Atmospheric methyl halides and dimethyl sulfide from cattle

Abstract

We have measured emissions of CH3C1, CH3Br, and (CH3)2S (DMS) from Holstein cows. In one experiment, two cows were studied in separate metabolic research chambers for a 24-hour period while on a normal diet and were studied for an additional 24-hour period 1 week later after being placed on a diet enhanced in chloride and bromide. Methyl chloride emissions ranged between 0.4 × 10-3 and 1.5 × 10-3 g cow-1 d-1, while methyl bromide emissions were much smaller, 3 × 10-6-2 × 10∼5 g cow-1 d-1. Daily emissions of methane from these cows were 134-180 g cow-1 d-1, quite similar to values found in many previous studies. A second 24-hour study of two different cows on normal diets yielded daily emissions of 0.6 × 10-3 and 0.9 × 10-3 g CH3C1, 0-1.0 × 10-6 g CH3Br, and 191 and 176 g CH4. If these emissions of CH3C1 and CH3Br are representative of the 1.3 billion head of cattle worldwide, then the global source of atmospheric CH3C1 and CH3Br from cattle would be 0.23-0.70 Gg yr-1 and (1-10) × 10-3 Gg yr-1, respectively. These emissions of CH3C1 and CH3Br represent <0.02% and <0.005%, respectively, of the total annual global atmospheric sources of these compounds; therefore, emissions of CH3C1 and CH3Br from cattle are insignificant contributors to their total sources. Discovered serendipitously, DMS emissions were between 0.17 and 0.24 g cow-1 d-1, and chloroform emissions were 2 × 10-4-3 × 10-3 g cow-1 d-1. DMS from cattle is not a major source over hemispheric or global scales but could be important in certain geographical regions. Chloroform (CHC13) emissions were similarly detected and quantified, as were those of C2H5X (X = C1 or Br).