Greenhouse Gas Mitigation Strategies through Dairy Forage and Manure Management

 Title Picture
 Photo by C.A. Campbell

Project Summary

The United States Dairy Industry voluntarily chose to pursue a cut in greenhouse gas emissions from milk by 20% by the year 2020. As a part of that goal, scientists across the Midwest are working with dairy farmers to improve manure processing and cropping management practices, including land applications of dairy manure from farms, manure storage options, and manure processing. As soil and dairy researchers at the University of Wisconsin, we have been asked to consider mitigation opportunities involved in the manure management continuum, comparing management opportunities to reduce emissions associated with manure storage, processing, and land application to dairy forages. Additionally, we are interested in understanding some of the environmental trade-offs associated with these management practices, especially related to their influence on contaminating groundwater and nearby water bodies with nitrate. Through an extensive literature review, we concluded that manure is most versatile for greenhouse gas mitigation when it is separated into solid and liquid (slurry) manure pools. Solid manure pools can be covered or sealed to capture emissions in piles or with anaerobic digesters and liquid (slurry) manure pools are best mitigated when they are acidified, covered, or routinely emptied from storage. Manure management practices are currently regulated based on total manure phosphorus loads, and in order to shift to greenhouse gas mitigation strategies, industry support--through policies and subsidies--will be necessary to maximize farmer participation in greenhouse gas mitigation. 

Research Objectives

To understand the mitigation potential and environmental impact of various manure management practices on dairy farms, we have three objectives.
  • Determine greenhouse gas emissions from baseline practice, namely, raw (unprocessed) manure, and manure broadcast applications.
  • Determine greenhouse gas emissions from mitigation strategies and evaluate these practices for differences.
  • Synthesis the best management practices for mitigating emissions throughout the mitigation process.

Introduction to Greenhouse Gases in Dairy Systems

Dairy production systems are particularly interested in mitigation greenhouse gas emissions because these systems can be vulnerable to effects of climate change, in terms of animal related heat stress, potential forage crop failures, and increase external costs from feed production, electricity costs, and transportation fees. To do their part, farmers are trying to reduce greenhouse gas emissions from their dairies. 

Unlike other industries, the dairy industry is a key producer of the greenhouse gases methane (CH4) and nitrous oxide (N2O). These emissions have 26 and 298 times the global warming potential of carbon dioxide respectively, which means these gases have a longer lifespan in the atmosphere and higher radiative forcing compared to emissions from other industries. With this in mind, the dairy industry is not only interested in mitigating all emissions, but also, when possible, changing emissions into less potent greenhouse gases, such as combusting methane gas to release carbon dioxide.

Sources of Emissions

Dairy related emissions (Table 1.0), are predominately related to manure management in dairy systems. Manure produces all three major greenhouse gases, carbon dioxide, methane, and nitrous oxide, while it is collected, processed, stored, and land applied as a fertilizer. Because the primary emissions from manure are methane and nitrous oxide, which have 26 and 298 times higher global warming potential than carbon dioxide, managing these systems to reduce emissions is an important step towards a more sustainable system. Chadwick et al.2011, suggests that manure management - among the phases outlined in this paper - are strongly interrelated based on the possible pathways of manure management. 

In essence, the manure management continuum suggests that each phase of the manure management process allows for chemical and physical alterations to manure, which can significantly change the bi-products of the manure, including greenhouse gas emissions. For example, manure slurry, collected without bedding materials, is predominantly liquid and thus, is stored anaerobically, which results in very high methane emissions. When a slurry is land applied, the manure has a lower C:N ratio because the N has been preserved in storage--resulting in higher nitrous oxide emissions from land application. Comparatively, manure that is stored solid allows more oxygen into the manure pile, and therefore, there will likely be higher nitrous oxide emissions from this storage option. The primary goal of this study is to review how varying mitigation options in manure management can be combined to synthesize the best possible pathway to reduce emissions. 

Table 1.0. Primary emission sources in dairy production systems in kg of CO2 equivalents of Energy Corrected Milk. 

 Source of Emission Methane Emissions Nitrous Oxide Emissions
 Enteric Fermentation
373.61 NA
 Manure Collection/Processing 25.32 4.782
 Manure Storage 25002 37.32
 Land Application
7.62 153.11

1Estimates from O'Brien et al 2014.

2Estimates from Aguirre et al. 2014.

 Manure Management Continuum
 Source: Chadwick et al. 2011

Manure Processing



 Manure Management Synthesis
 Figure 3.0. Manure handling and management synthesis, with all management pools considered for this study. Commonly, manure is managed through two distinct pathways: manure solids and manure slurry. Within these management pools, there are further options for manure handling to mitigate greenhouse gas emissions. Figure: C.A. Campbell
Manure processing is commonly separated into two distinct management pools: solid and slurry manures. Our findings show that this is the most effective, versatile manure handling processes result from manure solid liquid separation, which makes manure pools more versatile to work with--giving farmers more mitigation opportunities on their farms. Of the two management systems, solid manure is easier to process and handle at lower costs; the solid manure pool is also more stable--both chemically and physically, resulting in more mitigation potential. 

Solid Manure

Solid manure is handled two ways in most manure management systems: through air tight sealed storage, and through manure composting (also called piling). Capturing emissions from manure processing by sealing manure after solid and liquid separation is the most effective processing mechanism for solid manure handling. This allows producers to not only capture emissions, but also utilize the bio-gas produced from manure handling to heat buildings or barns and also can potentially be used to produce energy. Manure composting can be an effective way to create a value-added product to a farm, but if manure compost piles are not routinely turned, they result in higher nitrous oxide emissions than normally processed and stored manure solids. Anaerobic digesters are able to capture the most greenhouse gases from solid manures, but these can be cost prohibitive to many farmers. 

Slurry Manure

Slurry manures are much more difficult to process in order to reduce greenhouse gas emissions than manure solids. Because these manures contain most of the urine from dairy cattle waste, slurry manure is much more volatile and reactive, especially in conditions that do not allow oxygen to enter the system. In slurry processing and storage, nitrous oxide and methane emissions are a big concern. Our findings show that adding a cover to manure slurry during processing and storage can be an option to mitigate nitrous oxide emissions, as long as this cover does not increase the manure's temperature or make the manure anaerobic. Research conducted on differing manure covering systems and artificial films were able to reduce emissions from most systems. We suggest the best way to handle liquid slurry manure is to acidify manure during processing, which makes the manure less reactive with the atmosphere, and to frequently empty manure processing equipment to reduce bacteria loads in these systems. 

 Hou et al Manure processing
 Figure 4.0. Manure processing emissions as compared to reference, common manure practices. For manure solid (left), the best greenhouse gas mitigation strategy was to change the cows' diet, and manure processing practices, including compaction, and incorporation resulted increased emissions. For slurry manure (right), the more processing strategies were able to reduce emissions, including separation strategies (sep 1 = gravitational separation, sep 2 = screw press separation). When manure is separated, mitigation options become more versatile to dairy producers. Source: Hou et al. 2015

Table 2.0. Manure processing strategies and greenhouse gas emissions.
 Processing Strategy  Cost of System, $  Mitigation Potential, % Reduced
 Raw Manure (baseline) $10,000-$15,000 0
 Anaerobic Digestion $200,000-$900,000 20-40
 Solid-Liquid Separation $15,000-$75,000 5-10
(Cost of Labor) -30 to -50 (produces more emissions than baseline)

1System costs as reported by Lund et al. 2000. 

2Reported by Beddoes et al. 2007.

3All emissions reported in kg CO2 equivalents

4Percent reduced compared to baseline, raw manure, in terms of CO2 equivalents of total greenhouse gas emissions produced by the system as reported by Hou et al. 2015. 

Manure Storage


After manure undergoes a treatment process it must be stored before the timing is right for the manure to be applied to cropland as a source of fertilizer or utilized as a source of animal bedding. Generally, a farmer's end use of the manure (fertilizer or bedding) is a large factor in deciding how the manure will be stored. For example, slurry (liquid) manure would not be used as bedding and therefore is stored in a slurrystore or lagoon, but solid manure that will be used as a source of animal bedding would not be stored in a slurrystore or lagoon but rather piled to increase the drying of the bedding. In order to mitigate greenhouse gas emissions when storing manure, solids must be managed to mitigate methane emissions and liquid slurry must be managed to mitigate nitrous oxide emissions.  

Also, in today's dairy industry there are numerous regulations in place to protect the environment from pollution and degradation as a result of misplaced or leachate manure. With the scope of our research being modern commercial dairy farms, it is important to remember the standard that these farms are being held to in regards to environmental protection and that these regulations and standards are currently not an industry standard across all farm sizes.

Slurry Manure

Slurry manure, which is generally used as a source of fertilizer and spread on cropland as a nutrient source, is generally stored using two different but similar strategies: slurrystores (above ground) and lagoons (below ground - concrete, clay, plastic liners). Based on our research, we found that covers were the most effective way of mitigating greenhouse gas emissions from slurry manure because there use captures the greenhouse gases that are given off by the agitation of the slurry system. There are two general categories of covers used: permeable and impermeable, of which impermeable covers are known to be more productive. Permeable covers include a natural crust, straw, clay, geotextile, and ceramic or glass balls. Impermeable covers include plastic, concrete, and wood (Viguria et al., 2015). As mentioned above, slurry manure must be managed to reduce nitrous oxide emissions. In the absence of a cover, our recommended nitrous oxide mitigation strategies include reducing storage time, limiting manure moisture, increasing manure acidification, reducing ammonia volatilization, improving anaerobic digestion, reducing degradable organic matter, and composting (Montes et al., 2014).

Solid Manure

Solid manure, which is generally either used as a source of fertilizer high in organic matter or as a source of animal bedding, is generally stored via the strategy of solids piling/stacking/stockpiling. As mentioned above, solid manure must be managed to reduce methane emissions. Our recommended methane mitigation strategies include preventing anaerobic conditions, reducing storage time, limiting the moisture content of the manure, maintaining storage temperature, increasing manure acidification, composting, and covering the manure (Montes et al., 2014).

Manure Land Application

Land Application


No matter the timing of manure spreading or land application strategy, the primary greenhouse gas of concern manure land applications is nitrous oxide. When soil conditions become anaerobic—from moisture associated with irrigation, rain events, or manure applications—in the presence of high nitrogen concentrations, microbes will choose to respire using nitrate (NO3-) as their primary metabolic source. When NO3- is respired by microorganisms, nitrate is broken down in the process of denitrification. The primary end-product of the denitrification process is nitrous oxide. 

Three common land application strategies are used in the United States--broadcast application, injection, and banded manure application. Research has shown that though the most effective agronomically to efficiently make nitrogen resources from manure available to plants is manure injection or manure incorporation, however, these practices have proven to be the highest nitrous oxide producers from literature reviews. When manure is injected or incorporated, microbes have greater access to nitrogen sources, which results in higher denitrification and associated greenhouse gas emissions. Broadcast applications of manure are agronomically limiting, especially if rainy areas, which may wash away manure sources before it is utilized by plants as a fertilizer. Conclusively, we found that manure banding, which limits soil-manure interactions, is the most effective mitigation technique for manure application.  

Broadcast Application

Unprocessed dairy manure is most commonly broadcast applied to crop and forage land as the least labor intensive option for dairy producers. Broadcast application requires loading raw manure into spreaders or tractors, which are then driven over fields at relatively high speeds, with manure thrown from the spreaders. There are many draw backs to this system; manure cannot be applied at variable rates and in many cases, manure can be unevenly applied. To improve nitrogen use efficiency and reduce immediate nitrogen losses as ammonia volatilization from soil, many land managers will till in, or incorporate, manure after broadcast application. Though manure is commonly incorporated, incorporation increases nitrous oxide emissions because manure is immediately placed into soil at depth, where anaerobic (oxygen limited) zones are more likely to develop.

Injection Application

When manure is separated, liquid slurry manure can be injected into the soil surface, immediately placing the manure into the root zone at depth, where nitrogen is immediately available to plants. Slurry injection is advantageous because it is more cost efficient than hauling and spreading raw manure and does not require additional labor if manure has already been separated (Plastina and Johanns 2015). However, like incorporated manure, slurry injection results in increased anaerobic conditions, which promote nitrous oxide emissions. A 2015 study by Hou et al. found that manure slurry injection is responsible for a significant increase in nitrous oxide emissions, though carbon dioxide emissions decreased from this application treatment. This finding make manure injection a poor choice for greenhouse mitigation strategies. 

Banded Application

Banded application of manure is the most effective means of manure application for agronomic nitrogen efficiency and greenhouse gas emissions. Manure banding places bands strategically between planted rows of crops, which allows the rooting systems immediate access to manure nutrient resources. Because manure is applied in bands, it is not widely spread across the field, and by limiting the soil-manure contact area, greenhouse gas emissions can be reduced from this system because the manure is not easily accessed by microorganisms that can break it down to produce nitrous oxide. 

Influential Manure Management Factors


Current environmental regulations that drive manure handling processes limit all manure application by the amount of phosphorus present in the manure. Phosphorus is a primary concern for the Wisconsin Department of Natural Resources because as phosphorus is loaded into water that leaves farms and contributes to local, regional, and national watershed eutrophication in lakes, streams, and even the Gulf of Mexico. Eutrophication is detrimental to all aquatic communities, influencing fish populations, algal blooms, water temperature, and many other attributes that reduce overall water quality (Sturgul and Bundy 2004). Because phosphorus exists in very high concentrations in dairy manure, farmers must carefully monitor the amount of phosphorus applied, because large dairy farms are considered to be a point source of phosphorus pollution to water systems. Current regulations say that no more than 50 ppm P can be present in soil at any given time. Because liquid manure slurry is much lower than solid manure in P concentrations, this gives farmers some incentive to separate their manure stores, which consequently can reduce their greenhouse gas emission loads if handled properly (Sturgul and Bundy 2004).

Another issue of significant concern in areas with manure spreading is nitrate loading in groundwater. If nitrate is not taken up by plants or denitrified into a gaseous form, nitrate is easily leached from the soil’s root zone and in high concentrations (greater than 10% by volume), can lead to methemoglobinemia, or blue baby syndrome, a condition that converts nitrate into nitrite in infants, which results in the babies to become poisoned and their blood is inable to support oxygen (Bundy and Jackson 2004). Though rare, this is a huge health safety concern for the state of Wisconsin. 


Large scale dairy farms often have more manure produced each year than they have land to spread that manure on. To facilitate manure removal from their farms, dairy producers need community support from their neighboring farmers, particularly grain crop producers. In theory, by creating partnerships between grain and dairy farmers, both will receive benefits to their management systems; dairy farmers are able to remove manure from storage on their farms and grain producers have access to a relatively cheap, organic fertilizer source. However, there has been a historic cultural reluctance for grain farmers to partner with neighboring dairy farms for fertilizer products (Sanford et al. 2009). Though results of research trials studying grain yield productivity from these six Wisconsin farms showed promising increased in yields, farmers were hesitant to continue manure sharing practices with dairy farms due to concerns about manure availability, slurry run-off, costs of manure spreading, and environmental concerns of increasing nitrate and phosphorus levels from manure applications (Sanford et al. 2009). These concerns can create manure storage bottlenecks, which can limit a dairy farm’s ability to grow and maximize its production. 


Solid Manure Management

To mitigate greenhouse gas emissions from solid manure stores, we recommend storing manure in covered or sealed and compacted conditions or with anaerobic digestion and applying manure in bands. In order for manure solids to be separated, several options exist, including settling ponds, screw press separation, and gravitation lifts--all are feasible, at costs of $10,000-$50,000 of infrastructure. Anaerobic digesters, though the most effective mitigation option, are cost prohibitive for many farmers without further subsidies and incentives. To ensure the most mitigation potential, manure storage should be emptied frequently and captured bio-gas from storage can be further utilized to provide energy or heat for the farm. 

Slurry Manure Management

Slurry manure is most effectively mitigated by acidifying manure during storage, applying storage covers --like straws or synthetic oils, and routinely emptying the slurry tank to reduce bacterial loads, and applying manure in liquid bands. No matter the case, slurry manure application resulted in higher greenhouse gas emissions than solid manure land application, because liquids are more volatile when applied. Though the most efficient by agronomic N standards, manure injection results in the highest emissions source, and the most effective mitigation strategy was manure band spreading, to reduce the surface area of soil-slurry interface. 

Overarching Themes: 

  • Manure management is reliant on the type of existing management infrastructure. Solid-liquid separation allows for the most potential mitigation adjustments to each physical manure pool, and is the most versatile system. 
  • In every instance examined, capturing emissions by covering or sealing manure resulted in higher mitigation potential. 
  • Cool, dry manure produces the least amount of total emissions. 

Paper for Journal Club

Check out the paper we will discuss in class on Tuesday at this link.

Hey Fellow Food System Sustainability Classmates! We realize this article by Montes et al. is pretty lengthy, so we thought we'd give you some pointers to get ready for discussion this Tuesday. Please use the instructions below to guide your reading:

  1. Read thoroughly the Abstract and Introduction
  2. "Livestock Manure and Emissions": study up on Figures 1 and 2
  3. Skip "Animal Mangagement and Housing" but take look at Table 1
  4. Get the gist of "Manure Management and Treatment"

See you Tuesday!


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Meet the Team!

Hi, we're Claire and Jordan, students at the University of Wisconsin interested in mitigation strategies for dairy production in Wisconsin.

Jordan is a junior Dairy Science undergraduate student. He comes from his family's 3,000 cow commercial dairy farm in Northeastern Wisconsin. His family's farm is actively seeking strategies and technologies to further decrease the effects of manure on the environment and community. In his free time, Jordan enjoys watching and playing basketball and other sports, along with showing dairy cattle and maintaining a strong interest in agriculture.

Claire is a second year graduate student in the Department of Soil Science. When she's not petting cows, she's studying greenhouse gas mitigation strategies associated with manure land applications from dietary feeding trials for dairy cows. Originally from Tennessee, Claire has lived in South Carolina and Arizona before ending up in Wisconsin. In her spare time, she enjoys bike riding through Wisconsin's dairy land, reading good books, and hiking.

Keywords:greenhouse gas mitigation nitrous oxide methane carbon dioxide dairy forage manure tillage manure lagoon environment trade-offs   Doc ID:48421
Owner:Kate A.Group:DS 471 Food Production Systems and Sustainability
Created:2015-03-05 16:38 CDTUpdated:2019-03-04 13:40 CDT
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