Methane Emissions from Cattle

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In autumn 2005, the Department of Animal and Aquacultural Sciences was asked by the National Pollution Control Authority to improve the data basis for calculating methane emissions from cattle in Norway. This work was intended as a basis for calculating the Norwegian CO2 quota in connection with the Kyoto Protocol. According to the Protocol, industrialised countries have to reduce their greenhouse gas emissions in the period 2008-2012 by an average of 5.2 % below their 1990 levels.

Increasing atmospheric temperatures have been observed in the past decades. One of the causes is the rising concentration of the major greenhouse gases water vapour, carbon dioxide, nitrous oxide, ozone, methane and chlorofluorocarbons in the atmosphere. The greenhouse gases have different residence times in the atmosphere, thus affecting the gases' ability to absorb and emit energy in the form of heat. Each gas thus affects the temperature of the atmosphere to a varying degree. To facilitate the comparison of the different greenhouse gases’ warming effect, an international unit known as the Global Warming Potential (GWP) is used. GWP is a relative scale which compares the gas in question to that of the same mass of carbon dioxide, calculated over a specific time interval - usually a period of 100 years. Methane’s GWP is 21 times higher than that of carbon dioxide.

Ruminants have been identified as the largest single source of anthropogenic methane emissions. Methane is produced by the microflora in the rumen. The anaerobic fermentation in the rumen results in a surplus of hydrogen, which is then reduced to methane by methanogenic bacteria. Methane formation represents a loss of energy for the animal, as 2-12 % of the feed’s gross energy is transformed to methane. The total energy loss depends on the feed ration’s size and composition, and is mainly determined by how feeding affects the activity of the rumen microflora.

Through the UN’s Climate Convention and the Kyoto Protocol, we are committed to reporting the emissions of greenhouse gases. To standardise the assessment of greenhouse gas emissions, the UN Intergovernmental Panel on Climate Change (IPCC) established a committee that proposed a common international method for preparing national greenhouse gas inventories. Initially, IPCC described two procedures for calculating the amount of methane from livestock (Tier-1 and Tier-2). So far, Norway has applied the simpler Tier-1 method, which is not very advanced or robust. However, IPCC encourages countries to develop more advanced methods that take into consideration specific information about each country’s livestock production. In autumn 2005, the National Pollution Control Authority thus asked UMB’s Department of Animal and Aquacultural Sciences to improve the data basis for calculating methane emissions from cattle in Norway (Tier-3). About 90 % of all cattle in Norway are part of the dairy herd recording scheme, which gathers extensive data on milk yields and feeding. For growing cattle being fed to finish, the scheme records data on slaughter age and carcass weight. These data have enabled the development of a specific method for calculating methane emissions from Norwegian cattle. The new equations for assessing methane emissions thus take into consideration the dominant role of grass silage as cattle feed in Norway. Furthermore, they also consider the fact that the proportion of the feed’s gross energy that is transformed to methane decreases with an increasing concentrate to forage ratio and increasing feed intake. Based on information about milk yields and concentrate to forage ratio, the use of model simulations characterising Norwegian dairy production have led to the development of empirical equations that describe methane emissions from dairy cows. Similar models have been developed for cattle being fed to finish, where methane emissions are calculated on the basis of slaughter age and carcass weight. The new Norwegian assessment method for methane emissions is considerably more robust than IPCC’s Tier-1 and Tier-2 methods, as it responds much better to variations in production method and intensity.

The calculations show that between 1990 and 2005 there has been an overall decline in methane emissions for the three categories of cattle, i.e., dairy cows, beef (suckler) cows and finishing bulls (Figure 1). This trend is primarily due to the decreasing emissions from dairy cows, as emissions from beef cows have increased, while emissions from finishing bulls remained constant throughout the period. Changes in livestock populations account for most of the changes in total emissions within each cattle category.

Figure 1. Calculated annual emissions (tons) of methane from Norwegian cattle between 1990 and 2005. Photo: iha

For more information about this study, you can receive the report “Methane emission from enteric fermentation in Norway’s cattle population” by contacting Harald Volden at UMB.

Updated: 14.05.09
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