A process-based emission model for volatile organic compounds from silage sources on farms

Authors: Bonifacio, Henry; Rotz, Clarence - Al; HAFNER, SASHA - University Of Southern Denmark; MONTES, FELIPE - Pennsylvania State University; COHEN, MATHEW - University Of California; MITLOEHNER, FRANK - University Of California

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2016
Publication Date: 12/21/2016
Citation: Bonifacio, H.F., Rotz, C.A., Hafner, S., Montes, F., Cohen, M., Mitloehner, F. 2016. A process-based emission model for volatile organic compounds from silage sources on farms. Atmospheric Environment. 152:85-97.

Interpretive Summary: Silage contributes to volatile organic compounds (VOCs) emitted from dairy farms. In the presence of sunlight, these VOCs react with oxides of nitrogen forming ozone, which can contribute to air pollution and human health problems. A component model was developed and incorporated into USDA’s Integrated Farm System Model, for simulating the effects that various storage and feeding management strategies have on VOC emissions from silage. The revised farm model appropriately simulated emissions of the most important silage VOC (ethanol) from conventional silage piles, total mixed rations, and loose corn silage as measured at a commercial dairy farm. Simulation of a dairy farm showed that most VOC emissions were from feed lying in feed lanes, indicating that strategies to reduce VOC emissions during feeding will be most effective in mitigating farm emissions.

Technical Abstract: Silage on dairy farms can emit large amounts of volatile organic compounds (VOCs), a precursor in the formation of tropospheric ozone. Because of the challenges associated with direct measurements, process-based modeling is another approach for estimating emissions of air pollutants from sources such as those from dairy farms. A process-based model for predicting VOC emissions from silage was developed and incorporated into the Integrated Farm System Model (IFSM, v. 4.3), a whole-farm simulation model of crop, dairy, and beef production systems. The performance of the IFSM silage VOC emission model was evaluated using ethanol and methanol emissions measured from conventional silage piles (CSP), silage bags (SB), total mixed rations (TMR), and loose corn silage (LCS) at a commercial dairy farm in central California. With transport coefficients for ethanol refined using experimental data from our previous studies, the model performed well in simulating ethanol emission from CSP, TMR, and LCS; its lower performance for SB could be attributed to possible changes in face conditions of SB after silage removal that are not represented in the model. For methanol emission, lack of experimental data for refinement likely caused the underprediction for CSP and SB whereas the overprediction observed for TMR can be explained by uncertainty in measurements. Despite these limitations, the model is a valuable tool for comparing silage management options and evaluating their relative effects on the overall performance, economics, and environmental impacts of farm production. As a component of IFSM, the silage VOC emission model was used to simulate a representative dairy farm in central California. The simulation showed most silage VOC emissions were from feed lying in feed lanes and not from the exposed face of silage storages. This suggests that mitigation efforts, particularly in areas prone to ozone non-attainment status, should focus on reducing emissions during feeding. For the simulated dairy farm, a reduction of at least 35% was found if cows were housed and fed in a barn rather than in an open lot, and 33% if feeds were delivered as four feedings per day rather than as one.