agriculture isn't even the majority of methane production even in the US (which has as many cattle as China, but 4.5x fewer people). The single largest source of methane is from natural gas leaks.

Enteric fermentation is still a big problem, but it's a problem for another day- the only reason to solve it now is if it truly is as simple as feeding cows seaweed. That seems highly suspect but I have yet to see anyone accuse it of failing the sniff test.