There probably isn't an incentive to reduce methane emissions for farmers right now though, right? We'd have to pass regulations mandating a reduction, or an emissions tax or something.
It's an interesting puzzle as to how you'd monitor something like that for a global warming tax. Random cow inspections? Some kind of satellite thing?
This is how Norway encourages bottle recycling: There's no law mandating recycling of bottles, but there's a substantial tax on beverage containers that you get a reduction of in proportion to your return rate. This has resulted in a return rate of 99%+ of recyclable bottles by making it economically beneficial for retailers and distributors to participate in fairly elaborate and expensive collection and recycling schemes.
It takes some work to get mechanisms like this in place in a way that isn't easy to exploit, but get the incentives right and you can let economic interests do most of the work.