Under a compulsory carbon accounting scheme, methane gas promises to be agriculture’s most troublesome gas.
Burped—only about 2pc is farted—from ruminant animals, methane is considered 21 times more potent as a greenhouse gas than carbon dioxide, and responsible for about 11pc of Australia’s greenhouse gas emissions.
Speaking at the Carbon Farming conference in Orange, NSW, last week, methane researcher Dr Richard Eckard from the University of Melbourne suggested that the diary industry, in particular, would be hit most by methane costs, with the beef and sheep industries close behind.
Dairy, because of its intensive use of space and fertiliser dependency, is estimated to have an emissions profile of between four to 45 tonnes of CO2 equivalents (CO2e) per hectare.
Beef is plotted at 2-3 t CO2e/ha, and cropping just 0.2-1 t CO2e/ha.
Methane mitigation research is only just getting off the ground, covering breeding, diet and direct intervention in the rumen.
Dr Eckard, however, said a best guess is that methane production can be reduced about 40pc using current technology.
After Dr Eckard’s presentation, he fielded several questions from the conference floor (paraphrased for clarity), including:.
Q. Didn’t the great herds of ruminants that once roamed the earth—bison, wildebeest—produce as much methane as our domestic herds do now?
A. In Africa or the United States, an argument could be made that we have just replaced one source of methane production with another. The same can’t be applied to Australia and New Zealand, because there were no large herds of ruminants here.
The methane profile is vastly different here to what it was 200 years ago.
Q. A recent scientific paper said that levels of methane in the atmosphere had levelled off since 2000, while the number of ruminant animals has risen. What do you make of that?
A. We had some hope around that finding, but the measurements over the last three years have more than compensated for that levelling off.
Methane levels are now rising far steeper than they were before they levelled off. There’s a theory that methane trapped in ice is now being released.
Q. My sheep live on grass, which are part of the carbon cycle, and that’s all they get to eat. In order for them to be a problem in this global warming cycle, they have to emit more than they consume. Is that a credible proposition?
A. Ruminants do emit more greenhouse gas than they consume. There’s no extra carbon coming in, but when it is bonded to methane it absorbs infrared radiation (the mechanism considered to be behind global warming) more effectively.
The molecule acquires greater global warming potential. About 10pc of a ruminant’s total carbon intake is converted to methane. Methane is 21 times the global warming potential of CO2, so you’re multiplying that 10 per cent 21 times.
Q. What is the difference in methane production between a grain-fed cow and a grass-fed cow?
A. Grain adds soluble sugars and less fibre, so by supplying a bit of grain you are providing a diet that is more readily fermented and which makes the rumen more acid.
Acidity aids digestion and ensures a faster rate of passage. So you get about 15-20pc less methane by adding grain to the diet.
The flaw in that plan is that the grain was grown using nitrogen fertiliser, and grown, harvested and transported using fossil fuels.
Q. How does the maturity of the pasture an animal is grazing affect methane production?
A. To minimise the amount of energy the rumen needs to digest a plant, it should be grazed when protein-to-energy ratios are optimum—which varies between species.
Graze too early and the animal gets too much nitrogen. Graze too late and it gets more fibre, which slows its passage through the gut and generates more methane.
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