Emission of volatile organic compounds from silage: compounds, sources, and implications

Authors: Hafner, Sasha; HOWARD, CODY - University Of California; Muck, Richard; FRANCO, ROBERTA - University Of California; MONTES, FELIPE - Pennsylvania State University; GREEN, PETER - University Of California; MITLOEHNER, FRANK - University Of California; Trabue, Steven - Steve; Rotz, Clarence - Al

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2013
Publication Date: 5/9/2013
Citation: Hafner, S.D., Howard, C., Muck, R.E., Franco, R.B., Montes, F., Green, P.G., Mitloehner, F., Trabue, S.L., Rotz, C.A. 2013. Emission of volatile organic compounds from silage: compounds, sources, and implications. Atmospheric Environment. 77:827-839.

Interpretive Summary: Emission of volatile organic compounds (VOCs) from silage, a common type of fermented cattle feed, has been identified as a leading contributor to poor air quality in the San Joaquin Valley in central California. Emission measurements have shown that silage emits numerous VOCs (more than 50) that can contribute to ozone formation in the troposphere. The study of silage emissions has focused on the San Joaquin Valley; however, considering the widespread use of silage, it likely affects air quality in other regions. Interest in silage VOC emissions is a recent development, and there is little data on VOC production and emission from silage. At this time, it is not possible to make reliable estimates of the magnitude of VOC emissions from farms, or to identify management practices that can reduce those emissions. Based on a review of studies on silage VOC emission, concentration, and production pathways, we conclude that alcohols contribute more to corn silage total VOC emission and the resulting ozone formation than any other VOC group. Aldehydes, esters, and acids also may have a significant effect under some conditions. More research is needed on measuring VOC concentration and production in different silage types and under different management conditions. This should include accurate on-farm measurements of VOC emission, including an assessment of the importance of individual ensiling stages. Such measurements could confirm the importance of VOC groups, identify those stages that are most important for VOC emission, help assess the effects of management practices, and provide data for estimating VOC emission from farms or regions. Further work is also needed on approaches for reducing VOC production in silage, including the effect of biological and chemical silage additives. Better understanding of VOC production and emission from silage will lead to the development of practices for mitigating emissions and improving air quality.

Technical Abstract: Silage, fermented cattle feed, has recently been identified as a significant source of volatile organic compounds (VOCs) emitted to the atmosphere. A small number of studies have measured VOC emission from silage, but not enough is known about the processes involved to accurately quantify emission rates and identify practices that could reduce emissions. Through a literature review, we have focused on identifying the most important compounds emitted from corn silage (the most common type of silage in the US) and their sources by quantifying their production and emission potential in silage, and describing production pathways. We reviewed measurements of VOC emission from silage, and assessed the importance of individual silage VOCs through a quantitative analysis of VOC concentrations within silage. Measurements of VOC emission from silage and VOCs present within silage indicated that alcohols are generally the most important compounds emitted from corn silage in terms of mass emitted and potential ozone formation. Ethanol is the dominant alcohol in corn silage; excluding acids, it makes up 70% of the mean mass of VOCs present. Acids, primarily acetic acid, may be important when emission is high and all VOCs are nearly depleted by emission. Aldehydes and esters, which are more volatile than acids and alcohols, are also important when exposure is short, limiting the emission of more abundant compounds. Variability in silage VOC concentrations is very high. For most alcohols and acids, tolerance intervals indicate that 25% of silages have concentrations a factor of two away from the median, and possibly much further. This observation suggests that management practices can significantly influence VOC concentrations. Variability also makes prediction of emissions difficult. The most important acids, alcohols, and aldehydes present in silage are probably produced by bacteria (and, in the case of ethanol, yeasts) during fermentation and storage of silage. Aldehydes may also be produced aerobically by spoilage microorganisms through the oxidation of alcohols. Abiotic reactions may be important for production of methanol and esters. Although silage additives appear to affect VOC production in individual studies, bacterial inoculants have not shown a consistent effect on ethanol, and effects on other VOCs have not been studied. Production of acetic acid is understood, and production could be minimized, but a decrease could lead to an increase in other, more volatile, VOCs. Chemical additives designed for controlling yeasts and undesirable bacteria show promise for reducing ethanol concentrations in corn silage. More work is needed to understand silage VOC production and emission from silage, including: additional measurements of VOC concentrations or production in silage of all types, and an exploration of the causes of variability; accurate on-farm measurements of VOC emission, including an assessment of the importance of individual ensiling stages and practices that could reduce emission of existing VOCs; and studies to better understand the production of silage VOCs and possible approaches for reducing production.