Feature

Breathalising Sheep

Do all sheep breath equally?

Keith Lassey and Marc Ulyatt

A team of atmospheric scientists have been literally out in the field measuring methane "eructed" (or belched) by grazing livestock. New Zealand's sheep and cattle are major producers of this gas and, because we have large numbers of them and a relative small human population, New Zealand is among the world's highest methane emitters per capita.

Why the interest in methane? Methane (CH4) is a "greenhouse gas". This means that it selectively absorbs infrared radiation that would otherwise escape to space, so that much of the absorbed energy is retained within the surface environment. Because of this there is a widespread concern that ever-increasing concentrations of greenhouse gases in the atmosphere will lead to progressive warming of our environment. This concern has led to international agreements under the Framework Convention on Climate Change for signatory countries to set in place policies to limit the growth in emissions, and programmes to monitor those emissions. New Zealand is a signatory to that Convention.

From a global point of view, methane is the second most important greenhouse gas released by human activities behind carbon dioxide. But methane is the predominant greenhouse gas released from human activities in New Zealand. About 75% of this methane emission is generated by farmed livestock.

Although quite a lot is known about methane production by livestock during digestion, techniques have only recently become available to measure methane emissions while grazing. The major culprits are ruminant livestock -- those animals with an extra stomach (the rumen) to more efficiently digest cellulose. These include cattle, buffalo, sheep, deer and goats. They do not include humans and other omnivorous or carnivorous animals, and exclude some herbivores such as horses, pigs, elephants. The methane is a by-product of cellulose break down, generated by the microbial community in the rumen.

Methane emission represents an energy inefficiency in the digestive process, so reducing its production will benefit both nutritional efficiency and the environment -- a win-win situation. New Zealand's farmed livestock are estimated to emit about 1.5 million tonnes of methane annually. To put this into perspective, if every motor car in New Zealand were to be fuelled by compressed methane (natural gas is more than 90% methane), 1.5 million tonnes would propel each of them for about 14,000 km, a distance close to the average for the entire New Zealand car fleet each year.

How is the methane collected? The technique is both simple and stress-free. The animal carries a light yoke-shaped canister made from PVC, which is attached to a halter (such as might be used to lead the animal into a ring). The canister is equipped with a valve and connects through a fine capillary tube to an inlet near the nose, all mounted on the halter. The canister is pre-evacuated and, after connection, the valve is opened. This allows air from around the animal's nose and mouth to be sucked into the canister, but the capillary ensures that the flow rate is quite low, about 0.6 ml/min for a sheep, 0.9 ml/min for a cow. In this way an integrated breath sample is collected from each animal over 24 hours.

Needing to know the collection efficiency can be avoided by inserting a known source of inert tracer (sulphur hexafluoride) into the animal's rumen before starting. After detachment from the halter, the canister's contents are analysed for both methane and the tracer; from this the methane emission rate can be determined. The animal's faecal output is also collected and measurements made to establish the level of feed intake for the particular animal to be calculated. This in turn enables the methane output to be directly related to feed intake.

In March 1996 on a Manawatu improved ryegrass-white clover pasture, we measured daily methane emissions for five days from each of 50 young sheep (37kg) and each of 10 dairy cows, with about 90% collection success. The per-animal CH4 emissions averaged across the sheep flock and across the dairy herd were in accord with previous estimates of methane emissions: about 19g CH4/ head/day from the sheep; about 260g CH4/head/day from the dairy cows.

When we looked at how much variability there was about these mean values, we found that individual sheep did not vary much from day to day, but differences between sheep were considerable. A small amount of this variation was due to variations in the amount eaten, but there was surprising degree of variation among sheep eating about the same amount: "high emitters" produced about 40% more than "low emitters". What is the origin of this variation? That will be the focus of further research, as the answer may point to possible strategies to reduce such emissions.

Keith Lassey is at NIWA in Wellington.
Marc Ulyatt works at AgResearch in Palmerston North.