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.

Manure Application Timing

I’ve previously talked about nitrogen rate selection, using either the yield goal method or maximum return to nitrogen, and what that may mean from both a production and nutrient use standpoint. This time we are going to do something similar, but will look at a different aspect of it, the impact of when the nitrogen gets applied and how that may impact where it ends up.

The nitrogen cycle is complex; there is a lot going on and it is highly weather dependent – temperature, soil moisture, rainfall, and biology of plants and microbes. So at best, this is an incomplete nitrogen budget as not all sources of nitrogen are going to be accounted for as no measurement of soil nitrogen mineralization was made. Similarly, all the places nitrogen could end up aren’t measured, such as the amount of ammonia lost to volatilization, or nitrogen that ends up as N₂O or N₂, or the amount accumulated in soil organic matter. At best, this is a partial budget that looks at the amount of nitrogen ending up in tile water and in the crop.

So we are going to take a look at four treatments:  1) Spring UAN (corn-soybean rotation, chisel plow and field cultivate, N rate at 150 lb/acre to corn phase), 2) Early Fall Manure (corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as liquid swine manure in early to Mid-October), 3) Early Fall Manure with Cover Crop (corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as liquid swine manure in early to Mid-October), and 4) Late Fall Manure (corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as liquid swine manure in early to Mid-November).

As a first step, let’s take a look at average corn yield for these treatments between 2016 through 2018. On this figure, the first thing that stands out is the nitrogen application timing played a big role in the actually yield, with spring applied UAN out yielding late fall applied manure by around 35 bushels per acre and late fall manure out yielding early fall applied manure by around 40 bushels per acre on average. No difference in yield was seen between the early applied manure with and without cover crop (the cover crop in this case was cereal rye).

Figure 1. Average yield data for 2016 through 2018 crop years for corn in corn-soybean rotation with differing fertilization treatments (EFM – Early to mid-October manure application, EFM+CC- Early to mid-October manure application and a cereal rye cover crop, LMF- Early to mid-November manure application, UAN – spring UAN fertilizer application). All plots received 150 lb N/acre.

A second way to think of this data as what percent of the maximum yield was obtained and what this means for nitrogen utilization efficiency of the fertilizer source. One way to think about and visualize this data is as a function of where we fall on a typical yield response curve. While this curve looks different from year to year, I’m going to use the state average data yield response curve to look at and interpret what this means. The blue diamond shows the spring UAN application and suggests that it would have achieved 99% (or a little better than) of maximum yield. The late fall manure achieved about 84% of maximum yield and would have been equivalent to about 70 pounds of spring applied nitrogen fertilizer, while the early fall manures achieved about 67% of maximum yield and was similar in value to approximately 10 lb N/acre fertilizer application. I’ve marked these two points on the curve in Figure 2 with red dots.

Figure 2. Looking at a typical yield response curve to understand the effectiveness of manure fertilizer in this study. The blue diamond represents spring UAN, the red circles represent Late Fall Manure and Early Fall Manure applications respectively.

Looking at the next part, what did this mean for nitrate concentrations in the tile drainage water? In many ways, the results tended to mirror what we saw from the yield numbers, with one notable exception. Places where yield was higher tended to have lower nitrate concentration. The exception to this was the cover crop plots, where despite having lower yield, nitrate concentrations in the tile drainage remained low. The other thing of note was, in general, early fall and late fall manure showed more variability from year to year, indicating it doesn’t always increase loss, as much as it increases the chance of loss.

Figure 3. Average nitrate-nitrogen concentrations in tile drainage water for 2016 through 2018 crop years for corn in corn-soybean rotation with differing fertilization treatments (EFM – Early to mid-October manure application, EFM+CC – Early to mid-October manure application and a cereal rye cover crop, LMF – Early to mid-November manure application, UAN – spring UAN fertilizer application). All plots received 150 lb N/acre.

Manure Thoughts - beyond budgets

Last month we looked at manure budgets for different counties around the state, and I got a great question. In those counties where we see robust manure resources, what can be done to make sure they are using it, striving to be good stewards, and limit losses of nitrate from crop fields? I always view this question the same way; is there something we aren’t trusting about the manure alone to get the job done? In other words, what does the fertility program look like?

I think one common concern farmers have is how available is the nitrogen in manure. While the weather conditions in any particular growing season can vary, the more important factors are the species from which the manure comes from and the manure management system used. More information on estimating the nitrogen availability for Iowa conditions can be found in PMR 1003 – Using manure nutrients for crop production.

A second important area is the potential for loss of the nitrogen in manure, especially as related to application timing. Nitrogen in manure starts in either the organic or ammonium form, and while these generally aren’t susceptible to loss once they are in the soil, under warm conditions, they can be rapidly converted to nitrate, which makes manure application timing an important factor for estimating the amount of nitrogen in the soil and available to support crop production. Recent research has shown, in certain years, delaying manure application of liquid swine manure from early October until early November, could increase corn yields 30-60 bushels an acre, indicating if for some reason manure has to be applied earlier than intended, yield losses could occur. When farmers see this occurring, it can look like an availability issue, and cause loss of confidence in manure as a fertilizer course.

Finally, a third area of concern is uniformity of manure application and the consistency of manure. The precision age of agriculture has made farmers more aware of places in the field where yields are inconsistent. By its nature, manure has variability to it and while new technology, such as real-time nutrient measurement, may be able to help with this and correct this issue. Additionally, making sure those nutrients are applied at the correct rate and uniformly over the field is a critical component of using manure as a fertilizer, and trusting it will provide what you need, this needs to be done for both solid manure and liquid manure application equipment.

Figure 1. Image of manifold distribution evaluation

Yield Goal and MRTN – A look at what these recommendations mean

As you may be aware, Iowa State University has recommended MRTN for determining nitrogen needs for corn for a while now. This methodology uses data from numerous field trials to understand how corn responds to nitrogen in both continuous corn and corn-soybean rotations, as well as the price of both corn and nitrogen to determine what nitrogen application rate will, on average, provide the maximum profit per acre. This is the amount of nitrogen that results in just enough yield benefit to pay for itself in the extra yield it produces.

To get an idea of how this recommendation would have fluctuated with time, I looked retrospectively back at the average annual corn price for every year since 2005, along with the average price of anhydrous ammonia to ascertain nitrogen price. This was done for both corn and soybean rotations. The results did show some fluctuation, but in general, for continuous corn rotations, the recommendation was 190 lb N/acre with 140 lb N/acre recommended in a corn-soybean rotation, with a variation of about 5% in this recommendation based on specific crop and fertilizer prices. Another thing to note, there was roughly a 50 lb N/acre difference between the optimum N application in the continuous corn and corn-soybean rotation. While you might think of this due to a soybean credit, we generally call it a rotation effect.

The yield goal method, which is in the in the Iowa Manure Management Plan forms, uses a mass balance approach to estimate how much nitrogen is needed. In the yield goal method, we use the average of the previous five-year’s corn yields plus 10%. This is then multiplied by a factor, 1.2 lb N/bu corn for most of Iowa, to determine nitrogen need. If in a corn-soybean rotation, a soybean credit is also required which is suggested to be 1 lb N/acre per bu soybean/acre up to 50 lb N/acre. If we look at the N-recommendations over the same time frame, we see something interesting. The yield goal method suggested approximately 144 lb N/acre in a corn-soybean rotation and 188 lb N/acre in a continuous corn rotation, but the variation in the recommendation was higher at 20%. More importantly, while the MRTN methodology has remained relatively consistent, with perhaps slightly lower levels starting in 2000 as nitrogen prices increased, the yield goal method has shown the opposite trend, increasing consistently by about 2.5 lb N/acre-yr over this data set. This doesn’t come as a big surprise, yields have consistently shown an increase over this time phase, but what it is slightly more concerning, is that most data today shows optimum N application rate isn’t actually related to yield as suggested in the yield goal method.

Figure 1. N recommendations for Iowa as a function of time for the yield goal and MRTN method in continuous corn and corn-soybean rotations.

However, let’s look at and explore this another way. You may or may not be aware, but the amount of nitrogen in a bushel of corn has dropped substantially since the yield goal was first developed. In the late 80’s and early 90’s, it was generally accepted that corn had about 0.8 lb N/bushel (based on the USDA crop nutrient removal tool database) while now it has a bit under 0.6 lb N/bushel, at least based on the best data I seem to be able to find. You may wonder how this could happen – and it really comes down to what we use corn for and what we breed it to do. We want it for the starch or energy, both in animal diets and in making ethanol, so one of the things we’ve seen is larger kernels but with the same size germ, so more starch for the same amount of nitrogen. But the more important part is what does this mean to our nitrogen budgets when using the yield goal method?

Let’s take an example of 200-bushel corn (average of last three years in Iowa), 58-bushel soybean and compare our N budgets for when corn removed 0.8 lb N/bu (old removal estimate) and 0.6 lb N/bu (newer removal estimate) using both the yield goal and the MRTN methods. There are a few things to note; most notable, the yield goal method under high and low N content corn suggests N losses ranging from 30 to 70 pounds, which are in the range typically seen for Iowa soils. The MRTN numbers are substantially tighter budgets with allowable losses of -10 to 30 lb N/acre. This may slightly underestimate nitrogen leaching to put us in the approximate range. This suggests the expected nitrogen efficiency in production with the yield goal method was around 84% which is very similar to where the MRTN prediction of 80% now resides.

Table 1. Partial nitrogen budgets for high and low N content corn using both the yield goal and MRTN methods in corn-soybean rotations.

	Yield Goal		MRTN
	0.8 lb N/bu	0.6 lb N/bu	0.8 lb N/bu	0.6 lb N/bu
N applied (lb N/acre)	190	190	150	150
Estimated N removed with grain (lb N/acre)	160	120	160	120

I bring this up because as we try to put a value on our manure, it is important to place it in the context of our best recommendations for fertilization. It is important to consider both past methodologies for estimating need, and why they may or may not continue to be appropriate. For more discussion on this topic, I encourage you to take a look at “A historical perspective on nitrogen fertilizer rate recommendations for corn in Indiana”, which looks at a few more methods than this, but ultimately shows as we learn, we continue to see wisdom in how things were once done, but also in how we need to evolve to stay relevant.

Manure and Nutrient Budgets – How I see it

Recently, an article was published in The Gazette about a new publication on nutrient budgets in Iowa. The article discussed  work published in the journal, Ambio, entitled, “Livestock manure driving stream nitrate.” The paper looked at watersheds in western Iowa and compared the flow weighted nitrate concentration against several watershed parameters including a “nitrogen surplus.” This was a rough nitrogen budget comparing nitrogen additions and removals, various sources of nitrogen application rate, and the percent of the crop land in corn and soybean acres.

While all showed some positive correlation with the flow weighted nitrogen application concentration in stream water, the highest correlations were found with the nitrogen surplus, manure nitrogen application, and then the area portion of the watershed in corn and soybean. Based on this, the authors proposed manure was a key contributor in driving the higher nitrogen losses. However, let’s delve deeper into some of the results and closely examine some of the results.

The first thing to note is a strong relationship exists between the fraction of a watershed alone that is row crop (corn and soybean) and the flow-weighted nitrate concentration. This factor alone explains 65% of the variation in nitrogen content. This is in line with field plot trials conducted at Iowa State University that have generally shown similar trends with nitrogen reductions coming from land use change towards perennial crops or the use of cover crops. A big part of why these practices have nothing to do with an annual nutrient balance, as suggested in this paper, but more temporal dynamics of when nutrients are released from soil organic matter. These cool weather plants uptake nitrogen at times when more traditional row crops (corn and soybean) aren’t actively growing and as a result, can reduce nitrogen losses to waters.

They also constructed a nitrogen surplus budget for the watersheds (comparison of nitrogen applications and removals) and found it did a slightly better job of describing the average flow-weighted nitrogen concentration in the watersheds, describing 80% of the variation, or 15% more than land use alone. One important thing to consider, the nitrogen surplus budget itself is strongly correlated with land use, so these two measures aren’t independent. This isn’t to say nitrogen application rate isn’t important, it is, but to say land use alone may be a bigger component than nitrogen application rate, i.e., the correlation coefficient alone isn’t able to suggest how much of that effect is from land use and how much is from the effect of nitrogen application rate. Despite this, the result is clear, the higher nitrogen application rates will tend to result in higher nitrogen leaching losses, especially when certain thresholds of exceedance are met.

One last thing I found very interesting about the manuscript was manure nitrogen in the watershed was a much stronger predictor than commercial fertilizer. This result needs to be used with some caution, as the correlation efficient can be impacted by the range in date and in the manure nitrogen application rates had a much higher range than the commercial fertilizer application rate. There are several interesting aspects that manure has a stronger relationship. The first of which, is this represents excreted manure and not necessarily available nitrogen applied to the land.

While in most cases we consider nitrogen application to be a single year fertilizer application, with some manure types offering several years of potential fertility benefits due to the organic fraction which can be slow release. Could the higher loss of nitrogen from the high manure watersheds be a result of improved soil health and mineralization of nitrogen from soil organic matter? Potentially, but without good research or understanding of how long-term manure applications (especially of high organic manures like solid cattle manure) impact soil health, it is hard to anticipate how nitrogen needs and losses from the soil would change with much certainty . Alternatively, the improved nitrogen concentration in streams with manures could be indicative of challenges of using manure as a fertilizer; things like timing of application, certainty of the fertility it provides, or even application uniformity can all be issues that make it harder to trust manure fertility and in the right weather conditions could increase losses.

The important point to consider is this doesn’t necessarily mean we have an application rate problem, it means we have to find better ways to focus on the challenges we have with manure and how to overcome them.