Sustainable Dairy Fact Sheet Series

Methane Emissions from Dairy Cattle
An Overview

Introduction

Horacio Aguirre-Villegas
Biological Systems Engineering,
University of Wisconsin-Madison
Rebecca A. Larson
Biological Systems Engineering,
University of Wisconsin-Madison
Matthew D. Ruark
Soil Science,
University of Wisconsin-Madison

Methane (CH4) is a gas produced largely from bacteria that live in soil, water and thestomachs of ruminant animals (mammals that have a stomach with four compartments thatferment food as a major part of the digestion process). Methane is considered a greenhousegas (GHG) because it can trap infrared radiation in the atmosphere, causing an increase in airtemperatures. Methane is the second most abundant global man-made GHG, behind carbon
dioxide (CO2), which comes largely from fossil fuel combustion (IPCC 2014). Methane, once
emitted, will exist in the atmosphere for 12 years, which is shorter than the lifetime of CO2.
However, methane is able to trap more radiation compared to CO2 resulting in 28-34x greater
global warming potential (see inset on Global Warming Potential). Overall, methane represents
16% of annual GHG emitted to the Earth’s atmosphere based on its Global Warming Potential.
Figure 1. a) Global man-made CH4 emissions (Source: USEPA 2012), b) U.S. man-made CH4emissions (USEPA, 2016), and c) on-farm GHG emissions breakdown per gas type (Aguirre-Villegas et al. 2015). Mobile sources include on-road and off-road vehicles, ships and boats,trains, and aircraft. Stationary sources include factories, refineries, boilers, and power plants.
(Continued on next page).

SUSTAINABLE DAIRY PARTNERS
UNIVERSITY OF WISCONSIN-MADISONCORNELL UNIVERSITYa) 100%PENNSYLVANIA STATE UNIVERSITY

80%

UNIVERSITY OF ARKANSAS

70%

UNIVERSITY OF MARYLAND

60%

UNIVERSITY OF MICHIGAN

50%

UNIVERSITY OF WASHINGTON40%NORTH CAROLINA AGRICULTURALAND TECHNICAL STATE UNIVERSITY

Biomass combustion
Manure management
Mobile and stationary sources
Other agriculture
Rice cultivation
Wastewater
Coal mining
Landfills and solid waste
Natural gas and oil systems
Enteric fermentation

90%

30%
20%

INNOVATION CENTER FOR US DAIRY10%USDA ARS DAIRY FORAGERESEARCH CENTER

0%
Global methaneemissions

USDA ARS PASTURE AND
WATERSHED MANAGEMENTUSDA ARS NATIONAL LABORATORY FORAGRICULTURE AND ENVIRONMENTb) 100%

USDA NATIONAL AGRICULTURE LIBRARY

90%

Mobile and stationary sources
Other
Manure management
Coal miningLandfills
Enteric fermentation
Natural gas and oil systems

80%
70%
60%
50%
40%
30%
20%
10%
0%
U.S. methaneemissions1c)
Global Warming Potential
The atmospheric lifetime of a GHG,
or how long a greenhouse gas
remains in the atmosphere, determines how long it will increase
the temperature of the Earth. The
measure of how much energy a
GHG traps in the atmosphere overa period of time (commonly 20 or100 years) relative to CO2 is called
its global warming potential (GWP).
GWP is expressed in terms of CO2
equivalents (CO2-eq) (Table 1). For
example, if the 100-year GWP potential of methane is 28, then one
methane molecule (CH4) has thesame greenhouse effect as 28 molecules of CO2. The GWP of methaneis larger for shorter periods of time
when compared to CO2; the 20-year
GWP for methane increases to 84.
Standardizing GHGs based on GWP
provides a common unit of measure to compare all GHGs. This isvery useful in comparing different
scenarios that could guide emission reduction strategies.
The Intergovernmental Panel on
Climate Change (IPCC) has set thestandard values for methane’s GWPover the past 20 years as compared to CO2. These values havebeen increasing over time for two
reasons: the diminishing ability ofoceans and soils to absorb CO2 as
temperature rises (climate-carbonfeedback), and the production ofadditional CO2 from the oxidationof existing CH4 in the atmosphere.
Table 1. Global warming
potential of CH4
IPCC Report

IPCC 2013 – AR5

GWP 100 years28a – 34b

GWP 20 years

84a – 86b

Myhre et al. 2013a Without including climate-carbon feedbacks
b Including climate-carbon feedbacks

100%

CO2-eq off-farm
CO2 fossil on-farmN2O crops
N2O manure
CH4 manureCH4 enteric

90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Farm GHG
emissions

Methane can be emitted from both natural and man-made sources. Wetlands
account for 82% of natural sources. The high moisture and low oxygen
conditions of wetlands are ideal for methanogens (the bacteria that producemethane) to decompose dead plant material (USEPA 2010). But not all land
types are methane emitters. Dry upland soils act as sinks (the soil consumes
more methane than it emits) for atmospheric methane that is used by bacteria
in the soil as a source of carbon. Overall, man-made emissions of methane are
greater than from natural sources and account for 63% of the total methanethat enters the atmosphere (USEPA 2010). Man-made contributions of methane
include emissions from enteric fermentation, natural gas and oil production,
landfills and solid waste, rice cultivation, wastewater, manure management,
biomass combustion, coal mining, static and mobile combustion and otheragricultural activities (Figure 1a).
On a global basis, agriculture (enteric fermentation, manure management,rice cultivation, and other agriculture) contributes 41% of man-made methaneemissions. Livestock is the major single source, accounting for 73%, of agricultural methane emissions (USEPA 2013).

U.S. methane production and dairy
Man-made methane emissions in the U.S. are 731 million tons CO2-eq, which isequivalent to the emissions from 154 million passenger vehicles driven in oneyear (USEPA 2016a,b). Livestock represents 31% of the total methane emissions
produced from human activities in the U.S., with beef and dairy cattle as
the major contributors. More specifically, enteric methane from livestock isthe second largest source of methane emissions in the U.S. and dairy cattle
alone accounted for 26% of total enteric emissions (USEPA 2016a). Methane
production from enteric fermentation is part of the normal digestive process
in animals, especially ruminants, where microbes that live in their digestivesystem ferment the food they eat. These emissions are primarily the result ofanimals belching or exhaling, as only 5% of the total methane from a dairy cow
comes out the back end of the animal.
Manure is also an important emission contributor of methane (Figure 1b). Dairy
cattle alone are responsible for 53% of total methane emissions from manure
management in the U.S. (USEPA 2016a). Methane is produced by the microbial
decomposition of organic material in manure in the absence of oxygen. In
these conditions, anaerobic microorganisms dominate the decomposition
processes and produce methane as a part of their anaerobic respiration. As aresult, methane is mainly emitted from slurry and liquid manures as they are
2CO2(fossil)

NH3C02

CH4CH4(Enteric)

BarnNH3N2O

C02

CH4NH3N2O

Manure storage

C02 CH4CO2(fossil)

Land application

Figure 2. Sources of methane emissions.
handled under conditions with low oxygen levels. During
manure handling, methane is emitted mostly from storageand land application. Manure storage is common in large
confined operations where the anaerobic conditions andprolonged storage times are ideal for microorganisms toproduce methane. To a lesser extent, manure land application
also contributes to methane emissions, especially if manure
is injected in the soil as this creates anaerobic conditions(Chadwick 2011).
The life cycle assessment method can be used to estimate
emissions from a farm. Using this method, Thoma et al. (2013)
reported that 72% of the GHG emissions related to general
fluid milk production in the U.S. occur within the boundariesof the dairy farm, where more than 50% of these emissions
are from methane. The remaining 28% are emitted during
the transport, processing, distribution, consumption and final
disposal of milk and meat. Based on Aguirre-Villegas et al.
(2015), the production of one gallon of milk at the dairy farm
produces nearly 3.8 kg CO2-eq, which is equivalent to theemissions from driving an average passenger vehicle for nine
miles (USEPA 2016b). More than half of these emissions are in
the form of methane coming from both the cow’s digestive
process and the manure handling processes (mainly frommanure storage) (Figure 1c).

Mitigation
There are different strategies to reduce, or mitigate, methane
emissions from enteric fermentation and manure at the farmlevel. Enteric methane is difficult to mitigate as it involves thenatural digestion process of the cow, but ongoing work is
exploring the potential of genetics and changes in the cow’s
diet. Genetics involves selective breeding toward cows withhigher feed efficiencies to increase milk production without
increasing the methane produced. Animal performancecan be improved by exploring cow genomics to identify thegenes that a cow needs to grow and produce milk and byidentifying the management practices that tend to increase
productivity. Some strategies that consider changes in thecow’s diet to reduce enteric emissions include introducing
feed additives to maximize microbial fermentation andimproving feed efficiency through diet quality. For example,
Arndt et al. (2015) found that the amount and source of fiber
as well as the amount of starch fermented had an impact onenteric methane production. Corn silage yielded substantiallymore enteric methane than the fermentation of alfalfa silage
in the rumen of dairy cows.
To reduce methane emissions from manure, the focus has
been on changing the characteristics of the manure to reduce the degradation of solids or designing manure systems
to contain or capture emissions. Some strategies include3

composting, manure separation, manure storage covers andcapturing methane from manure using anaerobic digestion
systems to combust and convert the methane to carbon
dioxide. For example, Aguirre-Villegas et al. (2014) found that
GHG emissions from manure management can be reducedby nearly 50% with the installation of an anaerobic digester
without even considering the benefits of replacing gridelectricity. An anaerobic digester is a tank containing manurein which microorganisms produce methane in the absence ofoxygen. With this controlled process, the produced methane
can be easily captured to produce heat or electricity. Other
processing techniques, such as separating the solid andliquid components of manure, can reduce methane losses
during storage by 20%. Integrating these practices requires
careful consideration of many factors including cost, operational issues, regulations and other environmental concerns.

Summary
Methane is produced on dairy farms mainly through entericfermentation and manure storage. Methane accounts for
11% of greenhouse gas emissions each year in the U.S., ofwhich 31% is from livestock. Methane emissions from entericfermentation represent the largest fraction of the emissions
from a dairy farm, but they are difficult to mitigate. Methaneemissions from manure storage are also important, and there
are technologies that currently exist (anaerobic digestion
and liquid-solid separation among others) that can lead todramatic reductions in methane from a dairy farm.

References
Aguirre-Villegas, Horacio A., Thaís H. Passos-Fonseca, Douglas
J. Reinemann, Louis E. Armentano, Michel A. Wattiaux, Victor
E. Cabrera, John M. Norman, and Rebecca A. Larson. “Greencheese: Partial life cycle assessment of greenhouse gas emissions and energy intensity of integrated dairy production andbioenergy systems.” Journal of Dairy Science 98 (2015): 1571-92.
Aguirre-Villegas, Horacio A., Rebecca A. Larson, and Douglas J.
Reinemann. “From waste-to-worth: Energy, emissions, andnutrient implications of manure processing pathways.” Biofuels
Bioproducts and Biorefining 8 (2014): 770–93.
Arndt, Claudia, Mark J. Powell, Matias J. Aguerre, and MichelA. Wattiaux. “Performance, digestion, nitrogen balance, andemission of manure ammonia, enteric methane, and carbon
dioxide in lactating cows fed diets with varying alfalfa silageto-corn silage ratios.” Journal of Dairy Science 98 (2015): 418-30.
Chadwick, Dave, Sven Sommer, Rachel Thorman, David Fangueiro,
Laura Cardenas, Barbara Amon, and Tom Misselbrook. “Manuremanagement: Implications for greenhouse gas emissions.”
Animal Feed Science and Technology 166-167 (2011): 514-31.
Hristov, Alexander N., Kristen Johnson, and Ermias Kebreab.
Livestock methane emissions in the United States. State College,
PA: Pennsylvania State University Extension, 2014. http://extension.psu.edu/animals/dairy/news/2014/livestock-methane-emissions-in-the-united-states.
Myhre, Gunnar, Drew Shindell, Francois-Marie Bréon, WilliamCollins, Jan Fuglestvedt, Jianping Huang, Dorothy Koch, JeanFrançois Lamarque, David Lee, Blanca Mendoza, Teruyuki Naka-jima, Alan Robock, Graeme Stephens, Toshihiko Takemura, andHua Zhang. “2013: Anthropogenic and Natural Radiative Forcing
Supplementary Material.” Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change, editedby Thomas F. Stocker, Dahe Qin, Gian-Kasper Plattner, Melinda
M.B. Tignor, Simon K. Allen, Judith Boschung, Alexander Nauels,
Yu Xia, Vincent Bex, and Pauline M. Midgley. Cambridge, United
Kingdom and New York, NY, USA: Cambridge University Press,2013. http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/
WG1AR5_Chapter08_FINAL.pdf.
Intergovernmental Panel on Climate Change (IPCC). Climate
Change 2014: Mitigation of Climate Change. Contributionof Working Group III to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change, edited by Ottmar
Edenhofer, Ramón Pichs-Madruga, Youba Sokona, Ellie Farahani,
Susanne Kadner, Kristin Seyboth, Anna Adler, Ina Baum, SteffanBrunner, Patrick Eickemeier, Benjamin Kriemann, Jussi Savolainen, Steffen Schlömer, Chistoph von Stechow, Timm Zwickel, andJan C. Minx. Cambridge, United Kingdom and New York, NY,
USA: Cambridge University Press, 2014. https://www.ipcc.ch/pdf/assessment-report/ar5/wg3/ipcc_wg3_ar5_full.pdf.
Thoma, Greg, Jennie Popp, Darn Nutter, David Shonnard, RichardUlrich, Marty Matlock, Dae S. Kim, Zara Neiderman, NathanKemper, Cashion East, and Felix Adom. “Greenhouse gasemissions from milk production and consumption in the
United States: A cradle-to-grave life cycle assessment circa
2008.” International Dairy Journal 31 (2012): S3-S14.
United States Environmental Protection Agency (USEPA).
Methane and Nitrous Oxide Emissions from Natural Sources.
Washington, DC: USEPA Office of Atmospheric Programs, 2010.
https://www3.epa.gov/climatechange/ghgemissions/gases/ch4.html.
United States Environmental Protection Agency (USEPA). Global
Anthropogenic Non-CO2 Greenhouse Gas Emissions: 1990-2030.
Washington, DC: USEPA Climate Change Division, 2012.
https://www3.epa.gov/climatechange/Downloads/EPAactivities/EPA_Global_NonCO2_Projections_Dec2012.pdf.
United States Environmental Protection Agency (USEPA). Global
Mitigation of Non-CO2 Greenhouse Gases: 2010-2030. Washington, DC: USEPA Office of Atmospheric Programs, 2013. https://
www3.epa.gov/climatechange/Downloads/EPAactivities/
MAC_Report_2013.pdf.
United States Environmental Protection Agency (USEPA).
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 2014. Washington, DC: USEPA, 2016a. https://www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
United States Environmental Protection Agency (USEPA).
Greenhouse Gas Equivalencies Calculator. Washington, DC:USEPA, 2016b. https://www.epa.gov/energy/
greenhouse-gas-equivalecies-calculator.

Acknowledgement
This material is based upon work that is supported by the National Instituteof Food and Agriculture, U.S. Department of Agriculture, under award number 2013-68002-20525. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do notnecessarily reflect the view of the U.S. Department of Agriculture.

4(c) 2016 University of Wisconsin System Board of Regents and University of Wisconsin-Extension, Cooperative Extension. All rights
reserved.
Authors: Horacio Aguirre-Villegas, assistant scientist, Rebecca A. Larson, biowaste specialist, and Matthew D. Ruark, soil scientist,
University of Wisconsin-Madison.
Reviewers: Douglas J. Reinemann and Elizabeth Binversie, Brown County UW-Extension; and Mahmoud A. Sharara, Biological Systems
Engineering, College of Agriculture and Life Sciences.
Graphic design: Elizabeth Rossi, UW Environmental Resources Center.
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