Thursday, January 25, 2024

Optimizing Anaerobic Digestion Efficiency: Calculating Biogas Potential




Anaerobic digestion is a process that harnesses the power of microorganisms to break down organic materials without oxygen, producing biogas as a valuable byproduct. This process has gained significant attention as a means to manage organic waste and generate renewable energy. This blog will delve into the intricacies of optimizing anaerobic digestion efficiency, focusing on factors influencing different digester types and substrates – specifically, livestock manures and crop residues.


Factors Influencing Anaerobic Digestion Efficiency:


Temperature and pH:

Anaerobic digestion is a temperature-sensitive process with optimal efficiency and stability within specific temperature ranges. Additionally, maintaining an appropriate pH level is crucial for the activity of microorganisms involved in digestion. Different digester types may require adjustments in temperature and pH to maximize efficiency. Generally, designed heated digesters for agriculture are maintained at around 95-100ºF. As a general rule of thumb, microbial activity doubles for every 20ºF, so heating a digester allows significantly shorter retention times. While heating above 100ºF can further increase reaction rates, it also makes the process less stable as different bacteria and archaea populations respond differently to temperature.


Retention Time:

The duration for which organic materials are retained in the digester, known as retention time, plays a vital role in achieving optimal biogas production. Longer retention times generally lead to higher gas yields, but striking the right balance is essential to prevent process inhibition and to balance the initial cost of the digester against the potential yield for the substrate. For example, holding materials for an additional 30 days to make 5% more methane often can’t be justified.


Substrate Characteristics:

The type and composition of substrates significantly impact anaerobic digestion efficiency. Livestock manures, such as those from cattle, poultry, and swine, vary in nutrient content and organic composition. Crop residues, including straw and stalks, also introduce diverse characteristics to the digestion process. Understanding these variations is crucial for efficient biogas production and material handling considerations. Different tests, biochemical methane potential, and anaerobic toxicity assays are often used to characterize how desirable different substrates may be and if there could be issues with inhibition from chemical compounds. Physical properties are often characterized for solids content and particle size, with viscosity and settling rate sometimes characterized.


Digester Types and Their Influence:


Batch Digesters:

Batch digesters are characterized by loading organic materials in batches and allowing them to ferment for a specific period. These digesters are suitable for smaller-scale operations and can handle various substrates. However, optimizing efficiency in batch digesters requires careful consideration of loading frequency and substrate characteristics. In practice, few of these digesters exist, though more use has been seen with “high-solids” digestion.


Continuous Stirred-Tank Reactors (CSTR):

CSTRs maintain a constant flow of organic material into the digester, ensuring a continuous process. These systems are efficient for managing large quantities of waste. Factors such as temperature control, stirring mechanisms, and substrate consistency play key roles in optimizing CSTR performance.


Plug Flow Digesters:

Plug-flow digesters facilitate a unidirectional flow of organic material through the digester, promoting better mixing and higher gas yields. Achieving optimal performance in plug flow digesters involves careful design considerations and monitoring of substrate characteristics. Essentially, it needs a high enough solids content to act as a plug but not so high that it won’t flow through the digester.


Substrates: Livestock Manures and Crop Residues:


Livestock Manures:

Different livestock manures present unique challenges and opportunities for anaerobic digestion. Cattle manure, for instance, is rich in volatile solids, while poultry manure has a higher nitrogen content. Understanding the nutrient profiles of various manures is essential for tailoring digester conditions and maximizing biogas potential. In general, liquid manures are often preferred as little modification is needed to make them amenable to use in a digester. Many manures have higher nitrogen contents, which can make ammonia toxicity a potential concern.


Crop Residues:

Crop residues, such as straw and stalks, contribute to the diversity of anaerobic digestion substrates. These materials often have a higher lignocellulosic content, requiring special attention to enhance breakdown and gas production. Exploring pre-treatment methods can improve the digestibility of crop residues in anaerobic digesters. Particle size and maceration considerations, as well as the overall moisture content of the mix, are important to make these materials function in a digester.


Simple Calculation for Estimating Biogas Production:


A simple calculation for estimating methane production is based on the volatile solids content (VS), biochemical methane yield potential (BMP), and digester efficiency. The formula for biogas production (BP) is given by:




VS is the volatile solids content of the substrate.

BP is the methane yield potential, representing the volume of methane produced per unit of volatile solids.

DE is the digester efficiency, accounting for the proportion of methane produced compared to the maximum potential.


The complicating factor is getting good values for each of these parameters. The volatile solids and BMP vary based on diet, manure holding time, weather conditions, and other factors, making estimating for a specific farm difficult.


For lagoons, this can be complicated, as the temperatures and storage times vary regionally. In the figure below, the blue color represents lagoon digester efficiency if a yearly retention time is used, the red if manure is applied twice per year, and the purple the loss of efficacy from more frequent manure removal. Estimated lagoon efficacies reported in the EPA Ag Star database are provided to the right of the figure. Green dots represent states where reporting lagoon digesters are located (the majority in California). Heated digester efficiency is generally closer to 75%, with temperature, substrate, and retention time all impacting reported efficiency.

Figure 1. Estimated lagoon efficacies for livestock manures. Blue represents efficacy of annual application, red represents a twice a year application strategy, and the purple represents the loss in efficiency from twice a year application instead of annual application.


Optimizing anaerobic digestion efficiency is a multidimensional task that requires careful consideration of factors influencing both digester performance and substrate characteristics. By understanding the nuances of different digester types and the diverse nature of livestock manures and crop residues, we can pave the way for sustainable waste management and renewable energy production. The future of anaerobic digestion lies in the synergy between scientific understanding and practical application, offering a promising avenue for addressing environmental challenges while harnessing the full potential of biogas production.

Wednesday, December 20, 2023

Maximizing Efficiency: Aeration for Manure Treatment in Livestock Barns

 Managing manure is a critical aspect of modern livestock farming, and as the agriculture industry evolves, so do the techniques for handling and treating manure. One innovative method gaining popularity is aeration, particularly in deep pit manure storages for swine finishing, dairy cattle, and beef barns. This article delves into the science behind aeration, its impact on manure solids breakdown, nutrient content, ammonia and greenhouse gas emissions, and its influence on biological oxygen demand (BOD).


Aeration vs. Anaerobic Decomposition:

Traditionally, liquid manure management has relied on anaerobic decomposition, which occurs without oxygen. While this method is effective, it focuses most effort on storage, especially for deep pit facilities, with anaerobic decomposition occurring as a side effect of organic matter in the manure being held. In anaerobic situations, organic carbon molecules break down in cascading reactions towards carbon dioxide and methane. However, as little energy is released in these reactions, reaction rates are typically slower and can result in the loss of some partially degraded organic compounds that result in odor. Similarly, as these reactions are low energy they often are slow, which can mean slower solids decomposition.

Aeration introduces oxygen into the manure, fostering an aerobic environment. Aerobic reactions release more energy and, as a result, encourage greater microbial activity and faster reaction rates. Because reaction rates are faster, the breakdown of solids is encouraged. Carbon processing in aerobic conditions flows from organic carbon towards carbon dioxide, and while it is again a series of cascading reactions, the higher microbial activity limits the accumulation of carbon breakdown products and the potential for odor emissions.


Manure Solids Breakdown:

Why do elephants breathe in oxygen? I'll often start a lecture this way when discussing manure because it illustrates the concept of energy flows well. If you want to get big you need to be making energy in the reactions. Aeration enhances the breakdown of manure solids through microbial activity. Microorganisms thrive in the presence of oxygen, accelerating the decomposition process. This results in a more homogeneous and liquid manure, making it easier to handle and apply to fields as fertilizer. However, the extent of the change depends – on how complete was brake down under anaerobic conditions and how complete it is under aerobic conditions. Aeration can potentially increase solids breakdown and generally will, but the extent matters in these conversations about what the result will be.

The air flux through the manure also has the potential to suspend and mix manure solid particles back into the manure, making it more uniform. However, limited work has shown the impact at different air flow rates. Several years ago, experience with foaming manure indicated that higher methane fluxes lead to more uniform manure composition; similar impacts would be expected for aeration mixing, but results will vary based on changes in solids content, particle size, and the intensity the aeration mixing provides.


Nutrient Content and Availability:

Aerobic conditions promote the conversion of organic matter into free ions. Less organic matter (i.e., greater solids breakdown can free nitrogen and phosphorus making it more rapidly available in aerated manure. For phosphorus, this has little impact on overall fertility, as the phosphorus is held reasonably well in soil, and we can implement fertility management planning to handle availability. For nitrogen, the answer is more complex. For a manure like swine, where all the nitrogen is already first year, it is more immediately available at application, but it doesn't change our ability to take advantage of the nitrogen. With cattle manures where typically a multiyear mineralization sequence is used, it will push more of the nitrogen being available into the first year, where we have more opportunity to account for it and credit the manure appropriately.


Ammonia, Odor, and Methane Emissions:

Aeration can either increase or decrease ammonia emissions from manure, with no clear answer yet available, and the answer is at least somewhat dependent on the aeration system used. Aeration provides a relatively clear decrease in odor emissions for the reasons discussed previously. Additionally, methane emissions are diminished under aerobic conditions – with low levels of aeration generally as effective as methanogenesis, performing microorganisms are relatively easily disrupted.


Biological Oxygen Demand (BOD):

How much oxygen or air is required? It depends – what are the goals of the aeration system, and what are we hoping to achieve? However, in all cases, we are generally trying to aerate based on the biological oxygen demand of the manure. The biological oxygen demand, or biochemical oxygen demand, is the amount of dissolved oxygen aerobic organisms need to break down organic material in a given water sample at a certain temperature over a specific period. Essentially, it is a measure of the wastewater, or manure strength, with higher biological oxygen demand, meaning greater wastewater strength.

Historically, the recommendation has been approximately supplying enough oxygen to satisfy twice the biological oxygen demand in the manure. This recommendation is based on stabilizing all decomposable organic matter in the manure and assuming oxygen transfer efficiencies. However, low-rate (0.3-0.5x BOD) aeration has proven successful for odor reduction and methane emission mitigation, with lower potential benefits.


Full Rate Aeration vs. Low Rate Aeration:

The choice between full-rate aeration and low-rate aeration depends on various factors, including the type of livestock, barn size, and climate conditions. Full-rate aeration involves high airflow, facilitating rapid manure decomposition and more complete decomposition. On the other hand, low-rate aeration is a more energy-efficient option that may provide many benefits (reduced odor, reduced methane emissions) while reducing energy requirements for system operation.



The ASABE manure production standard provides estimated BOD excretion for different animals, with finishing cattle estimated at 1 lb BOD per day, dairy cattle at 3 lb BOD per day, and finishing swine at 0.3 lb BOD per day.

Looking at swine, let's do an example where we want to supply 2X the BOD for aeration and another at 0.3x.

We can make a rough estimate of the energy needed to achieve the oxygen requirement. While there is variation in oxygen transfer efficiencies, 3-5 lb O2/kWhr are typical.

Assuming an electrical cost of $0.10/kWhr allows operating costs to be estimated.


Full Aeration

0.3 lb BOD/pig- day x 2 lb O2/lb BOD = 0.6 lb O2/pig-day

0.6 lb O2/pig-day / 4 lb O2/kWhr = 0.15 kWhr/pig-day

0.15 kWhr/pig-day x $0.10/kWhr x 365 days = $5.50 /pig space-year


Partial Aeration

0.3 lb BOD/pig-day x 0.3 lb O2/lb BOD = 0.09 lb O2/pig-day

0.6 lb O2/pig-day / 4 lb O2/kWhr = 0.0225 kWhr/pig-day

0.025 kWhr/pig-day x $0.10/kWhr x 365 days = $0.82 /pig space-year


Further work is needed to quantify the difference in potential benefits of each system to understand how different aeration rates impact solids suspension, breakdown, and mixing.



Aeration is revolutionizing manure management in livestock farming, offering a sustainable and environmentally friendly alternative to traditional anaerobic methods. The benefits, from improved nutrient content and availability to reduced ammonia and greenhouse gas emissions, make it a valuable tool for modern agriculture. Farmers should carefully assess their needs and consider factors such as BOD levels, livestock type, and barn size when implementing aeration systems, ultimately contributing to a more efficient and sustainable farming future.


In next month's article, we will use this information to size a blower system and the airlines for an example aeration system.

Figure 1. Aeration lines outside the barn that actuate in zones to aerate the manure.

Wednesday, November 15, 2023

Balancing Act: Carbon Flows in a Continuous Corn System with Stover Harvesting for Livestock Bedding and Manure Reapplication

In the vast landscapes of modern agriculture, the intricate dance of carbon within ecosystems plays a pivotal role in sustaining soil health and fertility. One such intricate choreography unfolds in continuous corn systems where the harvest of corn stover for livestock bedding intersects with the subsequent application of stover-laden manure. Let us explore this delicate balance of carbon flows and its implications for soil health.

Harvest: The Carbon Exodus

Continuous corn systems rely on the consistent cultivation of corn year after year. In this cycle, corn stover, comprising stalks, leaves, and cobs, is a crucial component. However, the decision to harvest corn stover for livestock bedding initiates a subtle but impactful carbon exodus from the agricultural ecosystem. When corn stover is removed from the field, it takes a significant portion of organic carbon. This organic matter, a key constituent of healthy soils, contributes to soil structure, water retention, and fertility. The concern arises when this organic carbon, essential for microbial activity and nutrient cycling, is carted away, potentially leaving the soil in diminished fertility.

To help put some numbers to this, let us assume 200 bushels of corn, a harvest index of 0.5, and a shoot-to-root ratio of 0.21.

200 bu/acre x 56 lb/bu x 0.845 lb dry matter/lb grain = 9464 lb dry grain/acre

Using the harvest index, we can get stover produced.

0.5 = 9464 lb grain/(9464 lb grain + lb stover) gives 9464 lb stover/acre

Finally, the root-to-shoot ratio can be used to calculate how much root biomass is generated.

0.21 = root/9464 lb stover gives root = 1987 lb root/acre

The Livestock Connection: Stover as Bedding Material

The journey of corn stover continues beyond the fields. Harvested stover finds its way to livestock operations, where it takes on a new role as bedding material. Livestock farmers recognize the value of corn stover for providing comfortable and absorbent bedding for their animals.

A starting estimate for beef cattle bedding use in a bed pack barn is around 5 pounds per head per day or about 1800 lb per year. We want to harvest about 1 ton per head.

The Return: Manure Application

As livestock bedding breaks down, it decomposes, releasing nutrients and organic matter into the manure. When this nutrient-rich manure is reapplied to the fields, it returns a significant amount of organic carbon. This return of organic matter is vital to soil health and carbon sequestration.

A typical bed pack manure is around 20 lb N per ton (Assume 50% 1st year available, 5% volatilization losses), and each space generates 6 tons of manure annually. To make this easier, I will assume a manure application rate of 6 tons/acre at 70% moisture or 1.8 tons dry matter.

The carbon flow in this system resembles a cyclic rhythm, echoing the sustainable principles of nutrient recycling. The manure application not only replenishes the carbon lost during stover harvesting but also contributes to the overall organic matter content in the soil.

The Balancing Act: Where does this leave us?

So now, the fun part is making assumptions to track organic matter. There are many complicated models to help track soil organic carbon, but in the interest of time and ease, I will make assumptions to simplify what is happening. I am assuming 10% of above-ground biomass is transformed into soil organic matter and that, similarly, 10% of applied manure becomes soil organic matter. For roots, I am going to be more aggressive and say 30% of that material becomes soil organic matter. Some fraction of the soil organic matter also needs to mineralize per year, and unfortunately, I do not have a good rule of thumb on what this is – so I estimated it for a continuous corn system by assuming equilibrium at a known soil organic matter content, in this case, 5% soil organic matter which suggested annual organic matter mineralization of 1.5%. This organic matter mineralization was then used in the stover harvest-manure application case to estimate the soil organic matter content under this operation. Carbon flows estimated are shown in Table 1.

Table 1. Carbon flows in a continuous corn system and in a continuous corn system with stover harvest and manure application. 


Continuous Corn

Continuous Corn

Stover Harvest & Manure Application

Grain Yield



Corn Mass



Harvest Index



Corn Stover Dry Mass



Root to Shoot



Root Mass



Manure Applied



Stover Harvest



Respired Corn Stover



Respired Corn Root



Manure Respired



Corn Stover to SOM



Corn Root to SOM



Manure to SOM



SOM Added




Original Soil Organic Matter



Organic Matter in Acre



Percent OM Mineralized per Year



Organic Matter Mineralized



A few thoughts, this exercise is meant to be illustrative, and not determining of an exact field as the overall process was vastly simplified. However, with that said, our results indicated that higher organic matter would be expected in a field where stover is harvested for bedding and then that material applied as a manure. This should be expected, because on net, the more organic matter is added because of the cattle excrement and is removed from the field.

Of course, there are many complicating factors. Examples include, how does the use of manure impact tillage choices at the farm? If the farm was no-till and then choose to use manure, perhaps a tillage pass is added. This tillage pass would almost certainly increase the rate of soil organic matter mineralization. Does more organic matter in the soil increase the percent of organic matter mineralized per year? I think it almost certainly has to, but I didn’t model it as such here because I don’t have data to suggest a new rate of mineralization. With that said, even with these caveats I think the balance and process are illustrative as they help us understand some of the approaches we can use to get a feel for how different practices might impact our soil.

Conclusion: A Symphony of Sustainability

In the realm of continuous corn systems where corn stover is harvested for livestock bedding and later reapplied with manure, the carbon flows represent a dynamic interplay between extraction and replenishment. Striking a balance between the needs of crop production and livestock management is essential for ensuring the long-term sustainability of agricultural ecosystems. In many respects what I showed here is simple, I matched the harvested area with an area that is going to receive manure. There could be places where this isn’t the case. By understanding and respecting the nuances of carbon flows, we can foster agricultural systems that not only meet the demands of the present but also preserve the vitality of the land for generations to come.

Wednesday, September 27, 2023

Optimizing Crop Success: The Crucial Role of Manure Application Timing in Iowa

 As we embark on another agricultural season here in Iowa, it's time to delve into a critical aspect of farming that often goes unnoticed but plays a pivotal role in the success of our crops: manure application timing. While it might not be the most glamorous topic, understanding when and how we apply manure can significantly impact our yields and the environment.

Manure has long been regarded as a valuable resource in Iowa agriculture. It provides essential nutrients like nitrogen, phosphorus, and potassium to our crops, reducing the need for synthetic fertilizers. However, manure application timing is key to maximizing its benefits while minimizing potential drawbacks.

The Early Bird Gets the Worm: Spring Application

As the frost thaws and fields become workable in the spring, many Iowa farmers opt to apply manure. This timing aligns with planting, making it convenient to incorporate manure into the soil before sowing or transplanting seedlings. Spring application allows crops to access nutrients as they need them throughout the growing season, promoting healthy development.

However, spring application comes with its challenges. Manure can be challenging to work into the soil when wet, and timing must align with field conditions, which can be unpredictable in Iowa's temperate climate. There's also the risk of nutrient runoff during heavy spring rains, potentially impacting water quality in our rivers and streams.

Fall Application: A Sensible Alternative

Fall application allows farmers to take advantage of drier field conditions and can help reduce the risk of nutrient loss to runoff at the time of application. The nutrients have more time to break down and become available to crops over the winter months. However, there's a trade-off; some nitrogen may be lost to the environment, and more extended periods between application and crop growth allow the conversion of manure nitrogen to nitrate, a form that is highly susceptible to loss.

The Balancing Act

The choice between spring and fall application isn't one-size-fits-all. It depends on various factors, including the specific crop, soil type, and weather patterns. As we move forward in Iowa agriculture, it's crucial to continue implementing sustainable farming practices. Manure application timing is just one piece of the puzzle. By striking the right balance, Iowa farmers can nourish their crops efficiently while safeguarding our precious natural resources.

Manure application timing is not a minor detail but fundamental to successful and responsible farming in Iowa. Careful consideration of when and how we apply manure is essential to maximizing manure as a fertilizer resource and protecting the environment. Farmers are encouraged to wait to apply manure until soil temperatures are consistently trending cooler (towards 50ºF) to ensure the nitrogen will remain in the soil until crops are planted next spring.

Monday, August 21, 2023

Can Iowa Pork Offset Its Way to Carbon Neutral?

 Written by Jacob Willsea

Iowa is home to the largest swine industry in the USA, housing 23 million pigs at any time. Between feeding, processing, and manure handling/storage, the Iowa swine industry emits 0.5 metric tons (MT) of CO2e per pig space annually, totaling 11.5 million MT CO2e/year. Reduction of the carbon footprint for the pork industry has been a topic of interest for years, resulting in improved energy efficiency in animal housing and meat processing and greenhouse gas (GHG) reductions in manure management. Some farms are implementing onsite renewable energy systems to reduce their carbon footprint, including installing wind turbines, solar panels, and anaerobic digesters. This begs the question: Can installing renewable energy on farms make Iowa pork production carbon-neutral?

Let's start by looking at the current Iowa electricity sector:

Figure 1. Iowa electricity generation source.

Despite renewable energy sources supplying 57% of Iowa's energy, the energy industry still emits 0.38 MT CO2e per megawatt-hour (MWh) of electricity produced. Therefore, to offset the 11.5 million MT CO2e produced by the pork industry, an additional 30 million MWh of renewable energy would need to be supplied to the grid. One option for providing this energy to the grid could be the installation of wind turbines on farms.

Thirty-seven million MWh of Iowa's energy is currently supplied by the 12,500 MW of wind turbines installed across the state. To make up the required 30 million MWh to offset the swine industry, another 10,100 MW of wind turbines would have to be installed. About 10 MW of wind turbines can be placed on one section of land (1 mi2), so installing 10,100 MW would occupy about 650,000 acres. Although wind turbines can be farmed around, they still eliminate about 0.75 ac of farmable land/MW installed due to required access roads, concrete footings, and power substations. This loss equals $0.43/pig space from reduced corn sales (200 bu/ac and $6.50/bu). The wind turbines would produce about 1,310 kWh/pig space-year. On average, a swine farm uses only 26 kWh/pig space-year, so the farmer can return 1,284 kWh/pig space to the grid. The energy company's electricity buyback rate of $0.06/kWh would yield $77/pig space per year for the farmer. Wind turbines cost about $1,000,000/MW installed, so installing 10,100 MW of wind turbines would cost about $440/pig space. Assuming a project life of 10-years, a time value of money of 8%,  and that maintenance on the windturbines is $24,000 per MWh per year, then after selling the electricity back to the grid and accounting for crop loss the project would have a net present cost of $0/pig space.

An alternative renewable electricity source could be solar power. Solar panel efficiency is continuously improving. Could we offset the swine GHG emissions with solar power?

Technological developments have improved the efficiency of solar panels to about 20%. Full sunlight supplies about 0.93 kWh/ft2, so with Iowa's 4 hours of full sun each day, a solar panel could absorb 0.37 kWh/ft2/day and output 27.13 kWh/ft2/year. To produce the required 30 million MWh of electricity, about 25,500 acres of solar panels, or 48 ft2/pig space, would have to be installed across Iowa.

Like wind turbines, solar panels would make $77/pig space from selling electricity to the grid. Assuming the solar panels are installed on farmable land, the farms would take a loss from reduced crop production. If a farmer installs 48 ft2 of solar panels per pig space instead of planting corn, the farmer will lose about $1.44/pig space/year from reduced productivity. The capital cost for the solar panel installation is about $450,000/acre, which equates to $500/pig space. After selling electricity back to the grid, the total cost for this project would be $328/pig space over a ten year project life.

Recall that 30 million MWh of fossil fuel-based electricity must be replaced with renewable electricity to offset the swine industry fully. Let's revisit the overview of the Iowa electricity sector, this time with the electricity output in MWh:

Figure 2. MWhs of electrical generation form renewable and non-renewable sources annually in Iowa.

Fossil fuels only produce 28.7 million MWh/year of electricity in Iowa. Therefore, even if all non-renewable fuel sources were replaced with renewable electricity today, 30 million MWh cannot be offset. Furthermore, the value of 30 million MWh is estimated using the electricity sector's carbon intensity (CI) score. The CI score measures GHG emissions per unit of energy output. The CI score is the baseline for estimating the amount of renewable electricity it would take to offset fossil fuel emissions. With every improvement in the electricity sector, lower levels of GHGs are emitted for every unit of electricity produced, meaning a lower CI score. As the grid becomes greener, it will continually take more renewable electricity to offset the same amount of CO2e. The graph below illustrates how the solar panel area required to offset one MT CO2e changes as the electrical grid shifts toward renewable energy.

Figure 3. Installation of solar panels required to achieve different levels of carbon reduction as a function of a cleaner electrical generation grid resulting from clean energy installation.

                The calculations so far have all assumed a constant electricity demand in the coming years. Electricity demand will increase proportionally with the increase in electric vehicles (EVs) on Iowa roads. Today, EVs comprise only 0.2% of Iowa's 2.5 million registered vehicles. Every EV requires 3.9 MWh/year, so if Iowa had 100% EVs on the roads, the electric grid would need to supply an additional 9.4 million MWh annually. Although this increase in electricity demand would make the transition from fossil fuels for electricity production take longer, the full transition to a renewable electric grid is inevitable.

Here is the key takeaway: while adopting renewable electricity systems on farms will support our progression toward a fully-renewable grid and energy independence, it can be a good way to consider your swine farm "green" or "carbon neutral," that title will only be temporary. A future increase in electricity demand will allow farms to consider themselves carbon neutral for a longer period, but once the grid does become fully renewable, emitting no GHGs, your solar panels and wind turbines will no longer be offsetting any emissions, and your swine farm will be a net emitter of GHGs again. This highlights the unfortunate fact that farms will not be able to credit their way to net zero emissions, underscoring the importance of other on-farm strategies that must be implemented for emission reduction.

Manure management is one of the direct methods to reduce your carbon emissions. Upgrades to your storage systems, including covered lagoons and anaerobic digesters, can capture and utilize the methane emissions from your storage. Manure application timing can also make significant impacts on your carbon emissions. Spring and split applications of manure throughout the growing season can limit the length and amount of manure in storage, once again reducing the overall emissions from your farm.

Monday, July 24, 2023

Nitrogen Circularity in Swine Finishing


The circularity of nitrogen in swine finishing production refers to the management and utilization of nitrogen within the swine production system to minimize waste. Nitrogen is an essential nutrient for pigs, but its improper management can lead to environmental issues such as water pollution, ammonia loss, and greenhouse gas emissions. The concept of circularity refers to the percent or fraction of nitrogen that is kept within the cycle and can be used again.

Nitrogen circularity in swine finishing encompasses at least four areas.


  1. Feed Management: Balancing the feed composition with the pig's nutritional requirements minimizes nitrogen in manure. Nitrogen is lost during manure storage and recycling for crop production. As such, maximizing nitrogen retention in the pig is critical for improved circularity. However, the choice of managing manure as a waste or resource is often dictated by the ability of farmers to use the manure more cost-effectively than commercial fertilizer could be applied. If manure is treated as a waste, typically, circularity is reduced. Finally, diet ingredient selection impacts system efficiency by its relationship to crop production and crop selection and the nitrogen use efficacy in the cropping system. 
  2. Manure Management: Efficient handling and treatment of swine manure can minimize nitrogen losses and environmental impacts. During manure storage, nitrogen is volatilized to the atmosphere and lost. Technologies such as anaerobic digestion, impermeable covers, and acidification can help reduce this loss. Retaining nitrogen for recycling for crop production is critical for circularity.
  3. Nutrient Management: Implementing nutrient management plans on swine farms can guide the appropriate application of manure to cropland, ensuring optimal nitrogen utilization. By matching the nutrient content of manure with the crop's nutrient requirements, over-application can be avoided, reducing the risk of nitrogen loss and increasing circularity. Manure application method and timing strongly impact the ability to recover applied nitrogen.
  4. Crop Rotation and Cover Crops: Integrating crop rotation and cover crops into the farming system can improve nitrogen cycling. Selection of rotations can reduce nitrogen need for subsequent crops and increase yield relative to limited rotations. Nitrogen-fixing cover crops, such as legumes, can capture atmospheric nitrogen and make it available to subsequent crops. Cover crops have been shown to reduce nitrogen leaching, potentially keeping more nitrogen in the profile for use by subsequent crops.

 Nitrogen circularity is a complex issue that requires a holistic approach. Regulations, incentives, and education can play a vital role in supporting and encouraging the adoption of sustainable nitrogen management practices in the swine finishing industry, but a critical first step is understanding how different systems compare and how other options impact nitrogen circularity. Nitrogen circularity, and our understanding of it, is an ever-evolving topic that we can continue to better understand to make informed decisions that help keep our cropland in Iowa the most productive in the world at providing the food, fuel, and fiber we need.

 To illustrate these concepts, we will look at four systems focused on two different diets (Corn-DDGS and Corn-Soybean Meal), two different manure systems (deep pit and lagoon), and one manure application rate (yield goal rate with any purchased nitrogen fertilizer applied at the ISU maximum return to nitrogen rate) and cropping system focused on an optimized cropping system to supply soybean needed for the diet; manure is then applied to equivalent acres of corn to the amount of soybean that was raised with any manure nitrogen remaining above this level applied to corn raised in a continuous corn rotation.

 Schematics are shown in the four figures below. While the presentation is linear, showing a flow from left to right, the final crops on the right only need processing to become the feed ingredients on the left and begin the cycle again.


Figure 1. Corn-soybean meal diet with a deep pit manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 2. Corn-DDGS diet with a deep pit manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 3. Corn-soybean meal diet with a lagoon manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 4. Corn-DDGS diet with a lagoon manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.


Each figure shows a nitrogen flow through the swine finishing cycle, including an estimate for nitrogen fixation by the soybean. There are many ways of interpreting the cycle, but I like to start with efficiency as an engineer. Efficiency is defined as wanted output/inputs. In this case, the output would be the nitrogen in the pig. Nitrogen inputs include the amino acid and synthetic fertilizer nitrogen purchased to support crop production. Soybean complicates this slightly –the nitrogen they provide is an input, but some of the nitrogen comes from biological fixation and the rest from the soil. As I can't find a reference for how to include them, I will estimate biological fixation and use that fraction as an input.

Another metric that is receiving more discussion is circularity. One means of defining circularity is the percent of required nitrogen inputs obtained by recycling. This would be the amount of nitrogen land applied with manures compared to the total nitrogen needed for fertilizer (both manure and synthetic fertilizer) and the amino acids in the feed. This might be termed "self-reliance" circularity as it is the fraction of circularity an integrated farm could control.

An alternative circularity metric is the "recycling rate." The recycling rate is the percentage of recycled products available for recycling. In our examples, this would be the land-applied manure compared to the as excreted manure plus the nitrogen in the pig. At first glance, including the nitrogen in the pig sounds funny as it is the product, and we want it to be consumed. However, nitrogen is available for recycling at the slaughter and rendering plant and in the human wastewater treatment system, and including this term recognizes that fact and shows how urban locations and areas of consumption must find ways to recover this nutrient to create circular systems.


 Table 1. System comparisons on different performance metrics including efficiency and two circularity metrics.


System Efficiency

Self-Reliance Circularity

Recycling Rate Circularity

Corn-Soy Deep Pit




Corn-Soy Lagoon




Corn-DDGS Deep Pit




Corn-DDGS Lagoon





The recycling rate tells how well we return potential residues into places of value. Nutrients lost to volatilization during manure collection and storage are challenges that must be addressed. While deep pit storages are an improvement over lagoons, innovation is needed to help improve recycling rates further. Nutrients exported with the pig may or may not be recycled depending on what happens at the slaughter facility, the rendering plant, and the human wastewater treatment plant. Within this analysis, I excluded them as they are beyond my scope; however, for a better idea of how the agricultural system is performing, this needs to be included and must be addressed, as these nutrient exports are on the same order of magnitude as nitrogen losses during lagoon storage.

Self-reliance circularity indicates what percent of the nitrogen needs are supplied by items an integrated crop-livestock farm (or system) could control. An alternative way of viewing this number is 100 minus the self-reliance circularity is our reliance on nitrogen sources outside the farm's control. An integrated cropping and swine finishing farm with a deep pit manure storage is currently proving around 71% of the nitrogen it needs to feed a pig and fertilize its crops. An interesting take-home here is that using soybean to biologically fix some of the nitrogen for the diet improves self-reliance circularity significantly as it reduces nitrogen demand to the following crop and provides nitrogen to the manure that didn't originate from synthetic fertilizer or recycling materials. This metric could also be improved by reducing ammonia losses during manure collection and storage, improving rate selection for manure as a fertilizer (probably mostly through improved application timing), and innovating ways to recycle nitrogen that ends up at the slaughter, rendering, and human wastewater treatment facilities.

Finally, we have the system efficiency metric. This metric again favored soybean-based diets, though in this case, this was at least in part due to diet formulations that facilitated greater nitrogen retention in the pig and less excretion and at least in part from reduced nitrogen inputs needed to achieve corn production. This metric could also be improved by reducing ammonia volatilization during collection and storage, as demonstrated by the differences between the lagoon and deep pit systems.

A few caveats – this is by no means an end-all-be-all analysis. Many assumptions were made to facilitate the analysis; for example, I compared nutrient removal with corn grain to the nitrogen fertilizer application rate to facilitate calculations rather than trying to estimate N losses with nitrate leaching, surface water runoff, or denitrification losses. In doing so, I inherently assumed nitrogen additions through deposition didn't matter and that the soil nitrogen level was in equilibrium and unchanging. Similarly, I looked at nitrogen soybeans fixed from the atmosphere and didn't show nitrogen losses during their production. Water measurements in tiled fields show nitrogen leaching in these production systems. Similarly, co-products are produced from both diets – either soybean oil or ethanol. While neither contains nitrogen and thus doesn't impact my analysis with these parameters, recognizing that the system is more complicated and contributing other goods is important in using these metrics.

All this is a fancy way of saying things we already know. We, agricultural and agricultural researchers, need (1) to innovate new solutions that help reduce ammonia losses during manure collection and storage, (2) improve nitrogen rate selection with fertilizers but especially manures (again probably through innovations that facilitate improvements in manure application timing), (3) be mindful of how crop rotation effects alter nutrient needs and alternative in-field management practices (like cover crops) could alter losses and our ability to recycle nutrients, and (4) that on-farm management only goes so far and that recycling of consumed nutrients from urban areas where livestock products are consumed has almost as large of impact on nitrogen recycling metrics as the ammonia volatilization losses.