Or just measure the methane output and charge the farmers a tax on that directly. That would give them incentive to feed their cows that methane reducing seaweed.

> Can’t they just add a meat sales tax

No, because the (sinisterly clever) idea of ekianjo allows the implementation of something which is sought by some: progressive taxation of limited resources or externalities etc, as opposed to fixed one. It's akin to rationing. You drive 10000 miles/y, pay 1/u or 1000; if 20000 miles/y, pay 2/u or 4000...

Not if the tax is progressive (higher earning people, or people eating more meat get taxed at a higher rate).