Effects of intensification of pastoral farming on greenhouse gas emissions in New Zealand

In 2007, greenhouse gas (GHG) emissions in New Zealand were 16% higher than in 1990. Agriculture accounts for 48% of GHG emissions in New Zealand, and 10-12% of emissions in most other 'developed' countries. Methane (CH4) accounts for 35% of GHG emissions in New Zealand, mostly from ruminal fermentation. Nitrous oxide (N2O) accounts for 17% of GHG emissions in New Zealand, mostly from urinary N, exacerbated by excessive application of nitrogenous fertiliser. GHG are often expressed as carbon dioxide equivalents (CO2-e), and 1 kg CH4 has a similar global-warming potential as 21 kg CO2, whilst 1 kg N2O has the same warming potential as 310 kg CO2. Methane is derived from H-2 produced during ruminal fermentation, and losses account for 6-7% of gross energy in feeds. This is about 9-10% of metabolisable energy intake. Methane production tends to be lower when legumes, rather than grasses, are fed, and emissions are greater (per kg dry matter intake; DMI) when mature grasses and silages are fed. There are small differences between individual animals in their CH4 production (g/kg DMI) but there are few profitable options available for reducing CH4 production in ruminants. Emissions of N2O can be reduced by more strategic application of nitrogenous fertiliser, avoidance of waterlogged areas, and use of dicyandiamide in some cooler regions. GHG mitigation should be based on life-cycle analyses to ensure a reduction in one GHG does not increase another. Current and future strategies are unlikely to reduce GHG emissions by >20%. Food production is central to human survival, and should not be compromised to mitigate GHG emissions. Efforts should be directed toward increasing animal efficiency and reducing GHG emissions/unit edible food.