Overview of Ireland’s current Tier 2 approach
About 90% of Ireland’s agricultural land area is used for grazing or hay and grass silage production. Livestock products account for more than half of the agricultural economy and make major contributions to exports. Until 2006, Ireland’s GHG inventory used a Tier 1 approach for all livestock emission sources. Enteric fermentation from cattle and sheep, and cattle manure management are key emission sources. Since 2006 a country-specific Tier 2 approach has been used for enteric fermentation and manure management emissions from cattle.
Table 1: Overview of Tiers used for livestock methane emissions in Ireland’s national GHG inventories
|Livestock types||Tier used for enteric fermentation (CH4)||Year adopted*||Tier used for manure management (CH4)||Year adopted*|
*Year refers to the year of NIR submission
Approach used: Ireland’s Tier 2 approach was developed through a commissioned study conducted by the Irish Government under the National Development Plan 2000–2006 (1). The structure of the inventory and quantification approach was specifically designed to capture the diversity of Ireland’s grass-fed cattle production systems, and to make use of existing energy balance models used by extension services and farmers in the country.
Livestock characterization and population data: Livestock census data collected by the Central Statistics Office (CSO) categorize the Irish cattle herd into 11 main categories (Table 2). The country was divided into three geographic regions based on slurry storage requirements of local planning authorities and coinciding with the regions used for implementation of nitrogen pollution control measures pursuant to the EU Nitrates Directive. In each region, the length of winter housing and feeding practices vary. Because the CSO livestock statistics do not report numbers for each region, the number of cows in each region was obtained from the Cattle Movement and Monitoring System (CMMS). The total number of cows in the CMMS and CSO data differ, so the proportion of animals in each region in the CMMS data were applied to the total population reported by CSO. Emission factors were calculated for each of the 11 animal categories in each of the 3 regions, and a weighted average across the regions calculated for reporting in the inventory. The CSO undertakes two censuses of animal numbers each year (June, December), and for dairy cows and suckler cows, the average number in each category in June and December is used.
Table 2: Classifications for cattle used in Ireland’s national inventory
|Breeding cattle||Dairy cows||Suckler (beef cows)|
|Beef cattle||Male <1 year|
Female <1 year
|Male 1-2 years|
Female 1-2 years
|Male >2 years|
Female >2 years
|Other cattle||Breeding bulls||Dairy in-calf heifers||Beef in-calf heifers|
Estimation of gross energy intake: For estimation of gross energy intake, Ireland uses a system based on the French energy system (INRA 1989). For each animal type in each region, cattle production systems were characterized in terms of calving date, the dates of winter housing and spring turn-out to grass, milk yield and composition, forage and concentrate feeding level, cow live-weight and live-weight change, and lactation period. Based on these characteristics, the daily energy requirement of cows in each region is calculated by month, including requirements for maintenance, milk yield and composition, fetal growth, and gain or loss of bodyweight.
In the INRA system, net energy requirement is defined in terms of Unites Fourragere Lait (UFL), where 1 UFL is the net energy value of 1 kg of barley at 86% dry matter and is equal to 7.11 MJ net energy for lactation (NEl). (For growing beef cattle, net energy requirements are also determined using the same UFL as for dairy, but for finishing cattle, 1 UFV is the net energy value of 1 kg of barley for meat production and is equal to 7.61 MJ NEmg). For dairy cattle, the main equations used in estimation are:
- Maintenance NEl requirements (MJ) = 9.96 + (0.6 x LW/100), where LW is live-weight. A 10% activity allowance is added for the housed period and a 20 % allowance is added for the grazing period;
- NEl (MJ) required per kg milk = 0.376 * fat content + 0.209 * protein content + 0.948;
- Pregnancy: mean of 12.1 MJ NEl /day for the last 3 months of pregnancy;
- Live-weight change: each kg live-weight lost contributes 24.9 MJ NEl to energy requirements, while each kg of live-weight gained requires 32 MJ NEl.
The live weight of 535 kg for dairy cows was estimated by the Department of Agriculture, Food and the Marine. The composition of the diet of cows in each region was described on a monthly basis, and daily intake was calculated by reference to the daily energy requirement. In estimating diet composition, the concentrate allowance was fixed while forage intake varied according to energy requirements.
Daily methane emissions (MJ/day) were calculated from digestible energy intake using the equation of Yan et al. (2000):
CH4 = DEI * [ 0.096 + (0.035 x SDMI/TDMI) ] 2.298 * (FL 1)
where DEI is digestible energy intake (MJ/day), SDMI and TDMI are silage and total dry matter intakes (kg/day), respectively, and FL is feeding level (multiples of the maintenance energy requirement).
A constant methane conversion rate of 0.065 of gross energy intake is applied when the diet consists of grazed grass and 3 kg or less of concentrate supplement per day. This is based on a large New Zealand database of measurements for grazing animals on similar production systems to those in Ireland. Daily CH4 emissions are summed to give annual emissions for cows in each region, and a weighted national average emission factor is then calculated.
For beef cattle, emissions are determined by calculating lifetime emissions for the animal and by partitioning between the first, second and third years of the animal’s life. This approach allows the published CSO animal population census for June to be used directly as the activity data most representative of the inventory year for enteric fermentation while taking into account the movement of cattle from one age category to another (i.e. from 0-1 year old to 1-2 year old to over 2 years old), as enumerated by the June census, up to two times in their three-year lifetime. The most important parameter for beef cattle is live-weight gain, as it directly affects the energy requirement and thus the feed intake. Live-weight gain of different types of cattle was estimated by applying carcass weight of slaughtered cattle from government statistics to the various life stages of each animal category, such that when all categories are combined, that data is consistent with the national statistics for carcass weight (plus or minus 10 kg). Estimation of emissions from beef cattle were directly calculated using the software INRAtion, which is based on the French energy system.
As a result, the emission factors for dairy cattle reported in the NIR vary year to year by tracking milk yield. For other cattle types, the national emission factors vary depending on the average proportion of each animal type in the three regions.
Manure management: The Farm Facilities Survey (Hyde et al. 2008) provides detailed data on manure management practices to support the adoption of a Tier 2 method for estimating methane emissions from manure management. The Farm Facilities Survey was conducted on a representative sample of farms, the results of which are available at both national level and for each of the three designated Nitrates Directive regions. The partitioning of the year into pasture and housing periods is based on expert opinion in conjunction with the results of the Farm Facilities Survey for each production system identified in the inventory. Having derived the time spent at pasture and the time spent in housing for cattle, the Farm Facilities Survey is used to determine the partitioning of liquid and solid manures to manure management systems within the housing period, and the estimation of the number of animals that are out-wintered (i.e. at pasture all year round). The analysis of feeding regime used to estimate enteric fermentation was also used to estimate the excretion of organic matter by cattle. The methane production potential (BO) of manure, and the methane conversion factor (MCF) use the IPCC default values.
Improvements over time: Since the initial adoption of a Tier 2 approach for cattle, Ireland has used the same approach in its inventory. The Department of Agriculture, Food and the Marine has funded the establishment of The Agricultural Greenhouse Gas Research initiative for Ireland (AGRI-I). This is an organizational and collaborative framework designed to: build a critical mass of scientific expertise in GHG research, co-ordinate uniform measurement protocols, and address a specific set of research issues. The AGRI-I network has a specific set of research aims, primarily focussed on the inclusion of validated GHG emissions mitigation strategies into the national inventory. This research includes a review of feed intake parameters and assumed nitrogen content of feeds and updates as necessary. A separate but related research project investigated the development of country specific BO and MCF values using a range of cattle manures and environmental conditions. In addition the EPA has funded a research project aimed at reviewing the Tier 2 methodology used for the estimation of CH4 emissions from cattle.
(1) O’Mara. 2007. Development of emission factors for the Irish Cattle Herd.
Ireland. 2018. NIR.
O’Mara. 2007. Development of emission factors for the Irish Cattle Herd
Hyde et al. 2008. an extensive Farm Facilities (Manure Management) Survey.
INRA. 1989. Ruminant Nutrition. Recommended Allowances and Feed Tables. Jarrige, R. (ed.). John Libbey Eurotext, London and Paris.