Air Quality Issues with the Dairy Forage System

California is the nation’s leading dairy state. Unfortunately, it is also home to two of the three worst air-sheds with respect to ozone (smog) pollution. California is also the second largest contributor of Greenhouse Gases (GHG; e.g., methane) in the nation. Current regulatory estimates suggest that dairy cows in central California emit smogforming, so-called volatile organic gases (VOCs) at rates higher than those from vehicles, and thus contribute significantly to the region’s extreme ozone non-attainment status. During decades of VOC studies at UC Riverside and UC Berkeley, researchers identified plants as one of the most significant VOCs sources (e.g., oak trees emitting isoprenes). It is estimated that approximately 6% of net primary crop production in the San Joaquin Valley is fed to and consumed by dairy cows, which then carry out enteric fermentation leading to emission of VOCs during eructation. To investigate dairy cow and waste emissions, experiments were conducted in environmental chambers at UC Davis to simulate emissions from freestall cow housing conditions. State-of-the-art analytical equipment was used to measure a suite of smogforming VOCs emitted from dry and lactating dairy cows and their waste. Furthermore, greenhouse gases (methane, and nitrous oxide) and ammonia were measured. The total emissions for all measured organic carbonaceous gases (VOCs and methane) was dominated by methane. When cows were present in the chambers, VOCs were less than one percent of total organic gas (TOG), which is a factor of 10 smaller than that historical estimates used currently by the air regulatory agencies. Certain feed materials like silage emitted considerable amounts of VOCs (i.e. alcohols), which needs to be investigated further. Current air regulatory estimates for VOC emissions appear to be overestimated while greenhouse gas emission estimates (i.e. methane) are underestimated. Furthermore, the ozone-forming-potential (OFP) of the most abundant VOCs measured in our experiment is only about 20% those of typical combustion or plant-derived VOCs. This implies dairy cattle and their fresh waste have a smaller effect on ozone formation but a larger effect on GHG than currently assumed by air quality agencies in California.