Methane emissions
Measuring methane emissions from flocks of free-ranging sheep. Image - F Kellihier
Agriculture
The release of methane gas from ruminant livestock (sheep and cattle) amounts to almost 1/3 of New Zealand's greenhouse gas emissions, and it is the largest contributor. Methane also accounts for over 40% of all emissions in terms of global warming potential. However, internationally the dominant sources of methane are rice paddies and wetlands, not farm animals. New Zealand therefore has a special interest in the measurement and mitigation of methane emissions from livestock. The inventory of NZ's greenhouse gas emissions is calculated annually by the Ministry for the Environment and reported internationally. According to this inventory, methane emissions from ruminants have increased by 10 % since 1990. (Over the same period, carbon dioxide emissions from road transport have grown by 62 %, and nitrous oxide emissions from agricultural soils by 25 %.)
Measurements of methane emission rates on sheep and dairy cows have repeatedly shown that the variability of emissions between individual animals is large (e.g. for young sheep grazing the same pasture, emission rates varied from 9 to 35 g/day per sheep). While this variability may one day be exploited to reduce methane release by selecting for low-emitting animals, it is not well understood. It also leads to uncertainty in the national inventory calculations, which are based on measurements at the "animal scale", i.e. from small samples of animals (typically 20). In order to quantify this uncertainty, data representative for larger numbers of animals under typical farm management practice are needed, in other words, measurements at the "farm scale" or "paddock scale" are required.
Johannes Laubach and Frank Kelliher of Landcare Research have developed micro-meteorological techniques for measuring paddock-scale rates of net methane emissions from herds or flocks of freely grazing animals. "Micro-meteorological" means that wind transport of the emitted gas, away from the animals, is used as a vehicle to determine the emission rates. To this end, wind speed, direction and turbulence parameters are measured, as well as the air's methane concentration (see photo). For the animals, these techniques are unobtrusive, and are thus representative of normal behaviour and physiology. The techniques have been successfully used on commercially-managed farms.
When applying these techniques, Johannes and Frank collaborate with AgResearch colleagues who make measurements on selected animals from the herd or flock under investigation. This allows them to compare paddock-scale data to animal-scale data; so far there has always been good agreement. The work is currently focused on dairy cattle. On dairy farms in Canterbury, with herds of 270 to 550 cows, seasonal methane emission rates ranged from 284 to 427 g/day per cow.
Collaborators: AgResearch
Funding: Funding for this work was provided by the Foundation for Research, Science and Technology
Sheep
Methane (CH4) is the largest contributor to New Zealand's national inventory of greenhouse gas emissions, and it also comprises 45% of all emissions in terms of global warming potential. Per capita, New Zealand has the largest methane emission rate (0.6 t per person per year)—six times the global average. The methane comes primarily from enteric fermentation in ruminant livestock, and sheep are the greatest single source. The first measurements of methane emission rates from grazing sheep were made in New Zealand by NIWA and AgResearch during Autumn 1996. These emissions ranged four-fold from 9–35 g/day. Such large differences between individual animal's methangenetic response to digestion could potentially be exploited as an emissions mitigation strategy.
The wide variability in these measurements of methane emissions from sheep highlighted the need for larger-scale data that could be integrated into of greenhouse gas inventories. Frank Kelliher and colleagues, in collaboration with HortResearch, have developed a micro-meteorological technique for unobtrusively measuring paddock-scale rates of net methane emissions from flocks of free-ranging sheep. The technique also takes into account the oxidation or consumption of methane by soil bacteria. Collaboration with NIWA and AgResearch led to comparative micro-meteorological (‘top down') and sheep and soil (‘bottom up') measurements being made during Autumn 1997. There was good agreement by the two methods with an average net methane emission rate from sheep populated lowland pasture of 160 kg/ha per year.
Collaborators: AgResearch; HortResearch; NIWA
Funding: Funding for this work was provided by the Foundation for Research, Science and Technology