I think we will be able to detect methane emissions on a large scale from space sometime soon, e.g. [1]. At that point, farmers can't escape the law in any way, and they will have an incentive to reduce emissions.
[1] http://www.csmonitor.com/Science/2016/0615/NASA-detects-meth...
"Yes, there is a lot of work on finding methane emissions by satellite. This is good as a broad guide to spotting big emissions like the huge US leak: the real problem we have is in quantifying emissions at ground level from a wide range of much smaller sources, and in placing good accurate numbers on the fluxes.
Unfortunately, the emission is from the ground into the boundary layer, at the bottom of the atmosphere, while satellite observation sees through the total column. We use wind back-trajectory analysis to identify the source of the methane (running the weather backwards), but there have been bad examples in the past where the media latched onto claims of major methane anomalies seen from space (e.g. in the Arctic or in S America) and didn’t realise that the methane was in the mid-troposphere in air that had last been in contact with the ground many thousands of miles away.
What is needed is an integrated approach at all levels, using satellite observation, wind-tracking using chemical transport models, and careful ground measurement to identify sources."
You don't want to make the law something they must "escape" from, but rather something they can profit from by not polluting. Like tax credits.