Methane myth gives cattle a bum steer
Are cattle and other livestock the source of much of the greenhouse gases that concern us today? Dr Karl chews it over.
The atmospheric scientists tell us that we humans have affected our planet's atmosphere in two major ways.
We have punched a hole in the ozone layer; and we have, quite separately, set off global warming by dumping so-called greenhouse gases into the atmosphere.
Most of us know that the most villainous of these gases is carbon dioxide. Many of us probably know that methane from cattle and sheep is another significant greenhouse gas.
But practically all of us wrongly believe that this methane gas comes from the back end of the animals. And very few of us know that methane is 22-or-so times nastier than carbon dioxide.
Once again, let me start by saying 'everything is made from atoms'.
Carbon dioxide is made from one atom of carbon and two atoms of oxygen. Methane is a bigger molecule. It's made from one atom of carbon and four atoms of hydrogen.
Livestock, of which cattle are a significant proportion, produce about 20 per cent of the world's methane output. The rest comes from rice paddies, coal mining, landfill sites and so on.
There are about 1.3–1.5 billion cattle on Earth today. India has about 30 per cent, Brazil about 20 per cent, the USA about 10 per cent and Australia just three per cent.
The methane that comes from cows is not made directly by the cows. No, it's made by tiny bacteria-like critters that live in the gut of the cattle.
These critters have a happy symbiotic relationship with the cattle. The cattle give the critters a safe home and the critters turn grass into food for the cattle.
More specifically, these critters eat cellulose. Cellulose is a big molecule that makes grasses stiff and tough enough to stand upright. We humans would starve to death if we tried to eat grass.
But cattle can eat grass and grow big and healthy because the critters in their gut can digest the cellulose.
Animals that can do this are called ruminants. Cattle are ruminant animals, as are sheep, goats, deer, giraffes and moose.
They're not called ruminants because there is lots of room in it. No, the word rumen comes from the Latin for gullet, or throat.
Ruminants will partially digest the food in the rumen, and then vomit it back up into the mouth to chew it some more. A more polite word for vomit is regurgitate.
(By the way, another meaning for the word ruminant is a person who ruminates: they think deeply on matters, they contemplate and they meditate. They 'chew over' ideas and concepts, in the same way that cattle 'chew over' their food. The word ruminant comes from the Latin word ruminat, meaning chewed over, and it's closely related to the word rumen.)
So cattle can eat grass because they have not one but four digestive chambers at the top end of their gut. The first of these chambers is called the rumen. The average rumen can hold around 160 litres, which is two to four times the volume of your car's petrol tank. Tiny creatures (bacteria, fungi, protists and viruses, and bacteria-like critters called archaea) live in the rumen, and help digest the cellulose.
Some of these archaea can actually eat the cellulose, as well as the by-products of the other microbes living in the rumen.
The next chamber, the reticulum, is the smallest. This is where metal objects (barbed wire fragments, etc.) usually end up.
The third compartment is the omasum, which absorbs nutrients and water.
The fourth chamber, the abomasum, is similar to our human stomach, which is why it's called the 'true stomach'.
Getting back to the first chamber, the rumen is loaded with tiny critters, including archaea. These archaea are messy eaters and 'waste' about six to 10 per cent of what they eat. The waste comes out as methane.
If this potential source of energy were not wasted, it could be used to bulk up the cow.
But on the other hand, the strange combination of cow-and-archaea does have a marvellous, very special and very rare skill, so surely we can forgive their slight inefficiency?
Their special skill is to turn non-protein into protein, or grass into cattle. I have always been amazed that a cow can get so huge and meaty, while eating such a low-energy food as grass. It is all because of the archaea in their gut.
We humans do not have critters in our gut that can do that for us, so we have to eat protein.
But there is the unfortunate side-effect of the archaea making methane, which the cattle then release. And I'll talk more about that, next time.
Published 28 September 2010
© 2019 Karl S. Kruszelnicki Pty Ltd