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Methane Emissions from Free-Ranging Cattle: Comparison of Tracer and Integrated Horizontal Flux Techniques

^a Dep. of Chemistry, Univ. of Wollongong, Wollongong, NSW, Australia
^b National Inst. of Water and Atmospheric Research (NIWA), Wellington, New Zealand

^* Corresponding author (griffith{at}uow.edu.au).

Received for publication October 2, 2006. Accurate measurements of methane (CH₄) emission rates from livestock in their undisturbed natural environments are required to assess their impacts on radiative forcing (i.e., enhanced greenhouse effect) and the environment. Here we compare results from two nonintrusive techniques for the measurement of CH₄ emissions from cattle. The cows were kept in an outdoor feeding strip that allowed them to follow natural behavioral patterns but contained them within a well defined space. In the first technique, nitrous oxide (N₂O) was released as a tracer at the upwind edge of the feeding strip, and the downwind concentrations of N₂O and CH₄ were measured simultaneously using Fourier transform infrared (FTIR) spectroscopy. Average CH₄ emission per cow was calculated each half-hour on three separate days from the correlation between the two gases. The second technique was the integrated horizontal flux (IHF) or 1-D mass-balance method, in which we used the measured vertical profiles of CH₄ concentration and windspeed downwind of the cows to determine the total CH₄ emission. Comparing the IHF results to the known release rate of N₂O allowed us to test the IHF technique independently. We found agreement within 10% for all comparisons on all days. The daily CH₄ emission rate averaged over all tracer and IHF measurements was 342 g CH₄ head^–1 d^–1. This is within the range of previous measurements for mature lactating dairy cattle (200–430 g CH₄ head^–1 d^–1) but higher than expected for yearling cattle. The high CH₄ emissions are accompanied by high CO₂ emissions determined from the FTIR measurements. The bias is most likely due to the measurements being made during and after supplementary feeding of the cattle.

Abbreviations: IHF, integrated horizontal flux • FTIR, Fourier transform infrared