Photo by C.A. Campbell

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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.

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Introduction to Greenhouse Gases in Dairy Systems

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.

Source: Chadwick et al. 2011

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Manure Processing

Summary 

Solid Manure

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. 

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Manure Storage

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Manure Land Application

Summary

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. 

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Influential Manure Management Factors

Environmental 

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. 

Social

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. 

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Conclusions