Methane emissions from beef cattle - a comparison of paddock- and animal-scale measurements

Abstract

Methane (CH4) emissions from a herd of 58 grazing cattle were determined in a field experiment using paddock-scale (micrometeorological) methods. The emissions were also measured daily from each animal, using the sulfur hexafluoride tracer method. The paddock-scale methods exploit how the gas, once emitted from the cattle, is transported and dispersed by the wind. Hence, the emission rate may be calculated from measurements of windspeed, wind direction and turbulence statistics, as well as CH4 concentration upwind and downwind of the herd. The paddock-scale methods include a mass-budget approach, flux-gradient method and gas dispersion model. Accuracy can be assessed in unprecedented detail because the animal-scale (reference) method included all individuals in the herd, and the measurement site was ideal for micrometeorological methods (flat, usually windy and free of obstructions that alter the turbulent airflow). The cattle were hand-reared steers of average weight 325 ± 20 kg. Based on the animal-scale method, the average CH4 emission rate over 9 days was 161 ± 20 g/steer.day. The gas dispersion model, when utilising vertical concentration profiles, yielded on average 27% greater emissions. The other paddock-scale methods agreed with the animal-scale method, provided the cattle were at least 22 m away from the location of the downwind concentration measurements. When the cattle were allowed to graze as closely as 5 m from the instruments, the paddock-scale methods gave greater emissions than the animal-scale method; reasons for this are discussed.

Eddy covariance methane flux measurements over a grazed pasture: effect of cows as moving point sources

“…Direct measurements of enteric CH 4 emissions are commonly made on individual animals using open-circuit respiration chambers Kreuzer, 2006, 2008) or the SF 6 tracer technique (Lassey, 2007;Pinares-Patiño et al, 2007). Both methods are labor intensive and thus are usually applied only for rather short time intervals (several days).…”

“…These approaches are based on average concentration measurements: backward Lagrangian stochastic 3926 R. Felber et al: Eddy covariance methane flux measurements over a grazed pasture dispersion, mass balance for entire paddocks, and gradient methods (Harper et al, 1999;Laubach et al, 2008;Leuning et al, 1999;McGinn et al, 2011). They have in common that they integrate over a group of animals and are usually applied over specifically designed relatively small fenced plots.…”

Developments in micrometeorological methods for methane measurements

“…Laubach et al (2008) reported a good fit of CH 4 emissions from cattle in small paddocks when using the BLS, FG and IHF techniques (plus an enteric tracer technique) simultaneously, provided that a minimum separation between the paddock and sensor location exists when using the FG and IHF techniques. In their study, using 22 m separation, the BLS emissions were correlated with IHF (R 2 5 0.85) and FG (R 2 5 0.75); however, the BLS emissions were 20% higher than the two techniques.…”

“…Since then, this technique has also been applied to grazing cattle (Laubach et al, 2008). Like other micrometeorological techniques, FG is best suited to operate in the fully adjusted IBL; else a footprint model (Vesala et al, 2008) must be used to gauge the influence of the source configuration on the calculated emission (see the discussion below).…”

“…To be independent of wind direction, the MMD requires a horizontal line averaged of concentration along each source perimeter, at several heights on all perimeters (i.e., the face), thereby enclosing the entire source. Laubach et al (2008) used the IHF technique to determine the enteric emissions from cattle grazing ryegrass in small paddocks. They reported an average enteric CH 4 emission of 161 g/animal per day.…”