Research Program Area: Emissions Monitoring & Control
Previous work has shown that dairy silages are a major emission source of volatile organic compounds (VOCs) and to some unknown degree of oxides of nitrogen (NOx), both contributing to the San Joaquin Valley's (SJV) ozone challenges. In general, emission of VOCs from silage can be mitigated by either 1) reducing VOC production in the liquid/solid phase of the silage pile, or 2) reducing relative emission from the face of the silage pile or the feedlane. While NOx mainly forms in the initial phase of ensiling, the current research focused on the later stages of storage and feedout of silages; therefore, the NOx picture of the present report is incomplete and requires further research. The focus of the present research was on monitoring and modeling of VOC production using silage additives, as well as emissions mitigation via various silage storage methods, de-facing practices, and feed management approaches. Microbial and chemical silage additives were investigated using bucket silos, to reduce the production and emissions of volatile organic compounds in corn silage. The VOC concentrations were measured using headspace gas chromatography method. For the field monitoring of emissions from different silage storage and defacing methods, flux chambers and wind tunnels that were attached vertically on the silage face were used immediately after de-facing. These sampling devices were attached to a fully equipped mobile air quality lab, in which concentrations of all relevant gases were analyzed in situ. This set-up allowed the comparison of different storage methods (i.e. conventional standard pile vs silage bag), and defacing methods (e.g., perpendicular, lateral, and rake extraction), as well as various water inclusion rates for the feed all aiming at reducing emissions. The monitoring data was used to inform and validate a new VOC process-based model that was developed to predict VOC emissions from silage sources on farms using theoretical relationships of mass transfer and parameters determined through earlier (published) laboratory experiments and numerical modeling. The results for the silage additive studies showed, that most microbial and chemical additives actually increase VOC production and emissions. Only one chemical additive used at one particular concentration, reduced VOCs. The results for silage storage indicated that silage bags vs. conventional silage piles emit considerably fewer emissions. Furthermore, lateral defacing versus perpendicular- and rake defacing reduced emissions of most gases. Finally, reducing of emissions in the feed lane seems to be possible via inclusion of water to the TMR. Simulations of all relevant silage mitigation options that were studied on the commercial dairies, were conducted using the VOC modeling tool. These simulations clearly showed that most of the reactive VOC emissions on a California dairy occur from feed lying in feed lanes during feeding as opposed to the silage storage pile or bag. In conclusion, regulations aimed at reducing VOC emission could be ineffective or even increase emission if they promote silage additives without recognition of different types of additives. The monitoring results of the storage and defacing study results point at certain practices as being advantageous. However, one shall not view those monitoring results in isolation, because only the integration of other parts of the feed's life cycle, using whole farm modeling, explains not just the relative- but also the absolute effectiveness of mitigation techniques in reducing VOCs and NOx on the entire dairy. The whole farm modeling clearly showed that mitigation efforts should be applied to reducing emissions from feeding rather than focusing solely on those from the exposed face of silage piles.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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