Methane is a particularly powerful greenhouse gas and recent research suggests that it may have 33 times the effect of carbon dioxide when its interaction with airborne particles is taken into account. It is also very persistent in the atmosphere.
Calls for a tax on livestock emissions have been discussed half seriously in the European Commission, but now it has been advocate by the United Nations: Flatulence
What is evident more generally is that agriculture and the food chain has to be fully incorporated in attempts to mitigate and adapt to climate change. However, the farm lobby has been a powerful opponent of the cap-and-trade bill in the US.