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Mechanisms for Nitrogen Oxide Formation during Ensiling of Dairy Feeds
Citation:
Kosusko, Mike, S. Chilingaryan, P. Green, M. Cohen, AND F. Mitloehner. Mechanisms for Nitrogen Oxide Formation during Ensiling of Dairy Feeds. In Proceedings, Air & Waste Management Association 2015 Annual Conference and Exhibition, Raleigh, NC, June 21 - 25, 2015. Air and Waste Management Association, Pittsburgh, PA, 1, (2015).
Impact/Purpose:
Nitrogen oxide gases (NOx) are important air pollutants, particularly in regions with summer-time ozone and wintertime fine particulate matter (PM2.5) exceedances such as the San Joaquin Valley of California3, 4. This experiment investigated the mechanisms that may be causing the formation of nitrogen oxide gases that occur very early in the ensiling process and may be primarily enzymatic (i.e., pre-existing biochemical action on precursors present in the harvested plant) or microbial. This study pertains to mechanisms only, not a field study, nor an effort to measure emissions or plan mitigation or controls. To store animal feed from the time of harvest over a period of many months (often up to a full year), the feed is ensiled to produce an acidified product that, kept sealed tight from exposure to air, remains stable at ambient temperature. Ensiling is the process of converting the entire harvested feed plant such as corn, sorghum, or alfalfa into fermented, stable anaerobic animal feed (i.e., silage). The common practices of pickling or making sauerkraut are somewhat analogous. The ensiling process is completed in large air-tight structures (silos) or in large piles that are covered with plastic sheeting to minimize exposure to air and the elements. The anaerobic conditions in silos and piles allow anaerobic bacteria to partially break down and acidify the feed plant material both stabilizing it and making it more digestible for dairy cows. In many parts of the country, such as the San Joaquin Valley that can grow their own feed crops, silage (ensiled feed) is a dairy’s most critical feed commodity and its greatest operational cost. The continued use of silage is essential to a highly productive and economically viable industry. Previous work has shown that silages are a major source of volatile organic compounds (VOC) from dairies contributing to the San Joaquin Valley’s (SJV’s) emissions inventory5, 6. The emission of NOx has also been observed during the ensiling process1, 2 and at dairies7. Because NOx can be directly toxic and contributes to the regional air quality problem of ozone and PM2.5, its formation is important to understand. In addition, there is a California Air Resources Board (CARB)-funded project underway to measure the amount of NOx and VOC emissions from silage at dairy locations in the field in California. The co-authors of this manuscript at the University of California at Davis, are also involved in the CARB-funded project. The investigation described in this paper was completed to provide information on a side issue of the overall CARB-funded effort, the generation of NOx during the initial, aerobic stage of ensiling. Ozone formation and PM2.5 are long-standing air quality challenges in many areas of the country, especially in regions with hot sunny summers and cold winters with valley geography which traps air emissions near the ground. The San Joaquin Valley is such a region. As a result, California air pollution agencies have been diligently identifying, understanding and reducing all sources of VOC and NOx emissions.
Description:
Silage (ensiled feed), as a dairy’s greatest operational cost, is its most critical feed commodity. Ensiling is the process of converting entire harvested feed plants such as corn, sorghum, or alfalfa into fermented, stable anaerobic animal feed (i.e., silage). The continued use of silage is essential to a highly productive and economically viable industry. Previous work has shown that silages are a major source of volatile organic compounds (VOC) and a potential source of nitrogen oxides (NOx) from dairies contributing to the emission inventories for the San Joaquin Valley and South Coast Air Basin in California. Both VOC and NOx are precursors to the formation of ozone and PM2.5. The emissions of ozone and fine particulate matter (PM2.5) are long-standing air quality challenges in many areas of the country, but particularly in these California locations. As a result, California has been diligently identifying, understanding and reducing all sources of VOC and NOx emissions.The emission of NOx has been observed during the ensiling process1, 2. Since substantial NOx is not inherently present in corn, it is not released by the processing of corn NOx is generated during the early days of the ensiling process by an unknown mechanism. The underlying question for this investigation was whether the production of NOx is due to biological activity from the growth of microbes or whether the production of NOx is enzymatic, using precursor compounds already present in the harvested plant matter. Hence, our goal was to better understand mechanisms that could generate NOx emissions from silage. To understand the mechanism for NOx generation, NOx emissions during ensiling were compared between untreated control samples and treatment by (a) sterilizing the microbes that are inherently present in chopped corn, including all parts of the plant, to discern whether NOx formation during ensiling is microbial or is due to pre-existing plant enzymes, and (b) testing three chemical inhibitors to limit the activity of the peroxidase enzyme that is the most likely candidate to produce NO2 from nitrate.This paper will describe our test procedures, the results from testing, and conclusions and recommendations resulting from this effort.