Beef cattle methane emissions measured with tracer-ratio and inverse dispersion modelling techniques

The development and validation of management practices to mitigate greenhouse gas (GHG) emissions from livestock require accurate emission measurements. This study assessed the accuracy of a practical inverse dispersion modelling (IDM) technique to quantify methane (CH4) emitted from a small cattle herd (16 animals) confined to a 63 m x 60 m experimental pen. The IDM technique calculates emissions from the increase in the CH4 concentration measured downwind of the animals. The measurements were conducted for 7 d. Two types of open-path (OP) gas sensors were used to measure concentration in the IDM calculation: a Fourier transform infrared spectrometer (IDM-FTIR) or a CH4 laser (IDM-Laser). The actual cattle emission rate was measured with a tracer-ratio technique using nitrous oxide (N2O) as the tracer gas. We found very good agreement between the two IDM emission estimates (308.1 +/- 2.1 - mean +/- SE - and 304.4 +/- 8.0 gCH(4) head(-1) d(-1) for the IDM-FTIR and IDM-Laser respectively) and the tracer-ratio measurements (301.9 +/- 1.5 gCH(4) head(-1) d(-1)). This study suggests that a practical IDM measurement approach can provide an accurate method of estimating cattle emissions.