Comparing N-FACT to the Yield Goal Method: A Look at Nitrogen Recommendations

A few years ago, I took a deep dive into two common approaches for determining nitrogen application rates: the Yield Goal method and the Maximum Return to Nitrogen (MRTN) approach, illustrating which was higher in each county in Iowa and how similar or different they were. That comparison highlighted the strengths and limitations of each method, offering insights into how they influence manure management planning and nitrogen use efficiency. With Iowa State University’s recent release of the Nitrogen Fertilization Application Consultation Tool (N-FACT), it seems like the perfect time to revisit this discussion, comparing N-FACT to the Yield Goal method.

 

N-FACT represents a step forward in nitrogen management, leveraging updated on-farm data and modeling techniques to refine nitrogen recommendations. But how does it compare to the long-standing Yield Goal approach in the Iowa Manure Management plan form? My goal here isn’t to summarize either method but to compare what the two methods suggest. In so doing, it gives some insight into who may be selecting each technique and why that may be.

In this post, I’ll walk through how N-FACT differs from the Yield Goal approach, present some figures to illustrate their recommendations across different scenarios and share key takeaways on their practical applications.

 

Manure planners are probably extremely familiar with the Yield Goal method. Still, the county average corn and soybean yield were briefly estimated using the last five years of county yield data (2019-2023). The amount of continuous corn and corn following soybean in each county was calculated from the area of corn planted and the area of soybean grown in each county. Corn following soybeans are set equal to the amount of soybeans in each county (but not allowed to exceed the acreage of corn for that county); continuous corn acres were calculated from the excess corn acres not yet accounted for in each county. Corn yields in continuous corn and corn following soybean were estimated by assuming a 10% yield drag in continuous corn as compared to corn following soybean and requiring the county-weighted average corn yield to equal the reported county average corn yield from the survey of agriculture. Yield goal methodology was then applied, setting the yield goal for corn equal to the estimated yield for that rotation plus ten percent, multiplying by the area-weighted nitrogen use factor for corn (typically 1.2 lb. N/bu for most Iowa counties, but adjusting as necessary for areas that are 1.1 lb. N/bu and 0.9 lb. N/bu, see figure 1). For corn-soybean rotations, a “rotation” credit of 1 lb N/acre per bushel of soybean yields up to a maximum of 50 lb. N/acre was used.

Figure 1. Nitrogen use zones for corn in Iowa for use in the Yield Goal method in Iowa Manure management Planning. Zone 1 has a nitrogen use factor of 0.9 lb. N/bu, Zone 2 has a nitrogen use factor of 1.1 lb. N/bu, and Zone 3 has a nitrogen use factor of 1.2 lb. N/bu.

 

N-FACT works differently, following an approach similar to Maximum Return to Nitrogen (MRTN) but based on modeling efforts that simulate the yield response of corn to nitrogen in each county under various weather conditions, planting options, and management scenarios. For this tool, you select a county, the rotation, weather conditions for spring and summer, the amount of residual nitrate in the soil, a planting date, and a nitrogen and corn price. Inputs used in these simulations were done in all counties for corn following corn and corn following soybean, but then consistent weather patterns (average spring moisture, average summer moisture, a residual soil nitrate of less than 20 lb. N/acre, a planting date before April 30^th, a nitrogen price of $0.45 per pound N, and a corn price of $4.64 per bushel. N-FACT provides a range of outputs from the 25^th percentile economic optimum nitrogen rate, the 75^th percentile economic nitrogen application rate, and the average economic optimum nitrogen application rate. A modeled yield is provided for the 25^th and 75^th percentiles and the average economic optimum nitrogen application rates. The N-FACT tool uses a concept from MRTN: the response from applied N determines the economic optimal rate, not the yield itself. As such, even if actual yields differ from those suggested by N-FACT and the underlying modeling behind it, the response curve generated is still the best estimate of the N rate required. All that to say, when the N-FACT reports the 25^th percentile economic optimum N-rate, it suggests that 25% of the modeled site-weather years in the county would require a lower nitrogen rate to be economically optimal. Similarly, when it reports the 75^th percentile N rate, it suggests that 75% of site-weather years within the modeled scenario would have a lower economically optimum N rate.

 

An essential consideration for manure planning is how different locations will be impacted if a switch from the yield goal method to the N-FACTs recommendation is evaluated. For both rotations, we grouped counties into three groups: counties where the yield goal nitrogen estimate was less than the 25^th percentile economic N rate from N-FACTS (N-FACTS suggests considering higher nitrogen rates). In these counties, the yield goal nitrogen estimate is between the 25^th and 75^th percentile economic optimum N (N-FACTS and the yield goal method are similar), and locations where the yield goal estimate exceeds the 75^th percentile economic optimum N rate. These counties were colored blue (Yield goal lower than the 25^th percentile N-rate from N-FACTS), white (Yield goal is between the 25^th and 75^th percentile N-rates from N-FACTS), and orange (Yield goal is higher than the 75the percentile N-rates from N-FACTS), respectively.

 

(a)	(b)
Figure 2. Comparison of the Yield Goal Method (based on county average yields) and N-FACTS nitrogen suggestions for (a) corn following soybean and (b) continuous corn. Counties colored blue have a yield goal N rate below the 25th percentile economic optimum N suggestion from N facts, counties left white have a yield goal N rate between the 25th and the 75th percentile economic optimum N rate suggestion from N facts, and counties in orange have a yield goal N rate above the 75th percentile economic optimum N rate suggestion from N facts.

 

The maps in Figure 2 suggest that counties on the Des Moines Lobe, the Iowa Erosion Surface, and much of the Northwest Iowa Plains farms should continue to fill out their manure management plans using the yield goal method, but at the time of manure application should be considering opportunities to lower their actual nitrogen application rates (through both manure and commercial manure fertilizer) to be more in line with the guidance suggestions coming from N-FACTS. In the remaining regions of Iowa, primarily the Alluvial Plains, the Loess Hills, the Southern Iowa Drift Plain, and the Paleozoic Plateau, we should carefully consider our nitrogen application rates and obtained corn yields to evaluate improvement opportunities. Specifically, N-FACT suggests that higher nitrogen application rates than those indicated by the Yield Goal method of the Iowa Manure Management Plan form may be required to reach optimum economic thresholds. However, the N-FACTS tool suggests that these locations can achieve higher yields. As such, choosing to apply higher nitrogen rates should be accompanied by evaluating corn yields and practices that may help increase corn yields.

While the monolithic evaluation provided in Figure 2 helps answer the question of writing a manure plan with a yield goal compared to writing a manure plan with N-FACT, it doesn’t address the magnitude of the change. Overall, the results are similar; the counties in red on the Des Moines Lobe and the Iowa Erosion Surface are locations where the yield goal is higher than the 75^th percentile N suggestion from the yield goal. In many of the remaining counties, the green color indicates that the yield goal would need to increase by this much to reach the 25^th percentile N suggestion. While the graphs are similar, these figures illustrate the magnitude of change that could cause.

(a)	(b)
Figure 3. Comparison of the Yield Goal Method (based on county average yields) and N-FACT nitrogen suggestions for (a) corn following soybean and (b) continuous corn. The scales demonstrate the magnitude of difference between the yield goal method and the N-FACT suggestion. Counties in red show N-FACT is lower than the yield goal suggestion, and counties in green show N-FACT above the yield goal suggestion.

 

One important point to clarify is writing your manure plan based on a method and selecting a nitrogen application rate. Writing a manure plan to allow flexibility in decision-making is often helpful. Choosing methods that would enable higher nitrogen application rates and include more acres than the minimum required to write an acceptable plan provides the needed flexibility. Such an approach puts us in a position to make the best agronomic decisions within a growing season to respond to unknowing weather patterns; this means not treating the manure plan rate as a maximum rate but choosing a target rate below this to make the best use of manure. It also plans to increase the number of acres our manure touches to maximize its value.

Writing plans for higher nitrogen application allows flexibility without having to write amendments to the plan should weather conditions in any specific year put us in a situation where more nitrogen is required. An example of this might be if a wet spring occurs and additional nitrogen is needed to address this concern. One example of how to do this is that plans choosing to use N-FACT as their methodology could be written for average moisture conditions, but should wet spring conditions occur, the farm would be allowed to modify to wet weather conditions without filing an amendment.

One important question that remains is how a change from yield goal to N-FACT recommendations would impact overall nitrogen recommendations in the state. In the map, you can see areas where more nitrogen is required and areas where less nitrogen would be suggested, but it is hard to say what this means for state nitrogen use. Weighting the state by the number of corn acres in each county suggests that if all acres were applied based on a yield goal methodology and switched to the N-FACT suggestion, the average nitrogen application rate would decrease by about ½ lb. N/acre. As I follow up, it will be important to compare what this means for some of the manure-rich areas in the state and the implications for manure management planning.

Another aspect of N-FACT is the on-farm nitrogen rate trials. The on-farm trials provide data on the economic optimum nitrogen rate, yield at the economic optimum nitrogen rate, and the nitrogen use efficiency at the optimum nitrogen rate for each trial conducted. Thus far, data from two trial years, 2023 and 2024, have been available. The nitrogen use efficiency at the economic optimum nitrogen rate provides another point of comparison for the yield goal method. In 2024, the on-farm trials for corn following corn averaged an N use efficiency of 1.00 ± 0.19 lb. N/bu (ave.  ± s.d.). One interpretation of these results is that 84% of fields (assuming the fields evaluated represent Iowa fields) are below the 1.2 lb. N/bu.

As we look at the on-farm trials for corn following soybean, the obtained N use efficiency was 0.92 ± 0.19 lb. N/bu (ave.  ± s.d.). Again, this can be interpreted as 84% of fields (assuming the fields evaluated represent Iowa fields) are below 1.12 lb. N/bu, that on average corn following soybean crops would be allowed in Iowa Yield Goal based plans. This value is based on the 231 bu/acre corn yield (average of corn following soybean plots) and a 50 lb. N/acre rotation effect that allowable N rate is 1.10 N/bu.

Data is also available for 2023, but the previous crop isn’t listed. The measured N use per bushel of corn at optimum N rate was about 90% of the 2024 value, and the standard deviation was 0.14 lb. N/bu. As 2023 was drier in much of the state, these results are consistent with what we’d expect. Higher rainfall led to some nitrogen losses in 2024 and required higher nitrogen inputs per bushel. Moreover, the drier conditions led to more consistent results as the soil variation was slightly reduced. Assuming the 90% reduction is consistent for continuous corn and corn following soybean, then 0.92 ± 0.14 lb. N/bu was required. In continuous corn, 97.5% of all fields would be below the 1.2 lb. N/bu is used for much of the state for yield goals. For corn following soybean, the N needed was 0.85 ± 0.14 lb. N/bu. Given the yields in 2023, the YG method would have allowed 0.98 lb. N/bu. In 2023, 84% of fields in corn following soybean would be below the N level allowed by the yield goal method.

A few things come to mind as I look at the totality of the results. The yield goal method is essentially performing how it should be. It limits N application based on agronomically allowable amounts without constraining yield limits by limiting nitrogen application. I.e., those constraints were initially set to be a guide that didn’t limit N to the point yields would be unduly constrained, but it wasn’t meant to be a nitrogen recommendation system. It sets a maximum limit based on the best available science at the time, not an application rate suggestion. We should be thinking about facilitating better manure management and moving our rates to not the maximum allowable but more towards the best recommendation or best available guidance methodology. In some cases, this could mean increasing nitrogen application rates, but often, these must be accompanied by practices that are increasing corn yields.

Spring Manure Application: Setting Up for Success

As winter fades and the ground begins to thaw, spring manure application moves to the top of the priority list. But applying manure effectively in the spring requires more than just an open weather window—it demands planning to maximize nutrient availability, avoid compaction, and minimize environmental losses.

Evaluating Manure Storage and Application Timing

For many farmers, the first concern with manure is manure storage capacity. Earthen manures storages can be reaching near-full levels by the end of winter, creating urgency for spring application. While deep pit storages, which are often designed to hold close to a years’ worth of manure, won’t be full, it is important to evaluate how full the storage is and if there is sufficient space available to make it to fall or if you should be considering getting some manure used this spring. When soils are frozen there is greater risk of nutrient loss from both in both runoff and from volatilization. Even when the surface appears thawed, subsurface frost can prevent infiltration, leading to manure running off with early spring rains.

Soil Conditions: The Foundation of Smart Application

Beyond soil temperature, moisture content is another key consideration. Applying manure to saturated soils increases the risk of runoff and compaction. Wet soils soak manure in more slowly, and getting the manure nutrients in contact with the soil is critical for holding nutrients. Moisture conditions also play an important role in soil compaction. Compacted soils can reduce corn yields by up to 10% due to poor root development and restricted water movement in some growing conditions.

You can find more information on how soil moisture impacts field activities from Iowa State University Extension and Outreach and I talk more about how  moisture impacts manure movement in soil during injection in a classic The Manure Scoop post.

Manure Nutrient Content: Test Before You Apply

One of the biggest mistakes in manure application is assuming nutrient values without testing. The nutrient content of manure varies significantly based on animal diet, storage method, and agitation practices. The University of Minnesota, recently released a tool, ManureDB, showing results from many manure samples. One thing we can say is manure concentrations are highly variable from farm to farm. It is critical to know what is in the manure you are applying by getting it samples and tested.

·         Swine manure ranged from 30–65  lbs of nitrogen per 1,000 gallons, 10–30 lb P2O5, and 15–35 lb K2O

·         Dairy manure averaged 10–20 lbs of nitrogen per 1,000 gallons, 5–10 P2O5, and 15–25 lb K2O

·         Beef manure averaged 10–20  lbs of nitrogen per ton, 5–15  lb P2O5 per ton, and 10–20 lb K2O per ton.

·         Poultry litter contained 40–65  lbs of nitrogen per ton, 30–50  lb P2O5 per ton, and 26–45 lb K2O per ton.

Without testing, there’s a risk of either over-applying (wasting nutrients and increasing leaching risk) or under-applying (leading to nitrogen deficiencies in crops). Taking multiple samples and averaging, or compositing before analysis, helps reduce variability in sample results and ensure a more representative measurement of manure nutrient content.

Calibrating Equipment for Even Application

Uneven manure application can result in poor crop performance and increased environmental losses. Before heading to the field, check equipment to ensure:

·         Flow rates and pressure settings are appropriate for the manure consistency.

·         Injectors are distributing manure evenly across the application width.

·         GPS or guidance systems are calibrated to prevent overlaps or skips.

Minimizing Nutrient Losses and Protecting Water Quality

Spring manure application comes with an increased risk rainfall and wet soil conditions. To minimize loss:

·         Setbacks: Follow setback distances from streams, water sources, and designated areas.

·         Incorporation: Injecting or lightly incorporating manure reduces ammonia volatilization by 30–60% compared to surface application.

·         Cover Crops: Research shows cover crops can reduce nitrate leaching by up to 40%. We are currently evaluating cover crop control of nutrient loss with swine manure application data and will have more data available this fall on how it performed.

Regulatory Considerations and Recordkeeping

Keeping records of application dates, rates, field location, and application method is required for those with a manure plan. It is also a best practice to record field and weather conditions at application that can assist with compliance and nutrient management planning.

Final Thoughts

Spring manure application is about more than just emptying storage—it’s an opportunity to optimize nutrient use, improve soil health, and support crop productivity. By waiting for the right soil conditions, testing manure, calibrating equipment, and using best management practices to reduce losses, farmers can turn manure into a valuable asset rather than an environmental liability. With a little planning and attention to detail, manure can provide the nutrients crops need while protecting water quality and maintaining soil health for the long run.

How much manure is applied by commercial manure applicators in Iowa?

 The manure industry in Iowa is expansive, involving over 550 businesses that manage both liquid and solid manure applications across the state. In the spring of 2024, we surveyed 562 commercial manure application business. We received responses from 90 commercial manure applicators (16% response rate), offering a detailed look at the industry's practices, scale, and significance. These findings are valuable for farmers, businesses, and stakeholders engaged in manure management and decision making around manure utilization and fertilization. Over the next few months, we’ll be sharing information we learned from these surveys.

Manure Application Businesses by Manure Handling Type in Iowa

The survey revealed that the majority of manure applicators in Iowa deal with liquid manure. Of the 90 businesses that responded:

• 58 businesses handle liquid manure exclusively.
• 11 businesses handle liquid and solid manure
• 21 businesses handle solid manure exclusively

This implies for Iowa’s manure industry that 64% of businesses focus on liquid manure, 23% focus on solid manure, and 12% apply both liquid and solid manure. Taking this a step further, assuming this is a representative sample of the overall industry, this implies that of the 562 commercial manure businesses in Iowa 362 business apply liquid manure, 131 apply solid manure, and 69 apply both liquid and solid manure.

Market Share of Liquid Manure Application

Iowa generates approximately 13 billion gallons of liquid manure each year excluding rainfall. Given 2023 was a dry year, this estimate should be an accurate representation of manure volume available in the year the survey was conducted. According to the survey data, the responding companies apply about 21% of the state’s total liquid manure. With an estimated 562 commercial manure businesses in Iowa, and assuming 77% apply liquid manure, approximately 431 businesses are involved in liquid manure application. Extrapolating from the survey, commercial manure applicators handle roughly 8 billion gallons of the state’s total liquid manure output—about 62%.

If you are interested in more information contained within the survey, take a look at our video presentation where data is summarized.

Manure Matters: Carbon Intensity Scoring for Corn and Soybean Production

 The carbon intensity (CI) score of a farm's corn or soybean crop is a metric in evaluating agricultural sustainability. For farmers looking to improve profitability and environmental stewardship, understanding CI scoring—and how manure management influences it—can make a big difference.

In this post, we’ll break down what carbon intensity scoring is, why it matters, and how manure from various livestock operations (dairy, solid beef, swine, and poultry) impacts the score. We’ll even crunch some numbers to show how manure management can enhance a farm’s bottom line by improving the CI of its crops.

What Is Carbon Intensity Scoring?

Carbon intensity scoring is a way to measure the greenhouse gas (GHG) emissions associated with producing a specific product, such as corn or soybeans. The score is expressed as kilograms of CO₂-equivalent per unit of output—typically per bushel for grain crops.

The GREET model (Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies) developed by Argonne National Laboratory is often used to calculate CI scores. Key components of the calculation include:

• Soil carbon dynamics: Gains or losses in soil organic carbon.
• Nitrous oxide emissions: From soil or fertilizer (manure) application.
• Fertilizer inputs: Both synthetic fertilizers and manure.
• Yield: Higher yields generally dilute the CI score.
• Fuel use: Diesel for tractors and transport; grain drying

Why Does Carbon Intensity Scoring Matter?

Carbon intensity scoring matters because low-CI crops can open doors to new markets and revenue streams. For example, corn with a low CI score is highly desirable for ethanol plants aiming to reduce their own CI for carbon credit programs or low-carbon fuel standards. Similarly, soybeans with low CI may become more attractive for renewable diesel production or sustainable supply chains.

For farmers, this means practices that lower CI scores—such as manure use—could add value to their crops.

For livestock producers, low-carbon-intensity (CI) crops like corn significantly contribute to reducing the carbon intensity of animal production. In some cases, the low carbon intensity feeds may be fed to the animal. In others, the grown grain may go to an ethanol plant. Ethanol plants using low-CI corn not only create low-CI ethanol but also generate distillers’ grains with a lower CI. These distillers’ grains are widely used as livestock feed, creating a direct link between crop CI and the carbon footprint of the animals consuming them. This connection is especially important for farms pursuing carbon-neutral livestock systems, where every component—from feed to manure management—impacts the overall emissions profile. By integrating low-CI feed into their operations, livestock producers can reduce their greenhouse gas emissions and enhance their position in sustainable agricultural markets.

Manure’s Role in Carbon Intensity Scoring

Manure offers multiple pathways to reduce the carbon intensity of crop production:

·         Replacing Synthetic Fertilizers: Manure provides nitrogen, phosphorus, and potassium, reducing the need for synthetic fertilizers, whose production is energy-intensive and GHG-heavy.

·         Enhancing Soil Carbon Storage: Organic matter in manure improves soil structure and boosts soil organic carbon levels, which can lower CI scores by sequestering carbon.

·         Reducing Nitrous Oxide Emissions: Unlike synthetic fertilizers, certain manures release nitrogen more slowly, potentially reducing nitrous oxide emissions from soil.

Comparing Manures: The Math Behind the Benefits and potential economic benefits

The 45Z tax credit, part of the Inflation Reduction Act, is available to biofuel refineries producing low-carbon fuel from 2024 to 2027. This credit benefits facilities that produce fuel with emissions below 50 kg of CO₂ per million BTU. Specifically, this tax credit offers $0.20 × [1 – (kg of CO2e per mmBTU / 50)] per gallon produced; if certain wage and apprenticeship requirements are met the credit could go from $0.20 to $1.00. At this time, the tax credit is provided to the fuel refiner. As such, farmers may earn a premium for low-CI corn or soybeans if biofuel producers pursue these credits and use the CI score of the grain processed to qualify, with the percent of funds passing through to the farmer unknown at this time. Figure 1 provides an illustration of potential value per bushel of corn for both credit rates assuming 100% pass through.

Figure 1. Potential value added per bushel based on CI scoring of corn for ethonal.

Lowering CI often aligns with cost-effective and sustainable practices. Here are steps to consider:

• Fuel Efficiency: Optimize equipment and field passes to reduce fuel use.
• Nutrient Management: Match nutrient applications to crop needs, minimizing nitrous oxide emissions, and use sources of “green” nitrogen.
• Cover Crops: Capture carbon in the soil, reduce erosion, and boost soil health.
• Reduced Tillage: Minimizing soil disturbance can enhance soil carbon storage.
• Stover Harvest: Provides biofuel feedstock but must be balanced with soil health.
• Manure Use: Manure adds nutrients, builds soil health, and increases soil carbon.

These practices can improve your CI score, sustainability, and eligibility for incentives that reward low-emissions.

 

To help illustrate potential impacts on revenues, we estimated what may occur in certain situations.

 

Table 1. Example calculations illustrating how change in practices could impact CI scores, yield, and revenues.

CT – Conventional Till, CC – Cover Crops, M – Manure, RT – Reduced Till, NT – No Till

What Does This Mean for Farmers?

Incorporating manure into your fertility program not only reduces input costs but also improves the value of your crops in low-CI markets. For instance:

Corn sold to ethanol plants offering premiums for low-CI grain may yield higher profits.

Soybeans with improved CI may gain competitive advantages in renewable diesel supply chains.

To fully capitalize on manure’s CI-reducing potential, it’s important to sample and analyze the manure, apply it at agronomically optimal rates, and integrate cover crops or conservation tillage to maximize soil carbon gains.

If you are looking to get an idea of what the CI score of corn may be for your field, this calculator does a great job of getting you close.

Final Thoughts

Manure management is an opportunity to add value to your farm. By reducing synthetic fertilizer use, improving soil health, and lowering carbon intensity scores, manure can help farmers unlock new revenue streams while improving sustainability.

Revisiting the Yield Goal Method for Nitrogen Management in Corn Production

What Is the Yield Goal Method?

The yield goal method calculates nitrogen application rates based on an anticipated crop yield. The simplicity of using a historical average made it accessible and appealing for widespread adoption. From 1970 to 2005, university extension personnel almost exclusively used “yield-based” algorithms, often based on the work of Standford (1966, 1973). Essentially, this method estimates a field’s yield and then multiply by a Nitrogen need factor, subtracting off any “nitrogen credits” the farming system would provide (such as legume credits).

In the mid-2000s, Iowa State Extension switched its nitrogen rate methodology to the Maximum Return to Nitrogen (MRTN). Iowa State University is currently performing the Iowa Nitrogen Initiative to envision future nitrogen recommendation rates and tools further. However, most Iowa manure plans are still filled out based on the yield goal method. As such, it is important to revisit some of these factors and how they may influence what is allowed in manure plans compared to our best-recommended nitrogen practices. Within this article we are revisiting the yield goal method to help empower farmers and technical service providers to make informed decisions supporting productivity and environmental health. As yields continue to advance, so must the methods we use to estimate and meet their potential.

Implementing the Yield Goal Method

Nf = n * YG – Ncredits

where Nf is the per-acre N application rate in lb and n = 0.9, 1.1, or 1.2 (based on the Iowa DNR map in Appendix A), YG is the yield goal. Ncredits are the adjustments made to the N requirement based on N “credits” left behind by previous leguminous crops, such as soybeans or alfalfa.

Within manure plans, the yield goal is often derived from historical yield data and adjusted to account for potential improvements. In Iowa, the yield goal is typically set as the average yield from the last five years, plus an additional 10% to account for advances in crop genetics, management practices, and technology. Farmers could account for incremental yield improvements by applying a fixed adjustment without overly complicating the process. However, as yields continue to increase, this 10% adjustment factor for attainable yields does as well, effectively allowing more nitrogen application rate as a function of time beyond what increasing yields alone would otherwise allow.

How well does this estimate do at predicting corn yield?

To estimate this, I’m using Iowa Agricultural Survey data on corn yields for the state and then later at the county level. At the state level, we can compare actual yield data in any given year to the estimated “yield goal” for that year. If we use the current method, the average yield over the last five years plus ten percent, on average, the state-level deviation is 12.2 bu/acre. However, the best yield estimate is to add only 4% instead of 10%; in so doing, the average yield deviation is 11.3 bu/acre. Another approach is to use trendline yield to estimate the yield in the next year; while this method is slightly more accurate, the average deviation is still 9.5 bu/acre.

This same approach can be done on a county-by-county basis for all Iowa counties, but the findings are similar, the best yield fit is 4% with a 0.4% standard deviation among counties. However, even at the county level, county trend line yield was always a better indicator of the average county yield and didn’t require calibration for use as the estimated yield.

Implications

So why am I doing this or talking about it? Within the yield goal, we’ve used 10% to determine the achievable yield, but yields continue to increase, and as a result, this 10% factor is getting more significant as a function of time. However, there is no evidence that deviation from trend line yields is increasing, but instead has been constant with time (figure 1).

Figure 1. Corn yield deviation from trendline yield as a function of time for Iowa corn yields. Deviation from the trendline has been nearly constant.

Moreover, if we are trying to fertilize the crop we expect to harvest, adding this 10% doesn’t give us the best crop estimate; instead, a 4% increase would be better. Stronger still would be estimated based on trendline yield. If we use the yield goal method as a regulatory tool, this 10% increase makes sense to allow flexibility for individual fields. However, if you are using that as the basis to understand what yield you expect to achieve next year and how much fertilizer you need, there are better estimates. 

There is a lot of uncertainty in nitrogen fertilizer recommendations. I’m excited to see where the Iowa Nitrogen Initiative takes us. Still, in the meantime, if you are filling out Manure Management Plans using yield goals and this to estimate your manure application rate for next year, it might be time to rethink the yield number and if that is the best use of the manure resources on your farm, and hopefully the Iowa Nitrogen Initiative will help direct us towards more effective nitrogen application rate methodologies.

How to Buy and Sell Liquid Manure in Iowa: Key Steps and Requirements

 Buying or selling manure may sound straightforward, but liquid manure adds a layer of complexity due to specific regulatory requirements. Unlike solid manure, which is generally covered by more straightforward guidelines provided by 200A, liquid manure requires coordination with a farm's management plan, especially if the seller's operation has more than 500 animal units.

Here's a breakdown of the requirements and steps for selling liquid manure.

1. Understand the Role of the Manure Management Plan (MMP)

Liquid manure applications are tightly regulated for any farm with over 500 animal units. The field receiving the manure must be included in the farm's Manure Management Plan (MMP). This plan essentially documents where, when, and how much manure will be applied to stay within environmental and agronomic limits.

For liquid manure sales, this means:

Sellers must ensure the buyer's application fields are listed in their MMP. The field needs to be soil sampled and have Phosphorus Index tests run before manure is applied. These results are good for up to four years (assuming they align with the timing of the farm's manure management plan).

Buyers need to provide a "Statement of Intent" to specify the amount of commercial nitrogen they plan to use on the field receiving the manure.

2. Statement of Intent for Commercial Nitrogen

The Statement of Intent from the purchasing farm clarifies how much additional nitrogen they propose to apply. This document helps regulators and sellers confirm that the buyer follows appropriate nutrient management practices. The Statement of Intent also ensures that applications don't exceed environmental thresholds.

3. Why Liquid Manure Requires a Plan (and Solid Manure Doesn't Always)

In the case of solid manure, sales are often managed through simplified "200A regulations," which allow farms to record sales without extensive management plan updates. Generally, the solid manure analysis goes through a process to get registered with the Iowa Department of Agriculture and Land Stewardship (IDALS), which provides a "guaranteed" nutrient value basis. The guaranteed analysis is generally set lower than anticipated for nutrient concentration to ensure it always meets this level. Farmers can use this, or other sample analysis results, to negotiate a sale price based on the actual value, not just the guaranteed value.

Liquid manure, however, isn't sold through IDALS. The regulatory burden to ensure good use of manure nutrients is adhering to the MMP and ensuring a compliant nutrient application strategy.

4. Finalizing the Sale: Ensuring Compliance and Environmental Responsibility

The final steps involve ensuring both parties understand the value and nutrient content of the liquid manure. Although a nutrient guarantee isn't required, many sellers will still provide an analysis to give the buyer a reliable estimate. This analysis can help both sides negotiate a fair price and set application rates that respect crop nutrient needs and environmental limits.

In summary, selling liquid manure can be a practical and profitable move with some additional planning:

• Ensure the application field is in the seller's MMP if the operation exceeds 500 animal units.
• Buyers should provide a Statement of Intent for commercial nitrogen.
• Clarify the nutrient value of the liquid manure with a shared analysis.

By following these steps, buyers and sellers can take advantage of liquid manure's benefits, navigating the regulatory requirements smoothly while maintaining their farm's productivity and environmental compliance.

Manure and Soybeans - How many soybean are using manure and what's the right approach?

 

The other day, I got the question, how much manure is soybeans using, and what type is it? Right when I got it, I asked – do you mean manure applied directly to soybean, or since this was a lifecycle/greenhouse gas question, how much mined and manufactured commercial fertilizer is being offset because of how we use manure in our crop rotations? That may seem like a slight distinction to some, but it me that is a big difference. Why? Because soybean is a legume and, as such, is capable of fixing much of its nitrogen needs, applying manure to it, especially nitrogen-rich manures, may not be the best use of our manure resources (though this depends, in some cases, applying low nitrogen manures that have high P and K may still make sense). However, using soybean in a rotation, with, for example, corn, can be a great way to better match crop nutrient removal to the amount of P and K applied.

Within this post, I'll try to answer a few points

1.      Benefits and challenges of applying manure to soybean.

2.      How banking P and K from manure application within a rotation supports fertility for soybeans.

3.      How many acres of soybean are receiving manure, and an Iowa-centric estimate of how manure supports soybeans within the state.

Manure Application to Soybean: Pros and Cons

Pros of Applying Manure to Soybeans:

 Nutrient Supply: Manure, especially from livestock like swine and cattle, provides essential nutrients such as phosphorus (P) and potassium (K), vital for soybean growth. While soybeans don't require as much nitrogen (N), the P and K in manure can boost soil fertility and improve crop yield.

Soil Health: Manure applications can improve soil organic matter, structure, and microbial activity. Manure improves soil health and water retention, benefiting the cropping system, including soybeans.

Cost-Effective Fertility: Manure is a cost-effective alternative to commercial fertilizers, especially for farms with ready access to livestock manure. Farmers can reduce reliance on purchased fertilizers by incorporating manure into their nutrient management plans.

Improved Soil Fertility Over Time: Manure can provide slow-release nutrients that benefit subsequent crops. When manure is applied to corn in a corn-soybean rotation, excess nutrients not utilized by the corn can become available to soybeans in the following season.

Cons of Applying Manure to Soybeans:

Nitrogen Misalignment: As a legume, soybeans can fix nitrogen through nodulation. Applying nitrogen-rich manure directly to soybeans can lead to inefficient nitrogen use, potentially increasing nitrogen losses through leaching or denitrification, as the soybeans won't need as much of it as they are fixing their own, or it means reducing fixation as a result of high nitrogen levels in soil when that nitrogen could have been used to replace nitrogen in other, non-leguminous production systems.

Soil Compaction: If manure is applied under wet conditions or if heavy machinery is used during application, soil compaction can occur, detrimental to root development and water infiltration in soybeans.

Manure Application in Corn-Soybean Rotations: "Banking" of Phosphorus and Potassium

Although applying manure directly to soybeans may only sometimes be the most efficient use of its nitrogen content, manure is often applied in corn-soybean rotations with the specific intent of providing phosphorus (P) and potassium (K) for both crops.

Manure Application Before Corn:

In a typical corn-soybean rotation, manure is commonly applied to corn because corn requires higher nitrogen. When manure is applied before corn, it supplies nitrogen and deposits significant amounts of phosphorus and potassium into the soil. Since corn may not utilize all of the P and K, these nutrients remain in the soil, available for the following soybean crop.

Phosphorus and potassium are less mobile in the soil compared to nitrogen. As a result, these nutrients persist and become accessible to soybeans in the year following manure application. This method effectively "banks" nutrients, supporting soybean growth without additional fertilizer applications.

By applying manure to corn with the understanding that residual P and K will benefit soybeans, farmers can maximize the use of manure nutrients across the two-year (or longer) crop rotation. This approach helps balance nutrient levels and avoid over-applicating phosphorus and potassium, which could lead to environmental concerns like eutrophication.

Farmers can reduce or even eliminate the need for synthetic P and K fertilizers for soybeans when they take advantage of the nutrients from manure applied to corn. Doing so not only cuts production costs but also promotes more sustainable nutrient cycling within the cropping system and can lower the carbon footprint of soybean as both the energy associated with mining P and K fertilizers are eliminated as are application passes of these fertilizers.

While direct manure application to soybeans is not suggested due to the legume's nitrogen-fixing ability, it can still offer benefits in terms of phosphorus and potassium supply and soil health improvement. However, the common practice in corn-soybean rotations is to apply manure to the corn crop to use the excess P and K for the following soybean crop. This strategic approach ensures that nutrients are efficiently utilized over the rotation, benefiting both crops while minimizing the environmental impact and fertilizer costs.

Acres of Soybean Receiving Manure

The USDA ARMS survey estimates acres of different crops receiving manure. While not all done in the same year, it does provide a reasonable approximation of which crops are receiving manure. In 2020, they estimate that 2.3% of acres receive manure, or about 1.9 million acres of soybeans. It is ranked as the second most popular crop to receive manure, following corn, though corn receives about 80% of all manure produced, and soybeans receive only 10% of the manure. However, given the above conversation, it must be recognized that this is acres receiving manure in a given year and not accounting for carryover phosphorus and potassium from manure used to support soybean production in the following year.

 Table 1. USDA ARMS data on manure application rates by field crop across the US.

The second figure is also from a UDSA ARMS survey and estimates where the source of manure was for crop production. Of the manures applied to soybean, only about 7% was from swine, or about 130,000 acres across the US. At first, this may seem strange, but swine manure is typically the highest available nitrogen among manure sources and especially has much nutrient value (50%) related to nitrogen content. In contrast, other manures, cattle, and poultry generally only have 10-20% of their nutrient value tied to nitrogen with most value coming from phosphorus and potassium. Because of this, swine manure is probably the least likely to be applied to a legume like soybean. Beef and poultry manure also tend to be solid manures, which are more likely to be surface applied, especially in minimal tillage systems; timing the application after a corn crop can offer more residue and protection from runoff losses than applying after soybean (and before corn).

Figure 1. Manure source (by animal species) for the major crops receiving livestock manures.

However, that doesn't mean that swine manure isn't an essential source of nutrients for soybean production, just that it is being used as a multi-year fertilizer for soybeans in a corn-soybean rotation rather than the direct crop receiving manure. In Iowa, approximately 25% of all corn land receives manure, which is almost exclusively applied to fertilizer. However, even though continuous corn is more prevalent around livestock and swine farms than in other locations, most of the land (~80-85%) is in a corn-soybean rotation where manure is supplying P and K for the soybean crop). In Iowa, there are about 12.4 million corn acres, and assuming that 25% receive manure and 80% are in a corn-soybean rotation, there would be 2.5 million acres of soybean being fertilized by manure alone in Iowa. In Iowa, 80% of this would be from swine manure, so two million acres. The two million acres in a single state need to be reported on or captured in how USDA ARMS surveys farmers and collects manure application data. However, it represents a substantial amount of Iowa soybean acres (20%). While Iowa is a leader in acreage impacted in this way, similar patterns would be seen in Illinois, Minnesota, and Indiana, though with lower amounts of manure.

Soybean producers must proactively claim how this circular use of nutrients occurs across multiple years of a crop production system. It impacts the sustainability of soybean production in terms of both greenhouse gas scoring as related to reduced demand for mined phosphorus and potassium fertilizers, savings of energy use for a field pass specific to supply nutrients to soybean production, and a message of improved efficiency and reducing greenhouse gas footprints by more fully considering how manure is used.