Putting gas-masks on cows - how to measure the methane output of ruminants

One day last year I was in an archaeology lecture discussing whether the methane output of domesticated cattle was greater than that of the American Plains bison because if one species just replaced another then the methane contribution of cattle had to be offset by that of the bison. Then some-one asked the inevitable question: "How do you measure the methane?". Following a witty exchange of banter between students and tutor, I set out to research the question. Here is what I found out.

Cattle, which are ruminants, generate methane through the process of biological fermentation that takes place in the rumen and reticulum, the front two parts of the stomach which are involved in the familiar ruminant behaviour called "chewing the cud". This grinding and chemical processing is needed to extract the nutrients from the cellulose in grass. The good news is that over 95% of methane is expelled through the mouth in a process called 'eructation' - belching. I'll pass over the other 5%.

A jersey cow undergoing methane measurement (www.afbini.gov.uk)

One method researchers used for measuring methane output involved placing the animal in a respiration chamber and measuring the build up of the gas. Another involved placing a hood over the head of the test beast to capture the exhaled gases. These methods have now been replaced with a system which uses a chemical marker, sulphur hexaflouride (SF6), which is released at a constant rate from a bolus placed in the rumen and which is captured using a pipe system which connects the beast's nostrils to a collector. This system has been shown to provide an accurate measurement of the methane output of cattle and may be capable of measuring carbon dioxide emissions if correctly calibrated. The method has been successfully applied to cattle and sheep, though there are variations relating to breed, diet and lactation which may cause issues if we try and apply these measures retrospectively to Neolithic pastoralist practices.

The annual contribution of methane to the atmosphere from all this belching by the main domesticated species was estimated in 1983 as 74 Tg/ year of which 54 Tg/year came from cattle and compared with ~6 Tg/year emitted by wild ruminants. The contribution from 36 million cattle across 10 US states in 2008 was estimated at 2.5 Tg compared with a notional displaced population of 30 million bison which would have produced 2.2 Tg/year. This suggests that the cattle and buffalo methane contributions balance out. However, in 1983 there were 115 million cattle in the whole USA while the maximum population estimate for North American bison before 1492 is 60 million. The net difference means that American cattle account for around 8% of the total anthropogenic methane contribution from domestic cattle with about another 4% from Australia and 3% from Canada.

Of course, we can't just project these figures back 8,000 years; the number of cattle appears to be linked to the exponential rise in the human population and the ratio of people to cattle has undoubtedly varied over time. However, there is a promising area of research to understand to what extent early domestication of animals has contributed to the anomalous atmospheric concentrations of carbon dioxide and methane.