The Manure Scoop

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

200

200

Corn Mass

9,464

9,464

Harvest Index

0.5

0.5

Corn Stover Dry Mass

9,464

9,464

Root to Shoot

0.21

0.21

Root Mass

1,987

1,987

Manure Applied

0

3,600

Stover Harvest

0

2,000

Respired Corn Stover

8,518

6,718

Respired Corn Root

1,391

1,391

Manure Respired

0

3,240

Corn Stover to SOM

946

746

Corn Root to SOM

596

596

Manure to SOM

0

360

SOM Added

1,543

1,703

0

Original Soil Organic Matter

5.0

5.5

Organic Matter in Acre

100,000

110,400

Percent OM Mineralized per Year

1.5

1.5

Organic Matter Mineralized

1543

1703

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.

 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.

 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.