Atmospheric methyl halides and dimethyl sulfide from cattle

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).