Reduce CH4 & N2O emissions with livestock manure management

Livestock are produced throughout the world and are a significant contributor to global methane (CH4) and nitrous oxide (N2O) emissions. Manure management – the process in which animal excretion is captured, stored, treated, and used – is responsible for ~7% of agricultural emissions of CH4 and N2O worldwide, (the second largest source of GHG emissions after enteric methane), and makes up to 25% of the emissions generated at the farm level . Livestock wastes can be important sources of nutrients for crops, but manure must be managed properly to prevent loss of nutrients to the environment in air or ground and/or surface water.

Figure 1: Percentages of manure management emissions by animal, 2001-2011. Roughly 70% of emissions from manure result from cattle and pigs. Figure 4-6 from FAO Statistics Division, ESS Working Paper No. 2.

The magnitude of CH4 and N2O gases emitted from manure management is mainly affected by three factors (2):

1. Composition of manure: determined by livestock type (Fig. 1), diet quality, and condition of the digestive tract.

2. Manure storage and handling practices: Liquid management systems (e.g., lagoons) tend to promote CH4 production, whereas dry systems (e.g., piles) may increase N2O production by stimulating the nitrification process due to increased aeration (Fig. 2.)

3. Climate in which manure is kept: For liquid systems, warm temperatures increase both N2O and CH4 production by increasing microbial activity. For solid systems, rainfall can stimulate CH4 production, with wet climates having the greatest emission rate relative to arid climates. If adequate nitrate is present in the manure from nitrification, rainfall may stimulate the denitrification process, releasing N2O.

Figure 2: Sources of CH4 and N2O emissions from manure management. Thin arrows indicate the movement of manure between locations. Thick arrows indicate the relative emission rate (3).

Within livestock, cattle production systems can be broadly classified into confined, mixed (in which cattle can be in-house during part of the day or the year), and grassland-based systems (4).

In confined and semiconfined systems, manure can be stored and processed to be disposed in the field, whereas in grazing systems (also called pasture-based management), manure is deposited directly on pastures and is degraded under environmental and grazing conditions (5).

Opportunities to reduce N2O and CH4 emissions from livestock manure are diverse and can be addressed to different parts of the animal production cycle to control the production and emission of these two gases (6). The emissions of N2O and CH4 are highly variable and depend on multiple factors, which makes it difficult to use strategies that mitigate both gases simultaneously.

In general, liquid manure management systems lead to anaerobic conditions and increased methane production, and switching to practices that manage manure in drier, aerobic conditions reduces methane emissions.

This article will dive deeper on three strategies with a major mitigation potential (see below) - anaerobic digestion, daily spread and pasture-based management. These strategies were selected as focus area, as they are good examples of (respectively): a technology-based approach for farms able to spend capital; a management-based approach suitable for smaller farms; and a management-based approach for farms with a large area of land available.

However, other strategies of similar potential are:

• Separate solid-liquid manure content: Manure processing technology that can partially separate solid and liquid manure using gravity or mechanical systems such as centrifuges or filter presses. This process aerates manure storage conditions, which then limits the potential of emitted methane.

• Composting: the aerobic decomposition of manure or other organic material by microorganisms in a managed system. The process generally takes several weeks to months depending on the level of turning or aeration management. Composting manure produces fewer methane emissions than uncovered anaerobic lagoon or liquid/slurry manure management systems.

• Manure drying practices: involves any of a variety of methods to reduce the liquid content of manure to achieve a solids content of 13% or more. Manure drying is commonly used to facilitate the transport or storage of manure, and reduces the amount of manure entering uncovered anaerobic manure lagoons and thereby reduces the volume of methane emissions from lagoons.

Anaerobic digestion:

Anaerobic digestion (the most known technology for manure management) is a process through which microorganisms break down manure in the absence of oxygen and produce a mixture of biogas (mainly CH4 and CO2) and digestate. Where production is carried out on a large, intensive scale and manure is stored under anaerobic conditions, methane can be captured with biogas collectors. The captured methane can be flared or used as a source of energy for electric generators, heating or lighting.

Common designs include covered anaerobic lagoons, plug flow digesters, and complete mix digesters.

Best use case:

• Most common on swine and dairy operations that manage manure as a liquid or slurry and collect at a single location.

• Suitable for all climates, depending on the anaerobic digestion type selected.

• Suitable for large and small facilities (although there are economies of scale favoring large operations).

System requirements:

• Manure should be managed as a liquid or slurry and should not contain any materials that may inhibit the digester such as sand.

• Pretreatment may be required to reduce the size of the feedstock and remove contaminants.

• Needs infrastructure to process, transport, and destroy or use biogas and digestate products.

Daily spread:

In a daily spread management practice, manure is removed from a barn and is applied to cropland or pasture daily, decreasing the time manure is stored in anaerobic conditions.

Best use case:

• Suitable for smaller farms.

• Suitable for warmer climates as this practice is done daily, regardless of soil condition, weather, or time of year.

• Manure should not be spread near waterbodies or on snow to prevent runoff.

System requirements:

• Equipment to collect and land apply manure daily.

• Avoid spreading manure near wells, springs, sinkholes, terrace tile inlets, wetlands, or on slopes adjacent to streams, rivers, or lakes.

• Adequate land area to apply manure is needed.

Pasture-based management:

A pasture-based management system consists of keeping animals on fenced pasture. Animals are rotated between grazing areas to improve the health of the pasture and to spread manure. Manure is left as-is to return nutrients and carbon to the land. In livestock grazing systems, strategies such as the optimization of the diet, the implementation of silvopastoral systems and other practices with the capacity to improve soil quality and cover, and the use of nitrogen fixing plants are among the practices with more potential to reduce emissions from manure and at the same time contribute to increase carbon capture and improve food production (7).

Best use case:

• Best suited for ruminants, such as grazing cattle, which can rely on grasses as their main feed source.

• Suitable where forage is available year-round and animal confinement is not necessary for protection from the weather.

• In colder climates, feed would need to be supplemented when forage is not available, and confinement would be needed for protection from the weather.

System Requirements:

• Sufficient fenced acreage per animal to support the animals' nutritional needs, allow for rotated pastures, and manage the nutrient load of the manure.

• Supplemental manure management for milking areas in dairy operations and for areas of animal confinement.

In 2019, Cargill launched BeefUp Sustainability, an initiative committed to achieving a 30% greenhouse gas (GHG) intensity reduction across its North American beef supply chain by 2030. Designed to engage a diverse set of stakeholders including producers, customers and innovators; the initiative focuses on four key areas: grazing management, feed production, innovation and food waste reduction. The 30% reduction builds on the industry’s existing GHG efficiency efforts and will equate to removing 2 million cars from U.S. highways for a year (8)(9).

Climate impact

Targeted emissions sources:

• Manure management targets CH4 and N2O emissions. Scope 1 (farm owners):

• Reduced emissions from CH4 and N2O Scope 2 (farm owners):

• Reductions in energy if biogas is reused to power the farm and machinery Scope 3 (companies purchasing livestock):

• Category 1 (Purchased goods and services)

• Category 3 (Fuel- and energy-related activities not included in scope 1 or scope 2)

Decarbonization impact

Anaerobic digesters:

Up to 90 per cent of the CH4 emitted by anaerobic manure management systems can be captured and combusted.

Methane emissions are directly reduced from anaerobic digester systems used for manure management. Anaerobic digestion systems emit less methane compared to uncovered anaerobic lagoons because the methane emissions are captured and destroyed or utilized.

Nitrous oxide emissions may slightly increase but net GHG emissions are expected to decrease

In addition, when biogas is used for energy, methane emissions are indirectly avoided from reduced fossil fuel use.

Daily spread:

Methane emissions (and net GHG emissions) decrease when converting from an uncovered anaerobic lagoon or liquid/slurry systems. This practice produces less methane emissions than an uncovered anaerobic lagoon because the manure is applied daily and is not stored for an extended time in anaerobic conditions.

Pasture-Based Management:

This practice produces less methane emissions from manure management than an uncovered anaerobic lagoon because the manure is not stored in anaerobic conditions. In addition, where manure replaces fertilizer, emissions from production and use of fertilizer may decrease.

Carbon can be sequestered in the soil with this practice. However, research finds that nitrous oxide emissions may increase. Therefore, lifecycle analysis may be necessary to estimate net GHG emission reductions.

Business impact

Benefits

Effective manure management not only reduces GHG emissions, but it also contributes to reduce nutrient loss from livestock production systems. This applies to any manure management strategy.

Some benefits are strategy-specific:

Anaerobic digesters:

Will produce biogas which may be used as renewable energy for the farm facilities. The biogas generated can also be sold off, creating a new financial opportunity for the farm.

The digestate can also be used to create a variety of byproducts including livestock bedding, and soil amendments that can be used on-farm or sold to a neighboring farm. This can maximize the value of manure and diversify the revenue stream for the producer.

Daily spread:

Recycling manure to cropland may have added climate value when it reduces the use of synthetic nitrogen fertilizer, an energy-intensive product. This is also reduce the financial cost of purchasing fertilizers.

Spreading manure daily also reduces the need for long-term storage of manure, reducing capital expenses on storage facilities.

Pasture-based management:

As in the daily spread approach, pasture-based management returns manure to the land, therefore reducing the need to purchase fertilizer.

Pasture-based management also ensures a more uniform distribution of manure nutrients onto the land (8).

Costs

Just as opportunities to reduce CH4 and N2O emissions from livestock manure are diverse, the costs associated will depend on what strategy a company chooses:

Anaerobic digesters:

• Requires both CAPEX and OPEX investment

  • High initial expenses.

  • Requires staffing for regular maintenance and management.

  • May be subject to permitting requirements.

High capital and operating costs can be offset by the production of electricity, heat, and/or transportation fuel, the injection of biogas into existing natural gas pipelines, and the development of by products such as fertilizer or bedding.

Daily spread:

• There are daily labor and equipment costs associated with this management practice.

• Manure by itself typically does not contain the complete nutrient balance that crops need. Costs will be associated with the input purchase to rebalance the chemical composition.

Pasture-Based Management:

• May require acquisition of land suitable for pasture.

• Labor and investment required to:

  • Develop a grazing management plan.

  • Perform pasture maintenance such as the spreading of nutrients and weed management.

  • Provide water and any supplemental nutrients.

  • Manage temporary fencing needed for pasture management.

• Weather challenges must be addressed, such as animal confinement in the colder months and water and shade availability in paddocks.

• Grazing and rotating animals near streams or water bodies will require added fencing to protect water quality.

• A supplemental feed budget is necessary when the nutrient needs of animals cannot be met solely by grazing.

Impact beyond climate and business

Co-benefits

Manure management with anaerobic digestion can conserve agricultural land as it:

• Improves soil health by converting manure nutrients into a plant-friendly form.

• Safeguards local water resources by reducing nutrient run-off and eliminating pathogens.

Improved pasture management offers co-benefits such as:

• Enhanced soil drainage

• Reduced soil erosion

• Reduced invasions of noxious and poisonous weeds

Potential side-effects

Many of the most effective mitigation strategies involve expensive technology (e.g., anaerobic digesters) and increased use of inputs.

Handling manure can be difficult and unpleasant work, especially for low-income farmers without access to infrastructure or machinery.

Variability in manure characteristics and climate make it difficult to propose a one-size-fits-all solution to minimize GHG emissions from manure management. Mitigation practices need to be tailored to the system’s needs.

Some manure management structures contribute to embedded emissions (e.g. cement structures).

Manure by itself typically does not contain the complete nutrient balance that crops need, it is not a “complete” fertilizer. In some cases, unbalanced manure fertilizer application can lead to high soil test phosphorus (P) and potassium (K) levels beyond what crops can utilize. Both manure testing and soil testing are important to determine the necessary adjustments, and costs will be associated with the testing and the input purchased to rebalance the chemical composition.

Some side effects are strategy-specific:

Anaerobic digesters:

• When manure is anaerobically digested, the biogas produced is primarily composed of methane and carbon dioxide, with lesser amounts of hydrogen sulfide, ammonia, and other gases. Each of these gases has safety issues. Overall, biogas risks include explosion, asphyxiation, disease, and hydrogen sulfide poisoning (9).

Daily spread:

• Daily spread can result in over application of nutrients if there is not adequate land to apply manure.

• There can be concerns for the area's water quality when spreading manure during precipitation events or in colder climates due to runoff from frozen ground.

Pasture-based management:

• Some research has found that while pasture-based cows are less at risk of many health issues, they may be at more risk of internal parasitism, malnutrition and delayed onset of estrous activity postpartum than confined cows (10).

Typical business profile

This solution applies to companies with livestock in their supply chain, including input providers, farmers and producers, traders, manufacturers and processors, and retailers. Companies can play a critical role in reducing global GHG emissions by creating incentives and by investing in technologies that are tailored to the needs and concerns of farmers. By investing in the reduction of manure-related emissions in their supply chain, companies in the livestock sector can reduce their Scope 3 emissions significantly per kilogram of product.

Farm specificities:

• The manure management strategy to choose will depend on some key parameters of the farm. For example, a large farm with access to significant financial resources may select anaerobic digestion. Smaller farms with enough human capital will likely select daily spread. Finally, if a farm specializes in beef cattle and has access to large areas of land, a pasture-led approach is likely the best option.

Geographical specificities:

• The efficacy of manure management strategies will depend on the type of production system, associated costs, market availability, and localized social acceptance.

Approach

It is essential to have a comprehensive manure management plan that encompasses all aspects of manure collection, storage, treatment, and application. The best manure management strategy will change from farm to farm, depending on available capital and labor, waste sources, soil type, cropping practices, even personal preferences.

Some questions should be considered before developing a manure management strategy:

• Is there an optimal manure management strategy that is specific to;

  • your type of livestock?

  • your region of activity?

• Is manure management regulated in your region of activity?

  • If yes, is there any funding to support your transition to a more sustainable manure management strategy?

• What is the goal of your manure management strategy, beyond managing emissions

• What is the amount of capital and labor available?

• Is labor available year-round?

See the resource “Livestock & Poultry Environmental Learning Community. Evaluating Costs and Benefits of Manure Management Systems for a Decision-Support Tool” for a complete walk through of how to decide which manure management strategy is best for your farm.

Stakeholders involved

Buy-in from on-site farm workers is critical to ensure manure emissions are efficiently managed throughout the lifecycle of livestock. However, buy-in from an array of internal and external stakeholders is also needed to efficiently reduce manure emissions:

• Executive Management: To set decarbonization goals specific to methane and nitrous oxide emissions and integrate these into procurement requirements; approve investment in improved feed and additives, and in a novel animal productivity strategy.

• Finance & Accounting: To align budget availability amid a new methane and nitrous oxide emissions reduction strategy.

• Procurement: To implement a roadmap for engaging producers with new procurement requirements.

• Farmers: To introduce an improved feeding regime and feed additives and implement an animal productivity strategy.

• Government: To set methane and nitrous oxide emissions reduction targets for the agri-food sector; support the roll-out of policies that offer incentives for companies actively reducing manure-related emissions in their supply chain; support research and development into cost-effective manure-related emissions reduction.

Key parameters to consider

• Cost: Consider costs related to basic and specialized equipment, labor, installation, operation, and maintenance.

• Return on investment: Evaluate manure management practices for opportunities to offset costs or generate revenue.

• Goals and priorities: Account for the operation's water quality goals, labor requirements, nutrient utilization, or other priorities.

• Regulatory implications: Evaluate regulatory implications of changing manure management practices, such as need for permits.

• Safety protocols. Review and adjust existing safety protocols to account for new manure management practices.

• Unique operation characteristics. Consider the operation size, type and amount of manure, and the existing manure management system to determine the feasibility of switching practices. For example, daily spread is best suited for smaller farms that have time and resources to spread manure daily.

• Solution maturity: Livestock manure has been used as a tool to create a naturally derived fertilizer since the Neolithic era. Manure management has also been used to reduce impacts on climate and water quality for decades. However, quantitatively assessing the efficacy of one strategy compared to another is difficult, due to variables differing in each farm location.

Implementation and operations tips

There is no single strategy that is the “best.” Each technology and its components have advantages and disadvantages. The best manure-treatment technology depends on personal preferences, available capital and labor, waste sources, soil type, cropping practices, skills needed to use the technology, and a number of other factors.

The ways in which farmers choose to manage the manure created on their farm is driven by varying tradeoffs, including economic, social, and environmental; therefore the implementation of a manure management strategy will be unique to each strategy, depending on the goals of the strategy itself.