Yeah, that kind of thing is why the climate benefit of getting rid of cows is nonsense.  If the cows don't eat the grass, it eventually rots in the wet season producing amounts of methane and CO2 similar to what the cows produce.  But if one were to actually do a serious study on what happens in reservoirs, especially in somewhat arid areas, the methane produced by decay will be relatively little compared to the net suppressed carbon emission that equivalent fossil fuel plants would produce.  Yes, the fossil fuel plants would tend to produce CO2 while some rot channels produce methane.  Folks like to say methane is bad.  But here is the rub.  The half life of methane in the atmosphere is about a decade.  The half life of CO2 in the atmosphere is centuries.  The integrated effect of CO2 over its incredible lifetime makes simple comparisons of relative IR absorption strengths between the two gases incomplete to say the least.