Estimating Air Emissions on Dairy Farms
Dept. of Animal Science
Gases and particulates (dusts) are the two main concerns related to air emissions from animal operations. These emissions are a result of complex processes driven by the animal diet, manure handling practices, and environmental conditions such as temperature, wind and humidity.
Recently, EPA has required that large CAFOs report ammonia and hydrogen sulfide emissions under Emergency Planning and Community Right-to-Know Act (EPCRA).
Producers were uncertain what values to report as little data are publicly available to guide them in estimating how much of any gas their particular operation emits. A unified approach to estimating emissions from livestock operations is needed.
Estimates of air emissions are currently made by applying ‘emission factors’. Emission factors are production unit (i.e., per animal) multipliers that are used to calculate the total farm emissions.
A 2003 report from the National Academy of Science (NRC, 2003) Committee on Air Emissions from Animal Feeding Operations concluded that the existing emissions factors for livestock operations are generally inadequate and inappropriate because of the limited number of measurements on which they are based, as well as the wide emission variability across different kinds of operations.
In the absence of more appropriate values, regulatory agencies will continue to use the emission factors available at present.
The National Air Emission Monitoring Study (NAEMS) is currently being conducted at five dairies across the U.S. in order to contribute data to what is currently available. While the study is winding down (most sites concluded in 2009) it is unknown at present how emission factor values used by EPA will change as a result of the NAEMS data.
Emission factors are most commonly expressed as emissions per animal or per unit body weight. However, animal weight is not a good predictor of emissions. Rather, nutrient intake and the efficiency of its conversion to produce an animal product affect the quantity and composition of emissions and manure excreted by animals.
For this reason, American Society of Agriculture and Biological Engineers (ASABE) Standard D384.2—a standard that is widely used across the U.S. as the basis for determining nutrient excretion values for manure planning purposes—was revised in 2003. More than 30 scientists who contributed to the revision recognized that nutrient intake relative to nutrient needs for animal maintenance and production was the critical control point for determining nutrient excretions. The revised standard now estimates manure nutrients and mass as a function of diet inputs relative to animal nutrient needs. Expressing air emissions as a function of number of animals or total body weight is inappropriate because the number or weight of the animals is not a critical control point for emissions from animal housing. Rather, emission factors should be based on nutrient inputs and productivity of the animals.
Similarly, animal species may not be relevant when it comes to manure storage; loading rate and chemical and physical properties, including chemical composition, biodegradability, microbial populations, oxygen content, moisture content and pH, are the critical control points.
As one considers a systems-wide evaluation of environmental impact of the dairy industry and the impact that mitigation strategies will have on the environment, the manner in which emissions and changes in emissions are expressed is critical. Efforts are underway to develop a Life Cycle Analysis for the dairy industry, including impacts on air quality. Of particular interest is the dairy industry’s contribution to climate change, a topic addressed in recent issues of the Michigan Dairy Review (Bartlett, 2009 and Beede and Powers, 2009).
Dairy Operations & Climate Change
Climate change is a good example for illustrating the need to consider animal efficiency when expressing air emissions. As population increases and the inclusion of animal protein in human diets increases in developing nations, mass reductions in GHG emissions will be difficult to achieve because of enteric fermentation contributions along with fossil fuel use to plant and harvest food and fertilizer use to maximize food production for the growing population on a finite land base. However, large reductions in GHG emissions per unit of product are possible, particularly as use of dietary energy and nutrients improves.
A recent study (Capper et al., 2008) reported that use of rbST corresponds to 6.8 percent reduction in manure mass per unit of milk produced and a 7.3 percent reduction in methane output per unit of milk. In their analyses, industry-wide use of rbST reduced arable land requirements for grain production, soil erosion, nutrient excretions, and the global warming potential of the equivalent of 400,000 passenger cars.
As more research is conducted to mitigate air quality and climate change impacts from the U.S. dairy industry, particularly research to curb methane resulting from the dairy cow itself (enteric fermentation), it is imperative the methane per unit of milk produced be considered as the benchmark and not just gross methane emissions or emissions per animal.
In addition to using dietary means of improving efficiency of nutrient use, another approach is to extend the productive lifetime of the animal to distribute the emissions associated with birth to first calving over more lactations, thereby reducing the “fixed costs” associated with lactation.
While much research is underway to mitigate air emissions, improving animal efficiency and expressing emissions per unit of efficiency or product produced will be central to discussions.
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