Summary
Methane is a powerful greenhouse gas, with over 60% of Australia's methane emissions come from agricultural sources. Ruminant livestock are the single biggest contributors to methane emissions in Australia today. Methane is also a highly concentrated form of energy and a loss of methane represents lost production. Current knowledge shows that methane production from cattle can be influenced by feed intake, feed source, feed processing, addition of rumen modifiers or general changes in rumen micro flora and genetics (Johnson and Johnson, 1995; Hegartyet al, 2007). Pasture quality and type have been clearly demonstrated to influence the level CH4emissions from grazing ruminants.
To refine the emissions estimates from livestock, assess the impact of emission mitigation strategies and verify practice change as a carbon Farming Initiative (CFI) requires CH4emission measurement techniques suitable for use with a range of animal production systems, and with the precision to measure the anticipated changes in CH4emissions.
However options to determine methane emissions from grazing ruminants are limited. The Open-path FTIR plus tracer gas technique is a novel technique to estimate emissions of methane from ruminants in their normal, undisturbed free-grazing environment. The technique releases a tracer-gas at a known, controlled rate close to the mouth of the animal with both the tracer-gas and the emitted CH4measured simultaneously by OP-FTIR spectroscopy downwind from the gas source. The technique has been demonstrated to have increased precision in estimating emissions compared with other micrometeorological techniques (Baiet. al. 2009) and is of particular interest for measuring CO2and CH4emissions from smaller groups of grazing animals where the emissions from the source area are not uniform. The technique is less intrusive for the animals and provides a herd averaged emissions and, with a (typically) 3-minute temporal resolution, provides information on the distribution of emission over the day highlighting changes in emissions with animal behaviour.
This project aimed to demonstrate the operation, advantages and limitations of open-path FTIR spectroscopy to measure CH4emissions from livestock in a grazing environment with the objective of developing the capability within the research community in measuring greenhouse gas emissions using the technology. As livestock production systems in Australia are highly varied, this project measured methane emissions from four production systems: dairy cows in high rainfall region in Victoria, beef steers in north Queensland, representing the northern Australian rangelands; sheep grazing pasture of differing quality on the Tablelands of NSW; and sheep grazing two pasture systems in the sheep-wheat region of WA. By working with the staff from the University of Wollongong staff at each research institute gained an understanding of the advantages and limitations of the technique and the requirements to obtain quality emissions data and allowed researchers to assess if the technique can be of an advantage within their research programs.
In association with measuring emissions from the livestock systems, the University of Wollongong staff demonstrated the open path FTIR technology at four Field days. The demonstrations generated considerable interest with primary producers, giving producers an understanding of the technologies available to help them manage emissions from their production system.
The OP-FTIR tracer gas technique has been shown to have a precision of 5-10% under favourable conditions, and can provide an emission estimate each 3 minutes over 24 hours.
This highlights the relationship between CH4 production and animal behaviour. Emission measurements from sheep showed emissions were typically highest in the mid-morning decreasing in the afternoon, with a second but lower maximum in the early evening, and with lowest production at night, correlating with animal grazing patterns. In contrast emissions from the dairy cows increased dramatically from around 300 g CH4 animal-1day-1 before leaving the paddock, to 600 g CH4 animal-1day-1 on returning from the dairy where they received supplementary feed. Emissions from sheep on the New England Tablelands increased dramatically when introduced to the pasture after being constrained at the yards with limited available feed, with emissions reducing to normal levels over the following 12 hours. Emissions from steers in northern Queensland were greatest when leaving the pasture to access water, decreasing from 200 down to 150 g CH4 animal-1day-1 over the following 5 hours.
The project has provided baseline data for CH4 emissions from the four animal production systems across Australia. Comparison of emissions from sheep for the systems studied showed that emissions per animal were greatest for sheep grazing pasture typical of the region in WA (29.7±0.6 g CH4animal-1day-1), while sheep grazing the high quality pasture on high fertility soils on the New England Tablelands showed the lowest emissions (15.5±1.0 g CH4animal-1day-1), and emissions from sheep in the other systems were comparable (19.5±1.0 to 21.1±0.6 g CH4animal-1day-1). However until this data is compared with measures of production, animal live weight gain or feed intake, the difference in emission intensity cannot be confirmed.
The use of a tracer gas relies on placing a gas canister on or near an animal, which is not always feasible. An alternate method of retrieving an emission estimate from the measured methane concentration is to model the dispersion of the gas plume using a backward Lagrangian stochastic model, commonly in the software program WindTrax. However with WindTrax the error for individual emission estimates is reported to be up to 40%, limiting the usefulness of the technique. A comparison of the retrieved emission estimates for both the emitted CH4 and the tracer gas in this work has shown that it is possible to constrain the WindTrax bLs model using the controlled release of a tracer-gas in the area of the animals, offering a technique to measure emissions from remote animals with greater precision.
