Abstract
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 H2 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. |