Seamus Hogan posed an interesting question about methane emissions recently. He didn’t see how this was much of a stock problem (if you’ll pardon the pun) because methane, while a particularly nasty greenhouse gas, doesn’t hang around up there for long. He says
I am puzzled about why we should worry about methane emissions, given that they result from a circular process whereby carbon in grass is converted into methane by cows, but then carbon is reabsorbed from the atmosphere to re-grow the grass.
BK Drinkwater, I think correctly, pointed to fertiliser extracted from the ground and used to grow grass which (once eaten by cows) generates extra methane emissions; this is not part of a closed carbon cycle. But Seamus then observed that
my memory from high-school science and geography is an important source of New Zealand’s advantage in agrigculture is that we don’t have need for nitrogen fertilisers (something to do with clover), just for superphosphate. And a quick Wikipedia search confirmed that while Carbon is present in urea (a typical nitrogen fertiliser), it is not present in super phosphate.
It is true that clover is a nitrogen fixer, so we can & do get nitrogen from including it in the pasture mix (the salad bar). Also cow piss is unbelievably high in nitrogen, so we also get lots like that (too much actually, but that’s another story). However the key point is that, notwithstanding these facts, it still makes economic sense to apply urea because doing so accelerates grass growth and at current prices the value of the resulting milk is worth the cost of the urea.
So (in my current state of ignorance) it seems that the policy implication may be to tax urea.