The effect of methodology on estimates of greenhouse gas emissions from grass-based dairy systems

The objective of this study was to compare two standard methodologies, Intergovernmental Panel on Climate Change (IPCC) method and life cycle analysis (LCA), for quantifying greenhouse gas (GHG) emissions from dairy farms. Both methods were applied to model the GHG emissions from 9 dairy farm systems differing in strain of Holstein-Friesian cow and type of grass-based feed systems using the physical performance findings of previously published work. The strains of Holstein-Friesian cow used were; high milk production North American (HP), high fertility and survival (durability) North American (HD), and New Zealand (NZ). The alternate grass-based feed systems were; high grass allowance (HG, control); high stocking rate (HS) and high concentrate supplementation (HC). The milk production systems were modelled using a previously developed integrated economic-GHG farm model. The model calculated GHG emissions using the LCA approach and was extended to quantify GHG emissions using the IPCC method. The study found that the method of reporting GHG emissions (per unit of product or per unit area) affected the ranking of emissions of dairy systems investigated. Greenhouse gas emission were greater when calculated using the LCA method rather than the IPCC method. Both methods found reducing inputs or the intensity of dairy production reduced GHG emissions per hectare. When emissions were expressed per unit of product the methodologies did not rank farming systems in the same order. The effect of feed system on emissions per unit of product was inconsistent between methodologies because the IPCC method excludes indirect GHG emissions from farm pre-chains, i.e. concentrate production. Both methodologies agreed that animals selected solely for milk production (HP) had higher GHG emissions per unit of product relative to strains selected on a combination of traits. The results indicate that if dairy systems targeting a net reduction in global GHG for projected increases in meat and milk production are to be developed, a holistic approach such as LCA, should be used to assess emissions on a per unit product basis.