We already have a known source of methane emissions that is ever increasing: cattle. Cows and swine account for several metric tons of emissions every year, factory farming accounting for most of it. Reducing this is in most people's control.
The emissions from grains etc add up to roughly as much overall- you don't eat the entire plant and it either rots or gets fed to cows. Obviously reducing emissions in our food industry is a huge win- eliminating red meat would be awesome. It's nowhere near good enough. Fossil fuels are 30x more important, easily.