Methane emissions from pastoral systems: the situation in New Zealand

Methane is the major greenhouse gas of concern to countries like New Zealand and Uruguay, which have large ruminant and small human populations. The paper reviews the major factors affecting methane emission from ruminants. The relationship between DM intake and methane emission (g/d) is positive, but not strong. However, there is a stronger, but negative correlation when methane emission per unit of feed intake (kJ/100 kJ) is plotted against DM intake, suggesting that as intake increases the percentage of dietary energy lost as methane decreases. Starch and lipid are negatively correlated and fibre positively correlated with methane emission. The relationship between digestibility and methane is confounded with the effects of feed intake level: at low intakes methane increases as digestibility increases, but at high intakes methane decreases as digestibility increases. The SF6 tracer technique for estimating the methane emission of individual grazing animals is described and evaluated. Provided care is taken with SF6 permeation tube calibration it is considered that the technique gives reliable and repeatable estimates. Possible methane mitigation strategies are discussed including: reducing livestock numbers, increasing the efficiency of livestock production, exploiting natural between-animal variation in methane emission, dietary chemical additives that reduce methane, immunisation and manipulation of the rumen microbial ecosystem. While there are many interesting possibilities, more research is required before any of them is likely to be economically feasible. Methane mitigation strategies being developed in New Zealand to meet its obligations to the Kyoto Protocol are discussed. Research is focussed on developing accurate inventory methodology based on field measurements of various livestock classes and pasture types, and on possible mitigation technologies such as evaluating the cause of between-animal differences, plant inhibitors, and manipulation of the rumen microbial ecosystem