Reuben Crossan, of Balclutha, asks :-

Why is there such concern over cows farting? What do these gases do to our environment?

Keith Lassey, an atmospheric scientist at the National Institute for Water and Air at Lauder and who studies greenhouse gases from agricultural activities, responded.

Your phrase "farting of cows" presumably refers to the so-called "fart tax". In fact cows release very little gas through flatulence. The particular phrase "fart tax" was coined by those seeking to deningrate the proposed tax. The gases of concern are belched. However, while the proposed "fart tax" has been dropped, that phrase has inadvertently served to publicise the problem.

Cows are "ruminants" as are most farmed animals such as: cows and other bovine animals, sheep, deer, goats, but not horses or pigs (or humans). Ruminant animals have a fore-stomach (rumen) that helps digest fibrous diets such as grass more efficiently than do non-ruminant animals. However, a byproduct of rumen-digestion is methane, which is belched through the mouth and nose into the atmosphere. The technical term for this process is "eructation".

The problem with emitting methane into the atmosphere is that it is a greenhouse gas. In fact, the world's population of farmed ruminant animals is arguably the largest human-caused source of methane.

A greenhouse gas is an atmospheric gas that absorbs infra-red radiation that Earth would otherwise emit to outer space. That absorption directly affects the Earth's energy balance, with the result that natural greenhouse gases in the atmosphere (mainly carbon dioxide and water vapour) warm the planet by about 18 degrees on average, thereby making it habitable. Extra greenhouse gases from human activities are believed to contribute to global warming and thence to climate change. Human activities throughout the past few centuries, but mainly since the mid 1800s, have released a lot of greenhouse gases, principally carbon dioxide with methane the second most important.

Collectively NZ's ruminant animals emit about 20per cent as much methane as is recovered from NZ's natural gas fields. (Natural gas is mainly methane). Put another way, each dairy cow emits on average enough methane to power a car for about 5 km per day.

New Zealand researchers are investigating ways in which ruminant livestock can emit less methane without compromising production. This will also increase efficiency by diverting energy from methane production into products such as milk and meat.

You may like to consult the following document on NIWA's website: http://www.niwa.co.nz/rc/atmos/ghg-nis29.pdf.