Climate Change: Development of Emission Factors for the Irish Cattle Herd

ERTDI Report 46 - Special Report - O'Mara

Summary: Emission factors for methane from enteric fermentation and manure management were developed for the categories of the Irish cattle herd for which data on animal numbers are available from the Central Statistics Office.

Executive summary

The Intergovernmental Panel on Climate Change (IPCC) (1996) suggests that countries with large animal populations should develop specific emission factors for greenhouse gas emissions, and move their inventory from a Tier 1 (using default emission factors) to a Tier 2 (using country-specific emission factors) methodology.

Emission factors for methane from enteric fermentation and manure management were developed for the categories of the Irish cattle herd for which data on animal numbers are available from the Central Statistics Office (CSO). The process of deriving these emission factors for each category had the following main steps:

• The country was divided into three regions coinciding with the regions of the Nitrates Directive where it was possible to work out the proportion of animals in each region. This was possible for dairy and beef (suckler) cows, but not for the other categories of animals. In these cases, the country was treated as a single region. For most categories of animals, subcategories were created to give a more accurate assessment of their emissions. Thus 12, 18, 13 and 14 subsystems were modelled for dairy cows, beef cows, nonbreeding beef females and non-breeding beef males, respectively.
• The production system in each region was defined in terms of animal performance and characteristics such as size, calving date, and dates of winter housing and spring turnout to grass. In addition the diet of the animals was defined in terms of type and quality of forage fed, and amount and quality of concentrates fed.
• The energy requirements of the animals were defined for each month based on the animals’ requirements for maintenance and production, and from this the quantities of feed consumed were calculated.
• From the quantities of feed consumed, daily methane emissions were calculated and summed to give an annual emission. The annual emissions of the subcategories within a category were then combined on a weighted basis to give a single national emission factor for the category.
• From the quantities of feed consumed, daily output of manure organic matter (OM) was calculated while animals were at pasture, and during the ‘housing period’. For the latter period, the proportion of manure going into slurry or solid manure type systems was calculated (and account was taken of the proportion produced by out-wintered animals). Methane emissions for manure management were then calculated and the annual emissions of the subcategories within a category were then combined on a weighted basis to give a single national emission factor for the category.
• The process was repeated for 1990, changing any parameters where it was appropriate, and where data existed to allow the change to be made. The emission factors so derived are summarised in Table 1. There are many differences in comparison to the default factors recommended by the IPCC (1996). These differences reflect the different production systems in Ireland.

It was apparent that the use of the average of June and December livestock numbers from CSO data leads to an anomaly when these specific emission factors are used with non-breeding beef cattle. What is needed is the best estimate of the number of animals that fall into the category each year, and the June data give these estimates before significant numbers of animals are removed from the national herd for slaughter.

The implications for the national inventory of emissions from the cattle herd (both enteric fermentation and manure management) are shown in Fig. 1 in the report.

 The specific Irish Tier 2 values give higher total emissions than the current Tier 1 methodology for both 1990 and 2003, and using June-only numbers for non-breeding cattle gives a further increase. However, the drop in current emissions relative to 1990 is greater using the Tier 2 approach than the current Tier 1 methodology (3,600 t vs 10,500 t), and using June numbers further increases the reduction (16,900 t).

Full executive summary available in report.