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.
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...