Mitigation of methane emissions from the northern Australian beef herd

Summary

​Methane (CH4) emissions from cattle grazing pastures characteristic of northern Australia are yet to be reliably quantified. Poor quality pastures, marked seasonal rainfall and low animal productivity are characteristic of northern Australia, but are also associated with high methane (CH4) emissions intensity/unit animal product.​
Currently a number of methodologies are available to measure individual animal emissions; respiration chambers, SF6 (sulphur hexafluoride) tracer technique, or have been suggested; blood methane concentrations, whole body thermography, but these are difficult to use other than at an individual animal level and not applicable to estimating emissions for extensive grazing environments. Northern subtropical and tropical regions account for 54.5% of the national beef cattle herd. The smallest unit of measure to characterise livestock greenhouse gas (GHG) emissions across land and pasture types, bio-agronomic regions and or seasons may be at the herd scale. An on farm methodology is required to generate reliable baseline emission data and to assess the effect of mitigation activities at the herd scale. There are no suitable methods for measuring emissions for the northern beef herd, yet measurement is a critical component for mitigation and a carbon farming framework. This project was conducted to address this deficiency.
Expected outcomes of the project were:validated direct and indirect methods of measuring methane emissions from cattle under northern Australian grazing conditionsimproved understanding of the possible anti-methanogenic properties of important tropical legume plants widely used in northern Australia to improve beef productivityproof of concept to enable methane emissions benchmarking to the level required for carbon accounting systems.
 
This report details the validation and application of a methodology for estimating CH4 emissions at the herd scale using an open-path spectroscopic technique and a backward Lagrangian Stochastic (bLS) dispersion model. Inputs for this model included wind speed, direction, atmospheric turbulence data and line-averaged concentrations of
CH4 determined with an open-path laser to generate a series of unique herd scale data sets for 246 cattle including steers, cows and heifers grazing northern pastures.
Measurements were taken on properties across central and northern Queensland and the Northern Territory for a total of 98 days. In addition, methane production from 13
Brahman steers (mean ± sem liveweight; LW 227 ± 6.2 kg) maintained under animal house conditions and offered 22 diets from combinations of five tropical grass species and five legumes, with a minimum of three steers per diet, was measured using open circuit respiration chambers.
​Daily mean (± sem) CH4 emissions associated with five sites across northern Queensland and the Northern Territory ranged from 136 ± 21.5 g/hd/d for steers grazing an irrigated Rhodes grass pasture to 281 ± 22.3 g/hd/d for Brahman cows grazing a Buffel/ Sabi grass pastures. The lowest emissions were associated with young steers grazing an irrigated and improved pasture fertilised with urea (150 kg/ha) and managed intensively. In comparison, the higher CH4 emissions were associated with mature Brahman cows and heavier steers (LW> 200 kg) grazing either Buffel or Sabi grass dominated pastures, respectively. Methane emissions were within the range of values previously suggested for steers grazing improved and native pastures (Hunter 2007 and McCrabb et al., 1997), although generally higher than the values reported by Kennedy and Charmley (2012) for hay diets ranging from Spear grass (53.9 ± 4.44 g CH4/d) to Buffel grass (159 ± 13.7 g CH4/d) fed to steers under animal house conditions. Emission values, determined using an open-path spectroscopic technique and bLS dispersion model, similar to current IPCC Tier I emission factors (IPCC 2006) were limited to Brahman cross heifers grazing predominantly Native couch (Brachyachne sp.) and Golden Beardgrass (Chrysopogon fallax) dominated pastures indicating that current values used for national inventory purposes are only representative of a small portion of the northern beef herd.
This study was not conducted to assess emission reductions on each site although data collected in the field using an open-path spectroscopic technique/bLS dispersion method could populate a methane model to investigate mitigation outcomes. Emissions at the farm scale will vary depending on pasture type, seasonality and class of ruminant livestock, making the usefulness of herd scale emission data invaluable for national inventories, when a large number of properties can be captured in a series of measurement campaigns. Measurement opportunities that complement normal animal behaviour will be crucial in ensuring a low input methodology for extensive grazing environments. Further development of equipment rather than the methodology is required to have this technology more widely adopted by the industry. Developing a system that interfaces with current technology such as electronic identification systems (EIDs) to identify individual animals and source intensity (number of animals measured) will be beneficial and support long term measurements.