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

Mei Bai,Trevor W Coates,Thomas K Flesch,David G Mayer,Deli Chen,Nigel W Tomkins,Julian Hill,José I Velazco,Roger S Hegarty,Frances A Phillips

doi:10.5194/amt-14-3469-2021

Abstract

Abstract. 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 × 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 g CH4 head−1 d−1 for the IDM-FTIR and IDM-Laser respectively) and the tracer-ratio measurements (301.9 ± 1.5 g CH4 head−1 d−1). This study suggests that a practical IDM measurement approach can provide an accurate method of estimating cattle emissions.

Highlights

Agriculture is the main source of anthropogenic methane (CH4) emitted to the atmosphere, which includes emissions from ruminants, rice agriculture, waste treatment, and biomass burning (Solomon et al, 2007)
We are very confident in the tracer-ratio measurements given the conceptual simplicity of the approach, given that the Open-path Fourier transform infrared (OP-FTIR) is a very sensitive gas sensor, and given the agreement between the associated inverse dispersion modelling (IDM) measurements
The tracer ratio technique is a “gold standard” for measuring cattle emissions in an ambient outdoor environment. This technique is difficult to use given the need to outfit the animals with tracer sources and the requirement to monitor tracer gas concentrations downwind

Summary

Introduction

Agriculture is the main source of anthropogenic methane (CH4) emitted to the atmosphere, which includes emissions from ruminants, rice agriculture, waste treatment, and biomass burning (Solomon et al, 2007). Methane is an important greenhouse gas (GHG) with a global warming potential that is 28 times that of carbon dioxide (CO2) in a 100 year time frame (Myhre et al, 2013). Enteric CH4 from livestock is a major source of GHG emissions. A significant effort is being made to mitigate these emissions through diet modification, feed supplements, farm management, grazing strategies, and animal breeding (Min et al, 2020; Vyas et al, 2018), with ruminant nutritional management strategies seen as the most direct impact mitigation option (Cottle et al, 2011). On-farm CH4 emissions from beef cattle have been measured using three main techniques: 1. On-farm CH4 emissions from beef cattle have been measured using three main techniques: 1. Portable respiration hoods for tethered and non-tethered animals (Garnsworthy et al, 2012; Zimmerman and Zimmerman, 2012) directly measure the gas concentration of incoming and exhaust air from individual

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