claim that this is now already mainstream. That said, the focus of this is on methane as one of the GHG produced by cows. This doesn't count CO2 produced by cows or the rest of resource wastage of the meat industry, from the 7:1 grain:beef ratio to the huge industry of the barns and slaughterhouses, packaging, transport, ... All these factors are much lower with vegetables/grains or even plant based options.

And that is not to mention the moral side of animal cruelty in the industry, deforestation for that Argentinian beef, or the human suffering on the processing side.

1 https://e360.yale.edu/features/how-eating-seaweed-can-help-c...