How does uncertainty impact the manure nitrogen application rate you select?

 Selecting an appropriate nitrogen fertilizer rate is critical for optimizing profit from cornfields. Applying too little N reduces profit by reducing grain yield; too much N and you don’t get a return on the nitrogen you bought and can cause damage to the environment. In Iowa, most manure management plans are filled out using the yield goal method, with current university guidelines suggesting the use of the maximum return to nitrogen approach. If you are a long-time reader of this blog, you’ve probably seen both of these discussed before, so don’t worry, that isn’t the topic today. Instead, I’m focusing on uncertainties in the application and what that means for how we make decisions.

A lot of uncertainties exist when using manure as a fertilizer. Some examples include:

 · Nitrogen need of the crop (every growing season is a bit different)

· Spatial variation in nitrogen need to support crop production (because all soils aren’t the same)

· Nutrient content of the manure

· Nutrient variation from start to finish of manure application

· Application Rate Control and Variation in Application Rate

· Availability of the manure nitrogen to the crop

· Amount of nitrogen lost to volatilization

· Non-uniformity in application rate

 For now, I want to look at the manure application parts of this uncertainty and assume we know the crop response to nitrogen perfectly. How do all the variations and uncertainty impact the nitrogen application rate we should select? To answer this question, I first parameterized the yield response curve from the maximum return to nitrogen. The price of corn was set at $5.65 a bushel and nitrogen price at $0.40 a pound, which in a corn-soybean rotation gave an optimal nitrogen rate of 150 lb N/acre.

 The manure nitrogen content was set at 40 lbs N/1000 gallon, nitrogen availability at 95%, the nitrogen volatilization coefficient at 98%, and the desired application rate of 3706 gallons/acre calculated. A Monte Carlo simulation was then performed. For each variable that added uncertainty (manure N/content, Application rate, Volatilization coefficient, Nitrogen availability, and the knife-to-knife coefficient of variation), a normal distribution was constructed using the average value listed above and standard deviations of 2.75 lb N/1000 gallons, 250 gallons/acre, 0.01 % volatilization, and 0.05 % availability, respectively. Knife-to-knife variation varied between 0 and 100% were evaluated. I then performed 3500 simulations drawing randomly from the distributions I created to determine the nitrogen application rate for each knife (for distributions with natural limits, such as volatilization coefficient, no values over 100% were allowed).

 A lost value from application variability and uncertainty was calculated. If the actual amount of available N applied was greater than the MRTN rate of 150 lb N/acre, the value was set at the differences between the amount of N used minus 150 lb N/acre times a nitrogen price of $0.40 a pounds. If the nitrogen application rate was less than the MRTN rate, the value was set at the difference between corn yield at MRTN and the projected corn yield at the N rate applied times a corn price of $5.65 a bushel. The average loss in profit for all 400 knife simulations for each of the 3500 simulations was calculated, and then the average and standard deviation of the 3500 simulations were calculated.

A maximum return curve was calculated by taking the profit that would have been generated with perfect information (200 bu/acre x $5.65/bu – 150 lb N/acre x $0.40/lb N) minus the profit lost from uncertainty and application variability using the procedure listed above. Here we see an interesting trend – the uncertainty of ammonia volatilization and nitrogen availability and the variation in volumetric application rate and manure nitrogen content during the application, make it advisable to apply six pounds more available nitrogen per acre than if we didn’t have these variations. This occurs as the economics of nitrogen application is non-symmetrical, with the cost of being a pound short greater than being a pound heavy. Suppose we factor in any knife-to-knife application variability. The story gets more interesting, with the ideal application rate first increasing (until we reach a knife-to-knife application variability of about 40%, where the ideal rate is 167 pounds of N/acre, or 17 pounds/acre higher than the known nitrogen response curve we put in. Ideal nitrogen rate then decreasing to 137 lb available N/acre.

 

Figure 1. Impact of knife-to-knife variability of the effects of the maximum return to nitrogen for spring-applied swine manure to corn in a corn-soybean rotation. The ideal rate varies with our machinery variation.

 

But what about a fall application? As the MRTN curve is based on spring nitrogen applications, I added a term to the model to account for N-loss from fall to spring. For fall applications, I assumed an average of 15 lbs N/acre with a standard deviation of 15 lb N/acre and performed the same Monte Carlo simulation as above (but with the available N corrected for estimated nitrogen loss.

Figure 2. Impact of knife-to-knife variability on the impact of the maximum return to nitrogen for a fall and spring-applied swine manure to corn in a corn-soybean rotation and continuous corn rotations. The ideal rate varies with our machinery variation.

Almost all the curves looked the same. For example, the Maximum Return to Nitrogen in a continuous corn rotation was approximately 50 pounds higher in the continuous corn rotation than in the corn-soybean rotation, whether the manure was applied in the fall or the spring. The difference was impacted slightly by the knife-to-knife variability of the application equipment, but only slightly. Similarly, while the nitrogen loss from fall application was set at 15 lb N/acre, the fall application rates were on average 22 lb N/acre higher to hit the optimal rate.

So overall, where does this leave us. There is uncertainty and variability in every decision we make. The more confident we are about our equipment and manure, the closer our rate should be to the “true” MRTN. However, from an economic perspective, if there is uncertainty or variability in what we are doing, the right rate for us sneaks upward just slightly. This insurance N helps us in years we’d otherwise be short. And this is why we still talk about the 4Rs of right rate, right place, right timing, and the right type of fertilizer.

Manure Scoop: The Value of Real Time Nutrient Sensing

 

A few years ago, I became interested in the value of manure sampling and how obtaining good information helped us make better nutrient management decisions. I tried to use the theory of value of data to determine how much a manure sample was worth. Read a summary here.

 

Many factors cause variations in manure's average nutrient concentration: diet, housing type, manure storage type, environmental conditions, management techniques, and treatment practices. Just as critically, our ability to agitate and create a uniform, homogeneous mixture is often limited by our ability to stir manure storages.

 

A repeated sampling at five manure storages was used to assess the average, standard deviation, and coefficient of variability. The data were summarized as averages across the sampling data set to determine the variability in manure concentrations from each manure application event. Average manure nutrient concentrations were 28, 16, and 21 pounds of N, P, and K, respectively, per 1,000 gallons with standard deviations of 4, 7, and 3.

 

In determining the manure test's value, it is essential to understand how a farmer can use the information gained from the test results, i.e., how having this information alters the farmer's nutrient management and affects the farm profit. This is a complex topic, as almost limitless possibilities exist. This evaluation assumed the manure application method would be either injection or immediate incorporation to maximize N utilization. Additionally, we assumed best management practices for manure application timing were followed. As a result, the yield response to available N (defined here as the sum of ammonia N and organic N expected to mineralize in the first growing season) would be the same as the yield response to mineral N fertilizer. Finally, we limited crop rotation choices to continuous corn and corn-soybean rotations, as these represent the dominant rotations in the upper Midwestern U.S.

 

Our methodology was to estimate the profit that would have been made if the manure was assumed to have a "typical" nutrient composition and then to compare this to the profit generated if the actual nutrient composition was known. To make this evaluation, an economic model was developed as an Excel spreadsheet. The model compared the costs and revenue of corn production. Corn yield was calculated as the product of maximum yield and the estimated percent yield that was achieved.

 

For a corn-soybean rotation where the manure is going to corn, this means that the real-time nutrient correction for manure would be worth approximately $3.13 per acre. However, in a continuous corn rotation, which is more sensitive to nitrogen application in terms of crop response, it would be worth around $4.29 per acre. However, understanding just how big this variability is from load-to-load or pass-to-pass is critical for putting value to this technology.

Manure Nitrogen Availability from Manures and N Application Recommendations Around the Midwest

 

How much nutrient is there? While it seems a simple question, with manures where the only thing consistent about them is inconsistency, the answer isn't always easy. The place I like to start is what does 'availability' mean.

 I define availability as nitrogen present in a form able to be used. When talking about manures, we typically mean that this percent of the nutrient will cycle through a form that plants can use.

I'll use the term supply to specify how much is added; less the fractions are lost to volatilization, leaching, denitrification.

 Within the state of Iowa, our go-to document on the subject of nutrient availability is "Using Manure Nutrients for Crop Production," specifically table 1. A second correction is made for ammonia volatilization losses based on the type of manure (as this impacts the amount of nitrogen that is in the ammonium at the time of application) and the application method (as this influences how long the manure it's on the surface and is exposed for potential losses.

Table 1. Iowa Suggested Manure Nutrient Availability.

Manure Source	1st Year	2nd Year	3rd Year
Beef Cattle (solid or liquid)	30-50	10	5
Dairy (solid or liquid)	30-50	10	5
Liquid swine	90-100	0-10	0
Poultry	50-60	0-10	0

Table 2. Iowa Suggested Manure Nitrogen Volatilization Correction Factors.

Application Method	Incorporation	Volatilization Correction Factor
Direct Injection	-	0.98-1.00
Broadcast (liquid/solid)	Immediate Incorporation	0.95-0.99
Broadcast (liquid)	No Incorporation	0.75-0.90
Broadcast (solid)	No Incorporation	0.70-0.85
Irrigation	No Incorporation	0.60-0.75

Iowa nitrogen management recommendations either come from the Yield Goal Method or the Maximum Return to Nitrogen concept. In the Yield Goal Method. As a base case for comparison, I will look at deep-pit swine manure testing at 50 lbs/N per 1000 gallons. In Iowa, the average yield times 1.1 is 215 bushels/acre of corn and 56 bushels/acre of soybean. Assuming a nitrogen use factor of 1.2 lb N/expected bushel of corn and a soybean credit of 50 lb N/acre, nitrogen application rates would be 208 lb N/acre to the corn phase of a corn-soybean rotation and 258 lb N/acre in a continuous corn rotation. The Corn nitrogen calculator would be 140 lb N/acre in a corn-soybean rotation and 188 lb N/acre in a continuous corn rotation. I'll show a figure of these results in just a second, but I also wanted to compare them to two neighboring states, Illinois and Minnesota. Second-year N availability was estimated in the continuous corn rotation as what didn't mineralize the first year.

 Illinois uses the same nitrogen volatilization recommendations as Iowa (they come from Midwest Plan Service) and uses the Midwest plan service method to estimate nitrogen availability from the manure. For swine manure with approximately 70% of the nitrogen in the ammonia form and a 35% mineralization factor, 80% of the nitrogen will be available in the first year. The second-year availability for the continuous corn rotation is estimated to be half of the amount mineralized from the organic nitrogen fraction, which amounts to approximately 2 pounds. The desired Nitrogen application rate for Illinois is selected using MRTN, but rather than the optimum value, the maximum within a $1 profit is recommended.

Minnesota suggests that 150 plant available pounds of N per acre should be applied in a corn-soybean rotation and 195 in a continuous corn rotation. In Minnesota, they don't separate corrections for availability and loss but incorporate both into a correction factor. They also provide a second-year availability factor of 15% for swine manure.

What does this mean? I put together this figure of nitrogen application rate recommendations, two for Iowa (Yield Goal and MRTN), to compare the suggested rates for Minnesota and Illinois. Note this is only for nitrogen and doesn't consider any phosphorus limitations that may restrict manure application.

Figure 1. Summary of recommended manure application rates for the corn phase of a corn—soybean rotation. Swine manure with 50 lb N per 1000 gallons, 70% ammonium with error bars set based on high and low suggestions for nitrogen availability and volatilization losses suggested within each state.

 
Figure 2. Summary of recommended manure application rates for a continuous corn rotation. Swine manure with 50 lb N per 1000 gallons, 70% ammonium with error bars set based on high and low suggestions for nitrogen availability and volatilization losses suggested within each state.

 As we look at this data, I came home with a few takeaways. The first being, the factor ammonia loss with broadcast application in Minnesota is much higher than in Iowa and Illinois, based on the Midwest Plan Service Methods. Iowa and Illinois have volatilization factors of 15-30%, while Minnesota uses ~45%. The difference in volatilization assumption makes a substantial change in the recommended application rate.

The two recommendations for Iowa make an interesting comparison. When using the yield goal approach, Iowa's recommendations are similar to those provided with the Illinois method. In the case of continuous corn, the Yield Goal method for Iowa tends to be slightly higher, while in the corn-soybean rotation marginally lower, but overall, the results are similar.

When we look at the recommendations resulting from using the MRTN for Iowa, the results are more comparable to the Minnesota recommendations. In the corn-soybean phase, Iowa's MRNT recommendation would be lower thanks to both the lower mineralization suggestion and the slightly higher N application recommendation.

Overall, these results indicate that Iowa's approaches place us within the context of the surrounding states.

Shallow Burial for Mass Mortality Management

There is no best way to dispose of swine mortality carcasses. While some methods may work well for managing routine mortalities, due to capacity issues, they may not adapt to times when catastrophic mortalities occur. The optimum system for any particular farm location is based on a number of criteria, including the current state of the protein/oil market, the biosecurity required, the distance to processing sites, the local public's perception, the government regulations that apply to that location, the environmental conditions, and the ability of the farm to carry out the different procedures.

The death losses at a farm can be classified broadly as one of two types:  routine or catastrophic mortalities. Routine mortalities represent a small proportion of the herd and occur throughout normal production. Catastrophic mortality events involve high death losses within a distinct time. Four predominant methods of routine swine mortality disposal are burial, incineration, rendering, and compositing. Catastrophic losses present unique challenges because of handling large amounts carcasses within a short time (and if losses are due to disease, a higher biosecurity risk).

Burial

Burial can occur either on site or via transport of carcasses to approved landfills. Typically, on-farm burial of routine mortalities is performed using a trench method, which involves excavating a narrow and shallow trench, placing a single layer of carcasses in the trench and then covering with soil. Pigs slowly decompose until they are unrecognizable, generally after a few years. One concern is that burial can have negative environmental impacts if the sites are not selected carefully. In particular, depth to groundwater or sanding soils where leachate transport to groundwater is more likely. This method is not available when the ground is frozen and predators can uncover carcasses not buried deep enough. Typically, for routine management of mortalities, this method is often reserved for smaller operations.

In terms of catastrophic mortality, disposal burial is more common. With emergency disposal burial, the number of carcasses placed in a location is typically greater, increasing the potential for leachate, making location selection critical. The use of modern engineered landfilled equipped with leachate collection and treatment significantly reduces the risk of leachate concerns. The utilization of the landfill relies on the owner’s copperation and the transport of carcasses. Efforts to support bio-secure transport are required in cases where mortality is from a transmittable disease.

If mass burial is required on site, the combination of topographic, geologic, soil, and water resource data should be used to identify and map burials sites. Farms should work to identify locations for on-farm burial as part of emergency preparedness plans.

Shallow Burial

Shallow burial is a bit different, almost a cross between composting and burial. Often burial decomposition is slowed by lower oxygen concentrations and the fear of leachate movement. Shallow burial tries to address this by leaving the carcass near the surface.

The process is something like this – dig a long narrow trench approximately 20-inches deep by the width of an animal body for the length you need. You’ll want to put in a layer of organic material, like wood chips, approximately 12-inches thick. This absorbent material will help absorb any potential leachate from the animals and also can help facilitate air exchange and keep the zone aerobic.

The next step is to get the animals in the trench, one layer thick. The animals are probably mostly even with the ground. What we are trying to do with this approach is keep them in an area with high soil microbial activity to help promote decomposition. At this point, put the removed soil back on top, plant grass or perennial vegetation to keep it in place, and let nature help with the circle of life.

It will take a little time, but most work has shown in about a year, the animal’s decomposition will be nearing completion.

If you are looking into what you might need to get started on the process Michigan State has a spreadsheet to help you plan sizes and material needs. https://www.canr.msu.edu/resources/spartan-emergency-animal-tissue-composting-planner-v1-04.

The Value of Adding Small Grains and Hays to Improve Manure Management in Iowa

 Annual manure production in Iowa now exceeds 14 billion gallons of liquid manure, with most of this predominately produced on swine farms. This manure needs to be applied annually and its management is often cited as one of the critical factors impacting water quality. Some research suggests, this is in part to a mismatch in timing of when crop nutrients are needed and when the manure is applied. Moreover, unlike other fertilizer sources, the management of manure is complicated by the fact specific management activities at the facility does not always align with the most appropriate agronomic decisions. For example, manure application could be driven by a full storage rather than appropriate field conditions, or a turn at the facility that allows manure agitation and removal at a time when the facility is empty, improving withdrawal safety for employees and animals.

Moreover, given the predominately row crop (corn/soybean) agriculture that typifies much of the corn belt, manure application windows are typically limited to either spring after the soil thaws but before planting occurs or fall after harvest but again before the soil freezes. While these windows have typically proven sufficient, changing weather patterns, the expansion of livestock agriculture, and the separation of ownership of the cropping and livestock production portions of the operation have put new and greater stresses on the way the system is managed. Furthermore, the move from independent ownership of manure application equipment at the farm level, to a system where it is owned by an independent contracting business has taken much of the control away from the individual farmer and created a system.

In a system based on single ownership of the crop and livestock facility, the decision when to apply manure was a compromise for both the cropping production system and the livestock production side, with the farm manager typically wanting to balance the decision to maximize overall farm profits. There is great incentive to optimally manage manure as a fertilizer resource with more ideal application timing, but not to the extent it would prohibit the production of livestock. If the storage was full, there is incentive to perform emergency manure application, so animals could continue to be raised in the production facility. Since these farms often owned their manure application this would typically occur only to draw down the storage to an adequate level until more appropriate application timing.

However, in more modern setups where ownership of the crop and livestock is divided among different individuals, there are competing interests in different decisions. For example, the crop farmer still would want optimum timing for crop performance, but the barn owner often focuses his decision process solely on what is best in terms of barn management. When the fields open up, the livestock farmer may find themselves in a situation where they may be giving away or selling the manure at far below the market value of the nutrients it contains. This, in turn, allows the crop producer to view it as a free fertilizer only minimally impacts his other fertility decisions. While from an economic perspective this arrangement is perhaps beneficial to both parties, it also creates a situation where the environmental constraints on the system are not given priority.

This, along with the rising costs of manure application machinery, is putting the equipment out of reach for many farms. Thus, the greater stress to get all manure applied in shorter time windows. While I don’t have the answers on this, it is important as we go about facing these challenges, we look at all the options – new crop rotations, bigger and faster equipment, altering our manure management systems to make within season application possible, and potentially numerous others. It seems like a first step is to understand how different crops may open up new application windows, a first attempt at that, which I’ve shown below. Note: I’m not saying we need to have lots of acres devoted to other crops, the next fun steps are figuring out how much will be enough.

Figure 1. Cropping activity windows for different crops in Iowa.

The Science Behind Manure Management Plans - Nitrogen

Manure management plans are a tool used by both the farm to make sure they are getting the most from their manure and by society to ensure manures are managed in a way that is appropriate and only allows acceptable risk to environmental quality. I think we can all agree these are good things we want to occur but like most things, the devil is in the details. How do we, as a society and individual farmers, work to select the right nutrient application rates to balance both fertility decisions and impacts on yields and economics, with the potential environmental consequence?

In most manure plans, the yield goal method is used to define the maximum allowable nitrogen application rate. This approach is based on a mass balance approach where we are trying to match nitrogen application rates to removal and loss rates. In this sort of system, there is a fair amount of uncertainty in understanding where nitrogen ends up and how it moves. Changes in corn genetics have impacted how the existing factors in this equation may interact. Below I’ve constructed a partial nitrogen budget (estimate N application using the yield goal method minus the amount removed in grain) with two different grain nitrogen content. The old budget uses the 0.8 lb N/bu of grain which is what is listed on the USDA crop nutrient removal tool, while the newer partial budget uses 0.6 lb/N bu of grain which is just a bit higher than what newer research on corn nitrogen suggests is occurring. I’ve also marked two vertical lines; the gray line represents Iowa’s corn yield in 1995 (state average) while the black line represents Iowa’s corn yield in 2018.

What should we be taking away from this information? It isn’t that the yield goal method is archaic, but rather, as corn genetics have changed we need to be thoughtful about how it impacts the nitrogen use coefficients listed for different crops within the document. It is possible, at some point, these will need to be updated and deciding when, how, and what is the right factor is critical to providing a realistic yield estimate. While there is a lot to the nitrogen cycle and it can get a bit confusing, there are two things to note in this figure. The first is the two lines diverge from each other at yields get higher and the second is over the last 25 years the yields have greatly increased.

So what does this mean at the 1995 yield level? The old partial N budget application approximately matched the removal in corn grain. If we look to current yields, even at the old corn N removal rate, we were putting on around 25 lb N/acre more nitrogen than would be removed in the grain, but as grain nitrogen content has come down, this amounts to about 40 lb N/acre.

So what does this really mean? While the yield goal method is based on a mass balance approach for nitrogen, many of the factors in mass balance are hard to predict. Thinking about how our agricultural systems have changed over time and what this means for rate selection, is critical for making an informed decision. Moreover, this is one of the reasons Iowa State has switched to recommending the maximum return to nitrogen approach for rate selection.

Figure 1.  Partial nitrogen budgets (input – grain N removal) for old (corn with 0.8 lb N/bu) and new (corn with 0.6 lb N/bu) estimates. The vertical gray line represents corn yield in 1995, while the vertical black line represents corn yield in 2018. Representation for a corn soybean rotation with a soybean credit (rotation effect) of 50 lb N/acre)

Manure Management Plans – What are they?

Manure, waste or resource, is a question I like to ask when I get the chance to step in front of a class and hear what students have to think about the topic. Generally, I get enough answers of both to be satisfied. Manure can be a waste and it can be a resource: it comes down to how we manage it. It is a simple answer, but the best ones often are.

1. Students work to develop a manure management plan for a beef operations.

So what is a manure management plan? This is a tool that estimates all the manure a facility is going to generate and then looks at the crop fields available to make a determination about how much manure could be applied to each field in any given year, based on both the risk of phosphorus transport and the nitrogen needs of the crop. In the state of Iowa, they are required for confinement animal feeding operations with more than 500 animal units, which is 1,250 finishing pigs. Manure management plans can serve as a tool for the farm as a means to estimate manure application rates, but also a tool for society to ensure the farm has the capacity to manage its manure in a way based on legal standards we have defined as environmentally acceptable.

The original framework for these plans was developed in the early 1990’s and has been modified slightly. The livestock and the manure industry looked a bit different at those times. For example, manures typically had about $8-$12 per 1000 gallons nutrient value in it, whereas now we typically average closer to $30. While this change may not sound like much, when compared to typical application prices of $10-$20 per 1000 gallons, those differences can make a world of difference, changing it from a fertilizer source that isn’t cost effective for the farmer to utilize to one that is.

When manure plans were first developed, some farms manure looked like a product we had to dispose and find a way to manage, while minimizing environmental risk. Today, we have a greater opportunity, manure can be a resource, if we can find ways to manage it as such. Are the production systems we use today perfect? No, none are. We need to continue to get better and improve them to ensure livestock farms can remain an important part of our landscape in the future. We have made progress, and have to continue to do so in the future.

Manure management plans are typically filled out using the yield goal method. In this method, farmers determine a yield goal for each of their fields. This is a number selected based on previous yields either for that field or that county, and then multiplied by a nitrogen use factor, typically 1.2 lb N/bu of corn yield expected, less any legume credits their field would have. Is this method perfect? No. I’ve expressed different thoughts about it before. In a previous blog, I compared how yield goal method and MRTN estimates nitrogen application rates, looking at how corn nitrogen content per bushel has changed and what it means for these methods. In this blog, I looked at how the estimates compared in each county. However, nitrogen application is a complicated topic, related to weather, and soils, and timing, and there is a lot that goes into this decision.

Is there room for improvement? Absolutely! Take credit for those nutrients. Get manure to fields where all the nutrients have value. Work to minimize uncertainty in nutrient supply by testing the manure for nutrient content, calibrate the equipment and check the flow meter. Look at application uniformity, inject or incorporate, and try to apply at appropriate times. Will this alone solve our water quality concerns? No, but they are things we can do now to maximize the value of manure in our operations and take steps towards environmental improvements and continuing to build trust in using manure nutrients.