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How Does Manure Storage Impact Greenhouse Gas Emissions and What Can Farms Do?

Manure storage significantly impacts greenhouse gas (GHG) emissions on dairy farms, contributing to their overall emissions footprint. To effectively reduce these emissions, it's crucial to evaluate the entire manure management system and align strategies with the farm's specific goals. Mitigation efforts must be customized to each farm's unique conditions, including manure sources and infrastructure. Farms should work with experts in manure management and green technology to identify and implement the most effective emissions reduction strategies for their operation.

Learning Hubmanure emissionS

CONTENT CURRENTLY UNDER DEVELOPMENT

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Summary Presentation

SPEAKER: Dr. April B. Leytem, USDA Agricultural Research Service

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Summary of Key Points from the Presentation

Manure Greenhouse Gas Emissions

While much of the manure produced by cows is similar in composition, the way it is handled and stored significantly affects GHG emissions. Each stage of a manure management system —collection, transport, storage, treatment, and application — can trigger chemical and physical changes that influence the production of GHGs. The primary gases of concern from manure are:

  • Methane: Methane is produced primarily during the anaerobic decomposition of manure.

  • Nitrous Oxide: This is emitted when manure is applied to soils, particularly in aerobic conditions where nitrogen is transformed through nitrification and denitrification.

  • Ammonia: Although not a GHG, ammonia volatilization from manure can lead to indirect GHG emissions. Animal-derived ammonia can travel long distances in the air and eventually redeposit in waterbodies and natural terrestrial systems. When deposited in the landscape, ammonia can be converted to nitrous oxide, contributing to global GHG levels (Hristov et al., 2022). 

See Learning Hub Content for More Information

Options to Reduce Manure Emissions at the Farm Level

A carbon credit is the unit that is certified by a carbon credit program or standard for trade in carbon markets, representing one metric tonne of carbon dioxide equivalent. A carbon offset describes the process of using carbon credits generated outside of a company’s supply chain to compensate for that company’s emissions. A carbon inset refers to the process of using carbon credits generated inside the company supply chain to compensate for that company’s emissions by avoiding, reducing or sequestering emissions either upstream or downstream within its own value chain.

See the table below for an overview of some of the key mitigation options:

Program Goal

EQIP addresses specific problems or “resource concerns” such as erosion, nutrient run-off, etc. 

Practices Funded

New plans (including engineering), infrastructure, energy upgrades, in-field practices, edge of field practices, and more. 

Funding Rates

Payments are based on flat rate payment rates determined by NRCS based upon typical costs associated with the implementation of practices; rates differ by state.  

Funding Rates

See link for a full list of eligible practices and payment rates in each state. Find your state and click on the link for either EQIP or CSP  

Duration

1-10 years

Funding Distribution 

Max $450,000/ person or entity / Farm Bill EQIP distributes payments for specific conservation practice after they are complete and verified.  

Anerobic digestion

Microorganisms break down biodegradable material in the absence of oxygen. Anaerobic digestion creates biogas, a combination of methane, carbon dioxide, water vapor, and other gasses. Biogas can be used for electricity and heat, or even transformed into a natural gas replacement.

Gas capture and flare

Covering manure storage with an impermeable cover, like plastic, to capture gas, then combusting the captured methane to convert it into carbon dioxide which has a lower global warming potential.

Manure storage / management efficiency

Acidification

Reducing the pH of manure by adding acid or other acidifying agent. Lowering pH can inhibit the activity of methanogens that produce methane, if pH is low enough ammonia emissions can be reduced as well.

Manure storage / management efficiency

Convert flush to scrape

Instead of flushing manure to remove it from housing areas and utilizing liquid storage as a result, manure can be scraped and then managed as a solid.

Solid liquid separation

Chemical systems (combined with mechanical system); mechanical systems (e.g., centrifugation)

Separating manure into liquid and solid portions can reduce the methane emission potential of liquid manure in storage. Separated solid matter can also be utilized in different ways (e.g., composting, recycled bedding, land-application) or stored. The time that manure stays in the storage system may affect the overall reductions in methane.

Advanced treatment

Evaporation

In theory, evaporation can be used to dry solids enough so that, essentially, no methane generating microbial activity occurs while in (dry) storage. However, evaporating moisture takes energy, so incorporating systems that serve as a heat source is ideal for reducing the amount of energy required.

Advanced treatment

Vermifiltration

A biological treatment process that incorporates earthworms which stabilize carbon and nitrogen, move air and water through the manure, and produce worm castings (a value-added product).

Windrow, in-vessel, turning, static

Composting is the aerobic partial decomposition of manure organic matter. Emissions from composting vary by moisture content, temperature, carbon and nitrogen content, and the pH of the manure. Farms need to consider the tradeoffs between reducing methane emissions vs. ammonia and nitrous oxide emissions.

Categories for GHG Reduction

Practices and TechnologieS

Manure Myths Debunked

01

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one technology will enable me to achieve net zero.

False! There is no one magic bullet to reduce emissions!

Installing a digester may help to reduce overall farm emissions but that only addresses one source of GHG on the farm. Digesters will reduce the methane generated from liquid storage; however, depending on the type of digester and how it is operated, ammonia and methane emissions from the stored liquid can still occur. Depending on the type of housing system on a farm, not all manure may be able to go to the digester.​

02

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A crust on liquid storage will decrease methane emissions.

False! In many cases, the crust does little to reduce methane emissions because it is not perfectly sealed and may not have uniform coverage. A crust can also generate nitrous oxide emissions, which are an important trade-off to consider.

03

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Manure storage additives will reduce emissions. 

Don’t believe everything you hear! Companies sell all sorts of additives, and we often do not know if the products are viable or not.​

Learn More

Please note that we cannot confirm the accuracy or reliability of the materials found on external websites linked here.

References

Factors Influencing Manure GHG emissions

The amount of GHGs produced from manure storage depends on several key factors, including manure composition, storage conditions, and temperature.

  • Oxygenation: Anaerobic conditions (i.e., low oxygen levels), such as those found in liquid manure storage systems, promote the activity of methanogens, microbes that produce methane. These conditions can lead to significantly higher methane emissions, as oxygen levels are low, favoring anaerobic digestion (Peterson, 2018). Aerobic conditions (i.e., high oxygen levels), such as daily manure spreading or composting of manure solids, prevent the formation of methane because aerobic microbes dominate, reducing methane production. However, aerobic conditions can increase nitrous oxide emissions because the availability of oxygen facilitates nitrification and denitrification processes (Peterson, 2018)​.

  • Volatile Solids (VS): VS are organic carbon compounds in manure, primarily from undigested feed, that serve as precursors for methane production during anaerobic decomposition. The proportion of VS in manure is a key determinant of its methane emission potential, with higher concentrations leading to greater methane production (Peterson, 2018).​

  • Nitrogen Content: Nitrogen in manure exists primarily in organic and ammoniacal forms. In anaerobic conditions, nitrogen is more likely to remain in ammoniacal form, limiting the production of nitrous oxide. However, when manure is stored or treated in aerobic conditions (e.g., composting), nitrogen is more likely to convert into nitrous oxide through nitrification and denitrification processes (Peterson 2018). Ammonia losses during storage and field application can indirectly lead to nitrous oxide emissions, as ammonia deposited in soils or water bodies is converted into nitrous oxide over time​ (Hristov et al., 2022). 

  • Temperature: Temperature has a profound impact on methane production rates from manure. Methane emissions increase significantly as temperatures rise, particularly in liquid manure storage systems, because higher temperatures enhance microbial activity, including methanogenesis​. The relationship between temperature and methane production is exponential, meaning that even moderate temperature increases can substantially raise methane emissions​. For example, methane emissions from manure can be up to 20 times higher at warmer temperatures compared to cooler conditions​ (Sommer et al., 2007).​​​

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