Australia’s northern rangelands accommodate 54.5% of the national beef herd. However, poor quality pastures, seasonal rainfall and low animal productivity are associated with high methane (CH₄) emissions intensity/unit animal product (Charmley et al., 2008). A number of methodologies can measure individual animal emissions, but are not applicable to estimating CH₄ flux in extensive grazing environments. The smallest unit of measure to characterise livestock greenhouse gas emissions across N Australia will be at the herd scale. To date, methane emissions for herds in the rangelands of Australia have been determined using an open path laser methodology. This approach measures a line averaged methane flux, but is unable to correlate flux with pasture intake because this cannot be measured. Using live weight (LW) and LW change may be a suitable proxy for predicting emissions from grazing cattle. The objective of this study was to collate collected baseline CH₄ emissions for beef cattle across N Australia and relate these to LW to determine if a direct relationship at a herd scale could be established.
Methane emissions for 370 cattle including steers, cows and heifers (from 220 to 435 kg LW) grazing northern pastures at eight sites across Queensland and the Northern Territory were estimated using an indirect open-path spectroscopic / backward Lagrangian Stochastic Dispersion technique. Mean (±sem) CH₄ emissions were calculated from 10 min averaged data and ranged from 108 ± 4.5 to 340 ± 16.8 g CH₄ /hd per day. These are the first baseline values to be generated for a range of extensive beef production systems. 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 a range of tropical grasses. The relationship between methane emissions and LW explained ~ 33% of the variability, the remaining could be explained by pasture composition and quality. A combination of LW and LW change data obtained on a regular basis may yield a more accurate estimate of GHG emissions for beef production systems. Advances in recent technologies including remote weighing stations have the ability to measure both of these variables accurately and with minimal effort. Remote weighing systems deployed at CSIRO Lansdown Research Station and other properties in Queensland aim to calculate methane emissions following this approach and send the data through an internet gateway.