Keywords: Livestock characterization | dairy cattle
What data needs were addressed? Regional characterization of dairy cattle.
Why was the data needed? New Zealand’s initial Tier 2 approach allocated the livestock population to 3 different types of grassland (i.e. improved, unimproved and tussock) in 4 climate zones. Later analysis suggested that allocating all animals to 1 climate region and 1 of grassland changed total methane emissions by around 1% (Clark, 2001). So when a revised Tier 2 approach was adopted in 2002, detailed spatial categorization was no longer used. To account for differences in dry matter intake and methane conversion rates (Ym), weighted averages based on the proportion of animals in each pasture type were used. However, by the second half of the 2000s, the dairy sector had undergone rapid change, with large increases in the dairy population and productivity in some regions. This meant that a single national model for dairy emissions may no longer be accurate.
Methods used: Regional characterization of dairy cattle.
How was the data need addressed? A study (Clark, 2008) was undertaken to compare the differences in dairy cattle emissions estimated using a single national model and an aggregation of regional models. The study identified data on dairy cattle populations and animal performance that were both available at the regional level. Official population data were available for 73 Local Territory Authorities, while the animal performance data (i.e. total milk produced, milk fat and protein content, weight of dairy cows by breed and proportion of each breed in the national herd) were available from a farmer cooperative disaggregated into 17 regions. Both data sources provided time series going back to 1990. These data sources were already used in the national inventory. Therefore, the comparison was a test of whether the national GHG emissions model was linear or not; if it was linear, national and aggregated regional estimates would be identical.
The results showed that for 1990, the two models yielded very similar results. However, for 2006, differences were more significant, with the regional model estimating 2.3% lower national dairy cattle emissions than the national model. The trend in emissions is therefore non-linear. The reason is that emissions are most directly related to feed consumption, but feed consumption is estimated on the basis of animal performance (e.g. milk yield, live weight), which is not linearly related to feed consumption. That is, each additional unit of milk requires a smaller percent increase in feed intake, because maintenance requirements have already been met.
Using the results of this study, the national inventory adopted a regional model for estimating dairy emissions. Emissions from other livestock types are still produced using a single national model because regional data is not available.
Table 1: Comparison of dairy cattle emission estimates made using a single national model and an aggregation of 17 regional models.
|Enteric fermentation (Gg)||CH4 manure management (Gg)||N2O manure management (Gg)|
|Aggregated regional model||232.18||404.71||9.68||17.38||7.34||12.64|
Clark H. 2001. Ruminant methane emissions: a review of the methodology used for national inventory estimations. A report prepared for the Ministry of Agriculture and Forestry.
Clark H. 2008. A comparison of greenhouse gas emissions from the New Zealand dairy sector calculated using either a national or a regional approach. https://www.mpi.govt.nz/dmsdocument/2936-a-comparison-of-greenhouse-gas-emissions-from-the-new-zealand-dairy-sector-calculated-using-either-a-national-or-a-regional-approach