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2020 National Emissions Inventory Technical
Support Document: Agricultural Silage
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EP A-454/R-23 -001 q
March 2023
2020 National Emissions Inventory Technical Support Document: Agricultural Silage
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Air Quality Assessment Division
Research Triangle Park, NC
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Contents
List of Tables i
17 Agricultural Silage 17-1
17.1 Sector Descriptions and Overview 17-1
17.2 EPA-developed estimates 17-1
17.2.1 Activity Data 17-1
17.2.2 Allocation Procedure 17-2
17.2.3 Emission Factors 17-2
17.2.4 VOC Speciation 17-3
17.2.5 Controls 17-3
17.2.6 Emissions 17-3
17.2.7 Point Source Subtraction 17-3
17.2.8 Sample calculations 17-4
17.2.9 Improvements/Changes in the 2020 NEI 17-4
17.2.10 Puerto Rico and U.S. Virgin Islands 17-4
17.3 References 17-5
List of Tables
Table 17-1: SCCs for agricultural silage SCCs 17-1
Table 17-2: Sample calculations for VOC emissions from agricultural silage storage 17-4
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17 Agricultural Silage
17.1 Sector Descriptions and Overview
Agricultural silage is fermented cattle feed made from chopped forage like corn or sorghum. Corn silage
is a high energy forage that is readily consumed by cattle, and it is a major component of dairy and beef
cattle diets in the US. VOCs are formed during silage fermentation and can volatilize into the
atmosphere. VOC emissions occur during storage, mixing and feeding processes. During storage,
emissions occur when a silage stack, bunker silo, or silage bag is opened, and silage is removed. During
mixing, emissions occur from the exposed surface of a mixing wagon when the silage is mixed with other
feed components. During feeding, emissions occur from the exposed silage in a feed bunk or feed lane.
The only pollutants that are inventoried for this sector are VOC and VOC HAPs. Table 17-1 shows the
SCCs assigned to various ag silage processes; SCC level 1, 2, and 3 descriptions are "Miscellaneous;
Agricultural Crop Usage; Agricultural Silage" for all SCCs.
Table 17-1: SCCs for agricultural silage
SCC
SCC Level 4 Description
SCC Short Name
2802004001
Storage
Ag Silage - Storage
2802004002
Mixing
Ag Silage - Mixing
2802004003
Feeding
Ag Silage - Feeding
17.2 EPA-developed estimates
The calculations for estimating the emissions from agricultural silage are straight-forward. It involves
multiplying the livestock counts by emissions factors for VOC.
17.2.1 Activity Data
The activity data for this source category are based on dairy and beef cattle livestock counts (average
annual number of standing head) and population information by state and county used to develop U.S.
EPA's Greenhouse Gas Inventory [ref 1], This data set is derived from multiple data sets from the United
States Department of Agriculture (USDA), particularly the National Agricultural Statistics Service (NASS)
survey and census [ref 2], The USDA NASS survey dataset, which represents latest available, 2020
national livestock data, is used to obtain the livestock counts for as many counties as possible across the
United States. For a full description of the GHG livestock population estimation methodology, refer to
the above referenced citation for the EPA's GHG inventory document.
Generally, counties not specifically included in the NASS survey data set (e.g., due to business
confidentially reasons) were gap-filled based on the difference in the reported state total animal counts
and the sum of all county-level reported animal counts. State-level data on animal counts from the GHG
inventory were distributed to counties based on the proportion of animal counts in those counties from
the 2017 NASS census.
Pc,2020 = Ps,2020 X rc,2020 (1)
Where:
17-1
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Pc,202o = Estimated population of dairy or beef cattle in county c
Ps,2mo = NASS survey reported state-level population of dairy or beef cattle in state s
rc,202o = Ratio of county- to state-level animal counts from the 2017 NASS census for dairy or
beef cattle in county c
17.2.2 Allocation Procedure
The USDA survey reports the livestock counts at the county level for many counties, so no allocation is
necessary. The procedure for gap-filling missing county-level data using state-level data is described in
Section 17.2.1. One assumption is that agricultural silage is stored on all dairy farms.
17.2.3 Emission Factors
The emission factors (EFs) developed for ag silage are expressed in units of kg VOC per animal per year
and are the product of three estimated quantities:
1. VOC production within silage
2. Fractional loss of VOC by volatilization; and,
3. Silage feeding rates to cattle
Written as an equation:
EF (kg VOC / animal - year) = production (kg VOC produced [or available] / kg silage DM) *
(2)
loss (kg VOC volatilized / kg VOC produced) * feeding rate (kg silage DM fed / animal - year)
where DM = dry matter.
Emission factors are calculated for three stages: silage storage, feed mixing, and feeding. Estimates are
made for beef and dairy cattle, which are assumed to differ only in silage feeding rates. In total, 6 EFs
are calculated for the entire US, and are meant to capture average conditions. This approach does not
capture differences due to local climate or management.
Silage VOC production is based on the extensive compilation of VOC measurements presented in a
review paper (Hafner et al., 2013) [ref 3], Fractional loss of VOC is calculated by chemical group (acids,
alcohols, esters, and aldehydes) using the mass transfer model described by Hafner et al. (2012) [ref 4],
This model includes parameters for transport through silage and loss from an exposed surface, with
parameter values based on wind tunnel (Montes et al., 2010 [ref 5]; Hafner et al., 2010 [ref 6]) and mass
balance emission measurements made using silage representative of storage or feeding conditions
(Hafner et al., 2012) [ref 4], The inputs for the mass transfer model are average values that are assumed
to be constant for dairy and beef cattle for all US locations. All silage produced in the US is assumed to
be fed to dairy or beef cattle.
Silage feeding rates were estimated from NASS statistics for total silage production and estimated cattle
populations discussed in Section 17.2.2. Total national silage production was for corn, haylage, alfalfa,
and sorghum silage production, and is reported in tons. Reported mass was assumed to be in fresh
mass, and dry mass was calculated by assuming a dry matter (DM) content of 34%. Based on the
Integrated Farm System Model (IFSM) simulation results described in recent national assessments of
dairy and beef cattle (Rotz et al., 2019 [ref 7], 2021 [ref 8]), it was assumed that dairy cattle consume
17-2
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75% of US silage and beef 25% (C. Al Rotz, USDA-ARS, personal communication). Based on this approach,
effective average silage feeding rates were about 2,025 kg DM per year for dairy cows and 155 kg DM
per year for beef cows (5.5 and 0.4 kg DM per d, respectively).
Note that county-level emission factors are a composite based on the county-level distribution of dairy
cattle and beef cattle counts. Minimum, median, and maximum county-level emission factors are
provided in the "Wagon Wheel Emission Factor Compendium" on the 2020 NEI Supporting Data and
Summaries site.
17.2.4 VOC Speciation
VOC speciation is needed to define the HAPs for this sector, as well as to provide speciated VOC
information for air quality models. VOC speciation is determined as the product of VOC production and
the total fractional VOC loss for all three stages, determined individually for each compound. As
discussed in Hafner et al. (2013) [ref 3], the relative importance of individual compounds depends on
their production as well as emission conditions. It should be noted that ethanol is the dominant
component of the VOC. If used, this profile will be renormalized so that it represents 100% of the VOC
mass.
Information on VOC species for these sources are available in the "EIS Augmentation Datasets" on the
2020 NEI Supporting Data and Summaries site.
17.2.5 Controls
There are no controls assumed for this category. However, there are management practices (such as
chemical additives that inhibit yeast and controlling silage density and feed area) known to reduce VOC
emissions from agricultural silage and SLTs should consider whether the dairy farms in their areas
implement any of these practices [ref 9],
17.2.6 Emissions
To estimate VOC emissions from silage from each process, the livestock count of cattle is multiplied by
the emissions factor for the process.
/ ton\ ( ton\
Ec = [EFpdairy x Pcdairy x 0.0011—J + \ EFpheef x Pc>beef x 0.0011— J
Where:
Ec = Annual emissions of VOC in county c, in tons per year
EFP = Emissions factor for VOC for process p for dairy or beef cattle, in kg per cow
Pc = Population of dairy or beef cattle in county c
17.2.7 Point Source Subtraction
There are no point source-specific SCCs for agricultural silage; therefore, point source subtraction is not
performed for this category. If your state has reported emissions from agricultural silage as a point
source in the past, it should be calculated as a nonpoint source now.
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17.2.8 Sample calculations
Table 17-2 lists sample calculations to determine VOC emissions from agricultural silage storage. The
values in these equations are demonstrating program logic and are not representative of any specific
NEI year or county.
Table 17-2: Sample calculations for VOC emissions from agricultural silage storage
Eq.
#
Equation
Values
Result
1
Pa,c,2020 = Pa,s,2020 X ra,c,2020
N/A
dairy cattle
population is
available for this
county and does
not need to be
calculated using
NASS Census
ratios
1
Pa,c,2020 = Pa,s,2020 X ra,c,2020
N/A
beef cattle
population is
available for this
county and does
not need to be
calculated using
NASS Census
ratios
3
Ec — {^EFp dairy x Pc, dairy
ton\
x 0.0011
kg J
{^Pp,beef
x Pc,beef
ton\
X 0.0011
kg J
kg
Ec = 5.998-^41 x 2,616 dairy cattle
yr
ton
x 0.0011 —
kg
kg
+ 0.4581-^1
yr
x 27,098 beef cattle
ton
x 0.0011 —
kg
30.98 tons VOC
emissions from
agricultural
silage storage
17.2.9 Improvements/Changes in the 2020 NEI
This is a new source category for the 2020 NEI.
17.2.10 Puerto Rico and U.S. Virgin Islands
Since insufficient data exists to calculate emissions for the counties in Puerto Rico and the U.S. Virgin
Islands, we will base emissions for those domains on two proxy counties in Florida: 12011, Broward
County for Puerto Rico and 12087, Monroe County for the U.S. Virgin Islands. The total emissions in
pounds for these two Florida counties are divided by their respective populations creating a pound per
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capita emission factor. For each Puerto Rico and U.S. Virgin Island county, the pound per capita emission
factor is multiplied by the county population (from the same year as the inventory's activity data) which
serves as the activity data. In these cases, the throughput (activity data) unit and the emissions
denominator unit are "EACH".
17.3 References
1. U.S. EPA. 2018. Inventory of Greenhouse Gas Emissions and Sinks, 1990-2020. Chapter 5.2,
Manure Management. EPA 430-R-22-003. https://www.epa.gov/system/files/documents/2022-
04/us-ghg-inventorv-2022-main-text.pdf
2. United States Department of Agriculture National Agricultural Statistics Service Quick Stats.
https://quickstats.nass.usda.gov/
3. Hafner, S.D., Howard, C., Muck, R.E., Franco, R.B., Montes, F., Green, P.G., Mitloehner, F.,
Trabue, S.L., Rotz, C.A., 2013. Emission of volatile organic compounds from silage: Compounds,
sources, and implications. Atmospheric Environment 77, 827-839.
https://doi.Org/10.1016/i.atmosenv.2013.04.076
4. Hafner, S.D., Montes, F., Rotz, C.A., 2012. A mass transfer model for VOC emission from silage.
Atmospheric Environment 54, 134-140. https://doi.Org/10.1016/j.atmosenv.2012.03.005
5. Montes, F., Hafner, S.D., Rotz, C.A., Mitloehner, F.M., 2010. Temperature and air velocity effects
on ethanol emission from corn silage with the characteristics of an exposed silo face.
Atmospheric Environment 44, 1987-1995. https://doi.Org/10.1016/i.atmosenv.2010.02.037
6. Hafner, S.D., Montes, F., Rotz, C.A., Mitloehner, F.M., 2010. Ethanol emission from loose corn
silage and exposed silage particles. Atmospheric Environment 44, 4172-4180.
https://doi.Org/10.1016/i.atmosenv.2010.07.029
7. Rotz, C.A., Asem-Hiablie, S., Place, S., Thoma, G., 2019. Environmental footprints of beef cattle
production in the United States. Agricultural Systems 169, 1-13.
https://doi.Org/10.1016/i.agsv.2018.ll.005
8. Rotz, A., Stout, R., Leytem, A., Feyereisen, G., Waldrip, H., Thoma, G., Holly, M., Bjorneberg, D.,
Baker, J., Vadas, P., Kleinman, P., 2021. Environmental assessment of United States dairy farms.
Journal of Cleaner Production 315, 128153. https://doi.Org/10.1016/i.iclepro.2021.128153
9. Hafner, S.D., Buhler, M., Feilberg, A., Franco, R.B., Howard, C., Montes, F., Muck, R.E., Rotz, C.A.,
Weil?, K. 2018. "Volatile organic compounds and silage: sources, emission, and mitigation." XVIII
International Silage Conference (Bonn, Germany).
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United States Office of Air Quality Planning and Standards Publication No. EPA-454/R-23-001q
Environmental Protection Air Quality Assessment Division March 2023
Agency Research Triangle Park, NC
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