Measuring emissions of volatile organic compounds from silage

Volatile organic compounds (VOCs) are important precursors to smog and ground-level ozone production. While the largest anthropogenic sources of VOCs in the US are industry and motor vehicles, there is evidence that significant emissions may come from dairy farms, and in particular, silage. An experimental protocol was developed to obtain undisturbed silage samples from silage storages. Samples were placed in a wind tunnel where temperature, humidity and air flow were controlled to measure their influence on VOC emission rates from silage surfaces. Measurements of ethanol, methanol and acetic acid emissions were high immediately after fresh silage was exposed to circulating air, declined rapidly over the next 2 h, and then slowly declined over a 24 h period. A change in temperature from 5°C to 20°C caused up to a fourfold increase in emission rate, and the magnitude of this increase was different among the three compounds. The velocity of air moving over the silage affected VOC emission rate, cumulative emissions and the pattern of emissions through time. Predominant VOC emissions varied between corn, mixed grass-legume and alfalfa silages. Ethanol emissions were highest in corn silage, where 8-hour emissions sometimes exceeded 100 g/m². Eight-hour cumulative methanol emissions were as high as 39 g/m² from mixed grass-legume silage. These results have important implications for accurate field measurement of VOC emissions from silage.