Adjusting Manure Application for Surface Application - Considerations

It has been a challenging fall to get manure to the field and as a result, some farmers had had to consider switching manure application from injection to surface application. While that may be necessary, there are a few additional considerations you should make in your nutrient application.

In terms of nutrient management planning, look at updating the volatilization correction factor. Based on Table 2., of PMR 1003, "Using Manure Nutrients for Crop Production," a correction factor of 0.75-0.90 is recommended for not incorporated surface applied liquid manure and a factor of 0.70-0.85 is recommended for not incorporated surface applied solid manure, as compared to 0.95-1.00 for immediate incorporation and injection. There is a wide range for ammonia volatilization because there is considerable uncertainty about the process – the weather conditions we face, the characteristics of the manure, and how quickly it infiltrates into the soil all make a big difference on how much of that nitrogen is actually lost. The cooler temperatures we have this time of the year will slow ammonia loss. As long as the manure is infiltrating into the soil relatively quickly we should be on the lower end of the scale, probably losing around 10-15% of the nitrogen we apply to surface application.

The second thing to consider is setback distance requirements. With injection/incorporation, the required setback distances are often 0, but when switching to surface application setbacks of 750 feet from residences and public use areas, 200 feet from water sources and other designated areas, and 800 feet from high quality water resources will be required for liquid manure. If soils are wet, consider increasing setback distances to provide some insurance that no manure moves out of the field.

Finally, the last thing to remember is the soil hydraulic properties and weather conditions have a much greater impact on surface manure application. In some cases, if the soils are wet or we are on sloping topography we may have to adjust manure application rates down to ensure that no runoff occurs. When we inject we get immediate mixing of manure with soil and that can help to hold the manure in place. With surface application, we rely on the soil’s ability to infiltrate the manure, which can take more time for higher application rates. While applying watch from pass-to-pass to make sure the manure you are applying is soaking in and not moving over the field.

 

Figure 1. What happens when we can no longer inject and have to switch to surface application?

Composting or Stockpiling – What’s the difference and the science behind them

Solid manure from cattle and poultry facilities may require additional storage before fields are available for land application. Composting and stockpiling are two methods of storage and management available to store and treat solid manure. These handling methods can impact nutrient losses and manure characteristics, and as a result the fertility they can provide and their transport and application properties.

Let’s start with the basics, what is stockpiling? If you look up stockpile, you’ll find that it means to gradually accumulate something, in our case manure. A more specific definition for our purpose here is, stockpiling is a passive management of solid manure where the material is placed into a storage, which may be either inside a stacking shed or outside and exposed to the elements, where it remains until it is either land applied or moved. In either case, the important points to stockpiling are (1) this is a passive management system, once the manure is stacked it is left alone and not disturbed, and (2) as a result the pile will become anaerobic. It is only passive in the fact that we, the farmer or manager, won’t perform regular activities to alter the pile, but within the pile microbial activity will still be occurring. Despite this, stockpiling is essentially a storage technique, though some natural treatment may occur as a result. 

Figure 1. A stockpile of manure.

In contrast, composting is an active management and treatment technique, that encourages aerobic conditions to accelerate the breakdown of organic matter within the manure. This produces higher temperatures within the pile that encourages faster microbial activity and can also reduce the viability of pathogens, bacteria, and seeds within the composted material. The important parts here are that composting is (1) an active management process for treatment of the manure and (2) the process is aerobic.

The difference in aerobic and anaerobic may seem small, but there are some important distinctions between the two that result in vast differences in the two processes. In anaerobic conditions, breakdown of organic matter releases only very small amounts of energy and makes compounds like methane, carbon dioxide, ammonia, hydrogen sulfide, and many partially degraded organics (volatile fatty acids, alcohols, phenols). This means that while the pile may heat up a little, since there is little energy released, temperature increases tend to be small and breakdown tends to be slow. Also, the compounds we make tend to be ones that we can smell. Aerobic reactions tend to release larger amounts of energy; these exothermic reactions will cause the pile to warm up and accelerate biological activity and growth. In this situation we will still make carbon dioxide and ammonia but won’t make those other compounds.

What these differences mean to us is that we will have different amounts of break down occurring when we compost or stockpile manures. The amount of difference this makes is very dependent on the initial manure properties, with manures with high amounts of carbon in them (such as bedded manures) typically exhibiting a bigger difference. For example, studies of composting bedded cattle manure have suggested that a mass loss of 40-70% (water plus dry matter) can be achieved, while cattle manures from earthen feedlots typically range in the 15-25% range. A study on earthen lot cattle manure showed that composting the manure resulted in a 50% reduction in organic carbon while stockpiling the manure reduced organic carbon by 40%. However, composting reduced nitrogen mass in the manure by 40% while stockpiling only reduced nitrogen mass by 14%. This occurs because the warmer composting temperature can increase ammonia volatilization, while for the stockpile typically a crust develops that can reduce ammonia loss. Because of numbers like these, stockpiling has historically been the preferred storage technique, but for more carbon rich manure or situations where manure is hauled long distances this may not always be the case.

More recently additional topics related to manure stockpiling have come to the forefront. While poultry manures tend to have sufficient potassium in them to support crop production, certain management approaches can leave their potassium content lower than expected. Potassium is very water soluble and stockpiles exposed to the elements, such as rainfall can have the potassium within them leached into the soil below. While this poses minimal risk for water quality it is an important consideration for using stockpiled manure as a fertilizer source. Good stockpiling shaping, taller rather than wider, and with sloped surface to encourage rainwater shedding rather than water moving through the pile can help maintain potassium content. Additionally, stacking sheds can help keep rainwater off the manure and help hold potassium in the manure. Similar results have been found for nitrogen, with covered or roofed stockpiles only losing 5-15% of the nitrogen in the pile, while outside piles losing 15-25% of their nitrogen.

These nitrogen losses also have implications for crop production. In general, most of the nitrogen lost is from the ammonium fraction, which is first year plant available. Manures that have been stockpiled in ways that have minimized nitrogen loss from the pile (covered or roofed stockpiles) tend to result in a greater fraction of the excreted nitrogen making it both to the field and ultimately into the crop through uptake and utilization.

In summary, stockpiling remains a viable manure management strategy to help get the most fertilizer value from solid manures. However, opportunities to improve management due exist with covered and roofed storages potentially providing mechanisms to help hold onto nitrogen within the manure and minimize potassium loss during storage.

Maximizing Manure Value - Timing and Application Logistics for Value

“There is more than one way to skin a cat,” is a famous phrase telling us other options exist. It’s also a phrase murmured by this husband to Mrs. Manure when the initial plan for the day’s home renovation project hits an unexpected snag. However, the same idea applies when we think about capturing manure value, there is more than one way to achieve it.

In particular, I’d like to take a quick look at two approaches today, the bigger, faster, approach designed to reduce the cost of application per gallon and then side-dressing manure. The truth is, both have advantages and challenges to capturing manure value. Understanding the challenges of each is important to determine what may work best for you and your farm. While there are many considerations, today I’m going to focus on some of the economics behind value. To get started, I’ll be working on some of the concepts I first discussed in Manure Application Logistics – Rate and Cost, where I looked at how the application rate we are using impacts the cost of manure application rate.

To make this comparison, I’m going to consider a 4,800-head swine farm, which will generate about 1.75 million gallons of manure a year, or enough to cover about 695 acres (approximately 60 lb N/1,000 gallons and applying 150 lb N/acre). At this farm, we’d have an application rate of about 2,500 gallons per acre.

For illustrative purposes, I’m going to ballpark $500,000 in equipment costs (pumps, hose, drags, and a toolbar), but that is dependent on what you are using. In the case of swine manure, let’s assume we have a 30-foot bar and can drive through the field at 7 mph. This means they can cover an 0.42 acres per minute and to get 2,500 gallons per acre the flow rate would be about 1,060 gpm. This means to get all 1.75 million gallons applied would take 27.5 hours and assuming the crew was about 50% efficient, it would take about 55 hours overall. Just for fun, let’s assume run time costs about $500 an hour (tractors, fuel, wear and tear, etc.). If we figure a 5-year equipment life and 1.75 million gallons is about 10% of the total gallons they apply every year, then our cost for manure would be about $37,500 or about $0.021 a gallon of manure applied or about $0.36 per pound of N applied.

Figure 1. Manure application in the fall using drag line equipment and thinking about travel speed to lower application cost.

Now we need to do the same thing for a side-dressing type scenario. I’m going to keep the equipment cost the same and assume setup time remains the same at 27.5 hours (since I’ll have the same number of sets), but since we are side-dressing we are going to have to drive slower, here I’m going to assume a travel speed of 3 mph. At this speed, we will cover 0.18 acres a minute, or manure application will take 64.3 hours or 92 hours overall. Making the same assumptions as above for cost, that is $500 per hour in variable expenses and $0.006 in fixed expenses, per gallon for a cost of around $55,925 or $0.032 per gallon. This amounts to about $0.54 per pound of nitrogen.

When you look at these numbers it may be easy to say that the first case is maximizing manure value as the price per unit of nitrogen delivered to the field is cheaper, but there is a timing impact on how efficiently this nitrogen can be used by the plant. While I don’t have data on side-dressing manure and the impact it has on value, we do have data from the last two years on how three different application timings (early fall, late fall [50 degree soils and cooling], and spring manure application) impacted corn yield. While not a perfect comparison, they give us some idea of what the potential yield increase may be. In that study, we saw late fall versus early fall worth 45 bushels an acre on average, while moving to spring manure application versus late fall application was worth 33 bushels per acre. Given the weather and soils at that site, these are probably a bit higher than we’d see in much of Iowa, but provide a starting point to the conversation.

Figure 2. Side dressing manure, slows our gallon per minute rate, but what does it do to value?

The 33 bushels an acre we saw in that study, at $3 a bushel corn, would be worth $99 an acre. This improved timing added approximately $0.66 of value from the nitrogen applied. Thinking of this in a slightly different way, by changing timing we estimated a change in the cost of nitrogen delivery in these systems of $0.18 a pound increase, meaning the return on investment using the data we have, was about 3.6-to-1.

However, there are some concerns with this data – is that a good representation of the yield increase we could expect from switching from fall to spring, or because of the site and weather conditions, is this estimate a bit high? In coming up with the economics, I wrote off the cost of my equipment over the same amount of manure, but we saw firsthand in this example it took 1.7 times longer to apply the same amount of manure and if we have the same number of working days, this means we’ll apply less manure increasing our cost a bit more. Finally, we need to ask, given the time constraints of spring and side-dress manure season, what percent of manure could be applied this way given the number of working days available?

Value Adding to Manure - Manure to Energy

Manure is already used as a fertilizer on most farms, providing cost savings on fertilizer purchases and helping to build soil health on fields that receive this valuable amendment, but are there ways to get additional value? One that has received some attention over the years is manure to energy. In liquid and slurry manure systems this has typically meant anaerobic digestion for production of biogas, which is rich in methane. However, in states like Iowa, these systems remain relatively rare. Why is this? While there are many reasons, a good place to start is by exploring the economics of these anaerobic digestion systems. We are going to take a look at one specific type of digester system, the covered lagoon, to see how it may impact manure economics.

What is an impermeable cover? It is a plastic film placed on top of the manure storage that is impermeable to both gases and liquid. The idea is that this cover will keep rainwater out of the manure, hold in gases so odor and ammonia emissions are minimized, and capture methane made by the natural breakdown of organic matter in the manure.

There are several ways covers can add value to a farm.  For instance, reducing the amount of rainwater that needs to be hauled, holding onto nitrogen and increasing the fertilizer value of manure, and the value of odor control. By capturing the biogas, it can be used for the generation of electricity, heat, or additional processing (via pressure-swing adsorption to separate the methane and compression into pipeline quality gas in this analysis). In addition, doing so will add several new expenses for the operation. These include the cost of the cover, the cost of cleaning the gas to pipeline quality, and the change in manure application costs, as more land will be required as the cover maintained more nitrogen value.

Figure 1. Impermeable cover on a manure storage that allows collection of biogas, keep rainwater out of the manure, and help minimize odor and ammonia loss, but is it ever cost feasible?

So let’s start with some of the positives. Adding a cover to an outdoor manure storage (a Slurrystore, earthen, or concrete storage outside the building) would allow the design of a slightly smaller storage, as we’d no longer have to size for rainfall and the 25-year, 24-hour storm (about 5-5.5 inches throughout the state of Iowa). However, this has a minimal change on the construction costs of the actual storage. However, putting a cover on a storage does offer the potential for retaining nitrogen in the manure. In a typical deep pit storage, 7.8 pounds NH₃ is retained per pig per year. Switching to an impermeable cover would save about 5 pounds of NH₃ per pig per year, which on a 4800-head swine farm amounts to about $7,000 of nitrogen value every year. This would increase our manure application costs slightly as more nitrogen means more acres would be needed and manure would need to be moved a bit further, increasing application manure application costs by about $2,700 every year.

One advantage of the impermeable cover, it allows us to capture the methane the manure is making, the value of this captured methane would be around $9,000 through direct sales, and if marketed correctly to collect the RIN (Renewable Identification Number) credit which is granted for transportation fuels would add another $26,000 in value from the methane.

Comparing this to new expenses generated, we’d have an annualized installation cost (assuming a 5-year life of the cover) of $14,000 and a maintenance cost of around $3,500. Our largest new expense would be biogas cleaning, so we can inject onto the pipeline, which would be around $24,000 every year.

When you take all of these into account, this sort of system hovers right around the break-even point on an annual basis. While I’m not saying we should all start making biogas at our farm, it is nice to see that we are close to being able to say, in the right situation, it could make sense on a farm.

Manure Pumping – Viscosity, Flow Estimation, and going Beyond Hazen-Williams

When we design pumping systems, we are out to pick a pump and pipe combination that helps us to be efficient. In terms of the flow rate, we are trying to achieve the amount of pressure we need to get there and to get the pump to operate in an efficient range of it is operating rate.

There are several approaches to doing this, ranging from the Hazen-Williams equation (an empirical relationship which relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction) to the Darcy-Weisbach method (which is does essentially the same thing, but takes into account fluid density as well as viscosity, so it can be used with fluids other than water). So, this brings us to the question, which one should we use when we are trying to estimate manure flow rates?

The truth is even though Hazen-Williams is only for water, in most cases it will be accurate enough for manure and give us an idea of what is happening, but at least in theory the Darcy-Weisbach method is more accurate if viscosity becomes high. This leads us to a discussion of viscosity, how manures are different than water as a fluid, and what this means.

Viscosity is a measure of a fluid’s resistance to motion under an applied force. Our classic example of this is to compare water and honey; honey has a thicker consistency so when we coat our spoon with it and tip the spoon it flows off slowly, while the water slides right off, so we say the honey is more viscous than water. From this it may become clear that if we were trying to pump honey with the same setup we were pumping water with, the flow rate would be much lower, because the viscosity makes the honey harder to pump.

But what about manure – this is where things get even more complicated. Water and honey are what we call Newtonian fluids.  Their viscosity is dependent only on the temperature of the fluid, but manure is more complex than that and is a shear-thinning fluid, meaning its viscosity if dependent on temperature, but also on the shear rate (basically the flow rate). The faster we shear it the easier it becomes to shear it.

Here we are going to look at a slightly different question, how does the solids content of the manure, impact viscosity. You can see this relationship in figure 1, basically higher solids content leads to more viscosity. For reference, the viscosity of water is essentially 1 centipoise so if we take all the solids out of the liquid manure, it is really close to water, but a solids content of about 8% which is pretty typical of deep pit finishing swine manures we have a viscosity of 3.65 centipoise (or higher than the viscosity of water). This may sound like a lot, but it is essentially the same as kerosene (honey on the other hand is 1,500).

Figure 1. Relationship between solids content in swine finishing manure and the measured viscosity.

To see what this might mean, let’s assume we have an 8-inch diameter pipe and the fluid inside is flowing at 10 feet/second, we’ll just assume smooth pipe that is 1-mile long. Our question will be how much extra pressure is needed if the viscosity of the manure is 3.65 centipoise instead of the 1 centipoise it would be for water. This can be calculated using equation 1, where h_f is the head loss in feet, f is the Darcy friction factor, L is the length of the pipe, D is the diameter, V is the velocity of the fluid, and g is the gravitational constant.

                            hf = fLV2/(2Dg)                                      (1)

You may note that viscosity doesn’t show up anywhere in this equation and wonder what all the fuss was about, but it is hidden in the Darcy friction factor, f, which can be approximated using equation 2.

                   1/sqrt(f) = 2log(Re*sqrt(f)) -0.8                         (2)

For water under these conditions, I’d estimate we need to supply about 136 feet of pressure head. If we repeat the calculation for manure assuming a viscosity that is 3.65 times higher like we measured on in our viscosity analysis it would only reduce the pressure loss to 172 feet, or by about 25%. Just for fun, if you compare this to what we’d get using the Hazen-Williams equation, you’d estimate something like 160 feet of heat loss in those flow conditions.  All this to say, while in theory it seems like it would be nice to incorporate viscosity into our flow estimation, generally the uncertainties in all the variables and the variability of manure make using the Hazen-Williams approach good enough for flow and pumping problems.

Manure Sidedressing

As the warm summer heat pushes the corn taller, it seemed a good time to discuss sidedressing manure. Today, I want to look at three potential reasons people may want to sidedress manure:  storage management, nitrogen management, and equipment availability.

So, what is sidedressing manure?

Sidedressing is the application of fertilizer to an already established and growing crop. In the case of sidedressing manure, it simply refers to the fertilizer source being a manure. This can be done using either tanks or dragline application methods, though equipment must clear the emerged corn and move mostly between rows, giving only a short window.

Storage Management

Sidedressing opens up an additional spring window for manure application and thus potentially a chance to reduce storage pressure by having more room available going into fall. However, there are a few things to consider. Sidedressing with manure has a relatively short window. It probably isn’t a good idea to rely on it as your only land application window, as the weather during this short window can be unpredictable. From a nutrient management perspective, if we miss this window we can still get nitrogen applied using other sources and other equipment to provide it fertility, but that doesn’t provide an opportunity to use the manure.

Nitrogen Management

Sidedressing nitrogen allows it to be placed just before corn uptake is maximized and in so doing the risk of losses from earlier spring rains or long warm falls is reduced. There is some risk the weather during the sidedress window will not be suitable for manure application, but as other forms of nitrogen can be applied at larger growth stages, there is still options available to successfully manage the crop.

In terms of manure, though we often think of it as an organic nutrient source, much of the nitrogen, approximately 70% in the case of liquid swine manures, is available as ammonium. This fraction is immediately available for crop uptake and means this type of liquid manure is a good choice for sidedress fertilizers.

Equipment Considerations

While both tanks and dragline application methods can be used, the equipment needs to be set up so it will fit between the rows. For tanks, this means having tire widths that can move between the rows and injectors. For dragline application, it means making sure application is finished during or prior to the V4 stages so the corn plants are still springy enough they can bend over when the hose crosses over them.

If you’d like to try sidedressing manure with a dragline, consider planting corn at a 45-degree angle to the field, so it follows the natural pattern applicators would use with draglining.

Finally, if you are sidedressing manure, be sure to let us know, we’d be glad to come watch, collect some pictures, and even some crop performance and water quality data if you are willing. Let me know at dsa@iastate.edu or 515-294-4210.

Figure 1. Manure sidedressing using a dragline application method.

Prepping for the Manure Applicator Training Advisory Meeting

Generally, when I’m writing it is about the science of manure, but I thought today I’d write something a little different. Today is the Manure Applicator Training Advisory meeting for the 2019 Iowa Manure Applicator Training Program. Every year we get to interact and share information on manure with about 5000 individuals in this program, help them understand the current state of the industry and the science, and hopefully encourage them to make the best possible decisions on how to utilize their manure.

I consider it a great privilege to help provide this program. Manure is an important topic in Iowa and one that touches on technology and machinery, agriculture and the environment, human and animal safety, soil science, and so many more. There is great diversity in the topics each individual farmer or manure applicator will find important, and the challenge is how do we take what we know about them and their farms, there application companies, current and future regulations, and provide them with knowledge that they find useful, interesting, and engaging.

While I by no means have it figured out, we have been working on engaging, exploring active learning. Last year one activity we explored using active learning was compaction. Those present were divided into teams and set around to discuss and answer four questions related to compaction and how it impacts the manure business. This provided a great chance to stretch their legs, but also some peer-to-peer discussion and a chance for sharing of their experience. You can get an idea of what was happening in the photo below, and while discussion may have started slow, as we went along it picked up and we got plenty of great comments. As we are planning for the upcoming year I thought it would be fun to look back on the activity and see what we heard.

 Figure 1. Groups discussed and provide answers to each question, spending 5-7 minutes discussing and summarizing.

The four questions we asked were: 1. What causes compaction? 2. Why do we care about compaction? 3. What are your or your client’s expectations about compaction? And 4. How can we reduce compaction (primarily related to manure application)? For each of the questions we compiled the answers as a “wordle.” For those of you like me who may not know, a Wordle is a toy for generating word clouds from test that you provide. The clouds give greater prominence to words that appear more frequently in the text, or this case the answers provided at all the different sites. The important thing is that the give a quick and elegant way of providing a visual clue summarize what people were talking and discussing in their answers to the questions. So let’s take a look at what they had to say.

Figure 2. What causes compaction?

Figure 3. Why do we care about compaction?

Figure 4. What are you or your client’s expectations about compaction?

Figure 5. How can we reduce compaction?

My goal here won’t be to dissect these answers, but you can see that there were themes that emerged and we’ll try to focus more on those, how they relate to the science we do know on compaction, and more importantly how the mitigation strategies they mentioned rely on that science. Overall, though the activity proved worthwhile, provided a few smiles, and we well received, so something we’ll continue to pursue and work on.

Human Waste Treatment compared to Livestock Manure Management

A while back I wrote about why human and animal waste are treated and managed differently. In many respects, this was an economic rationalization why we chose to do things so differently with the same goal in mind, protection of water quality. The foundation behind it was right, but it stopped short of the question I get more often, which one is more effective.

While it’s important to keep in mind that there are good reasons to manage them differently, I’m going to make a simple comparison between the two systems. Granted there is a lot more we could focus on than just BOD (biological oxygen demand, the amount of oxygen needed to break down the waste which tells us something about the risk of a fish kill) and the amount and forms of nitrogen that comes out of each system.

To recap what was covered in the first post, there are significant differences in how human and animal wastes are managed.  Human waste (assumed here to be domestic waste only, no industrial in this post) is typically treated and discharged to receiving waters.  Animal manures are typically stored and land applied.

Several factors influence the difference in approaches including:

• Wastewater characteristics
• Regulation
• Economics

The following table compares typical waste characteristics and volumes for a 10,000 population city) and a 10,000 head hog farm, for both total volumes produced and the characteristics of it.

Table 1 – Human and Animal Waste Comparison (1,2)

Parameter	10,000 Population City		10,000 Head Hog Farm
	Value	Assumption	Value	Assumption
Volume	1,250,000 GPD 456.3 MG/yr	125 gpd/capita	12,000 gpd 4.4 MG/yr	1.2 gpd/head
BOD	1,900 lb/d 693,500 lb/yr	0.19 lb/d/capita	3,030 lb/d 1,105,500 lb/yr	30,350 mg/L
Nitrogen	300 lb/d 109,500 lb/yr	0.03 lb/d/capita	700 lb/d 255,000 lb/yr	7,000 mg/L
Phosphorus	80 lb/day 29,200 lb/ yr	0.008 lb/d/capita	95 lb/day 76,500 lb/yr	2,100 mg/L
Oxygen Demand	3,470 lb/day	1.1 lb O2 per lb BOD and 4.6 lb O2 per lb nitrogen	6,550 lb/day	1.1 lb O2 per lb BOD and 4.6 lb O2 per lb nitrogen

Regulation and Estimated Nutrient Loss

All wastewater treatment plants (WWTPs) must meet the requirements of their discharge permit as part of the Clean Water Act.  Typical permits include limits for BOD, solids, ammonia, pH, and disinfection to kill pathogens.  Focus on the harmful effects of nutrients (total nitrogen and phosphorus) in watersheds (depressed oxygen levels, algal blooms, fish kills) has led to increased regulation of nutrients for many treatment plants, often requiring advanced and expensive treatment processes.  Most treatment plants land apply treated solids and must meet regulations with limits on pathogens, nutrient loading rates and application practices. 

While this definition of the process is helpful, what we typically want to know is how many pounds of BOD, N, and P we are allowed to discharge per person. That is what is the actual effect we have?  Looking at the city of Ames wastewater permit, our municipal treatment facility is allowed 2018 lb/day of BOD, and 284 lb D of NH3-N lb of N per day. For fun let’s say Ames has a population of 66,000. This is 11 lb BOD/per person per year and 1.6 lb NH3-N/person per year discharged.  They don’t mention nitrate, but it’s probably about 9.4 lbs NO3-N as very little is denitrified using the current technology they have (though this is subject to change).

Most animal operations, once they hit a size threshold of 1000 animal units, are subject to an NPDES permit if they propose to discharge. Iowa law actually doesn’t allow confinement farms to discharge, so on the point source side the regulations are pretty stringent and the number would be zero except in extreme weather conditions. However, land application of animal manures is an important part of nutrient transport. Let’s work off a pig space, so at 1.2 gallons per day and N content of 60 lb N per 1000 gallons of manure. This works out to about 30 lbs of N per pig space per year, so about 0.2 acres fertilized with the manure. We lose about 30 lb N per hectare as nitrate leaching when we grow row crops, so we are losing about 5 lb N per pig space per year, so about half of what we lose per person. Losses of NH3-N in water and BOD in water are very minimal do to the effective treatment of soil.

Figure 1. Water quality is important to all Iowans. Different treatment approach can both help achieve desired water quality objectives.

Final Thoughts

1.      If we didn’t recycle manure as a fertilizer would the nutrient load to streams increase, decrease, or stay the same?

2.      If we say there are no point source losses from collection to storage (a mostly true assumption) how much does manure contribute to the nutrient loading?

If we treated manure like municipal waste, the nitrogen loss actually goes up as we just replace the manure with other synthetic fertilizer (meaning non-point source losses stay similar, though some change is possible). We’d also have additional nutrient loss; though we may remove most of the BOD and almost all the ammonia in the manure, we’d still have nitrate released from the treatment plant into our streams and rivers.

The other interesting take away was that while the two approaches for treatment were drastically different, they both seemed to be equally effective at removing BOD and ammonia from water, but have some difficulty with nitrate though losses per pig space are estimated to be about half of that from a human.

The Real Scoop on Manure

Recently an article was published in The Storm Lake Times titled Nitrate levels expected to rise with hog numbers. The topic of nutrient management and the role animal production is multifaceted, rarely having any easy answers, and almost always having complex interactions that need to be carefully considered. Here we will take a look at some of the implications animal agriculture has on nutrient management and as a result, its impact the concentrations of nitrate and phosphorus in the Raccoon River. As animal agriculture looks to expand, an important question is, ”How will waters of Iowa be impacted?”

An important starting point for any conversation on manure nutrients is, how effectively they can be used as a fertilizer resource. Most manure in Iowa is managed this way. In terms of nutrient losses, most university studies have indicated when appropriate application timing guidance is followed, and when similar nitrogen application rates are selected, nitrogen losses via leaching will be similar to those from commercial fertilizer sources. For example, the Iowa Nutrient Reduction Strategy science assessment team summarized all the research comparing both swine manure and poultry manure to spring applied commercial fertilizer and found nitrate-N leaching losses were similar as were corn yield. Similarly, in terms of phosphorus management, given the same soil conditions and runoff shortly after application, Iowa State University research shows using manure, instead of commercial phosphorus fertilizers, actually reduces phosphorus loss from that first runoff even by about 50%! The best research we have says, under appropriate management strategies and rate selection, the nitrogen use efficiency is not largely impacted fertilizer source. Rather, it is more controlled by crop rotation selection, soil properties, and weather conditions of a particular growing season and it will actually improve phosphorus management.

A second important question is, “Will the addition of new confinement operations result in too much manure?” While there are many ways to define and assess what too much manure is, an important starting point to the conversation is, “What crop land is available to which the manure could be applied as a beneficial fertilizer?” As a state, Iowa currently obtains between 25-30% of the nitrogen, phosphorus, and potassium we need for crop production from animal manures, with the rest of the required fertility coming from other commercial fertilizer sources.  This does vary by county, but currently all counties harvest more nutrients in the crops they produce than are available in the manure produced within that county.  Figure 1 below shows how the amount of manure nitrogen available for crop production compared to the amount of nitrogen removed with non-legume harvested crops (i.e., doesn’t account for nitrogen removal with soybean or alfalfa).

Figure 1. Percent of nitrogen available in animal manures relative to the nitrogen removal with harvested, non-legume crops.

In the recent article, one of the comments made was that CAFOs can change an area’s nutrient balance by importing feed for the livestock. While this does have the potential to create nutrient imbalances, it is only a piece of the puzzle. The Manure Management Plan (MMP) was developed to make sure, even if feed for the livestock was being imported, sufficient land was available to appropriately use the manure. While it can still serve as an indicator, it is one of many indicators of how nutrient management is working. Manure management plans ensure the same thing in a more measured and complete way.

We all know our land resource base is finite. No matter what, Iowa is only going to have an area of 55,857 square miles (or just under 36 million acres); of this, about 26.2 million acres, or about 73% of the state, is farmland. How we choose to use this land can have profound impacts on our ability to produce food, fuel, fiber, and the impacts we have on the environment. Because of this, it was suggested adding animals would cause more row crop production to produce the feed for these animals. Yet, as a state, Iowa counties show very little correlation between the amount of land that is planted to corn and the amount of manure produced in that county.

Figure 2. Relationship, or lack thereof in this case, between the amount of manure produced in a county and the percent of cropland planted to corn within a county.

In terms of nutrient management, this is only a piece of the puzzle as there are some additional differences between manures and other commercial fertilizers that may make their nutrient losses a bit different. In particular, manure is a complete fertilizer, in that it contains all the macronutrients crops need, but not necessarily a balanced fertilizer. That is, the nutrient ratios in manure may not be balanced to crop removal. Historically, manures have been relatively high in phosphorus as compared to plant available nitrogen. The Iowa Phosphorus Index has been used in Manure Management Plans (bill passed in 2002 and implemented in 2008) and uses information about how much phosphorus is currently present in the soil, how much will be added, and its risk of transport to an Iowa water body to determine if manure application should be limited by supplying nitrogen or phosphorus. This is a risk based approach that focuses on water quality in making a manure management decision. With this said, changes in farming and feeding practices in the swine industry have reduced the amount of phosphorus in swine manures relative to its nitrogen content making its nutrient content approximately balanced for corn-soybean rotations and reducing the risk of phosphorus build-up.

So what’s all this mean? While there are certainly challenges to managing manure that make it a unique fertilizer option. Farmers strive to get value from this manure in their operations, and in so doing typically make application decisions that result in nutrient losses to water similar to those of other fertilizer option. Though it may seem like Iowa is livestock rich, it’s important to remember that adequate land to utilize our manure researches exist. Livestock operations play a vital role in Iowa’s agriculture economy and continue to strive to do so in ways that decrease environmental impact, that are more sustainable, and more importantly these farms continue to strive to do better.

Manure Scoop – More on Variable Rate Manure Application

Last time we started the conversation on precision manure application and some of the challenge of variable rate manure application, this time we’ll dive into an example and see how the economics play out.

Precision agricultural methods and variable rate application can improve nutrient management and help reduce environmental impact. We talked last time about doing this with nitrogen and the challenges it presents, but as manures also provide phosphorus and potassium. These nutrients and their need for upcoming crops can be well predicted by soil sampling. This means grid soil sampling can be used to tailor phosphorus and potassium recommendations across the field and put down a sufficient, but not an excess amount anywhere. In some cases, this may allow us to cover more acres with manure and save the purchase of commercial fertilizer for those additional acres.

The process of soil sampling and developing the “to apply” maps are similar to what would be done for commercial phosphorus or potassium fertilizers. The first main difference from commercial fertilizer is you get a guaranteed analysis, whereas with manure you have to sample, sample, sample to get a handle on its composition. This could range from testing every load to taking a composite sample, the key being you have to trust the analysis to have less variability than the rate change over the field. In some cases, this may be easy, if the manure was well mixed as it is loaded and stockpiled it will be more uniform than if it was loaded from different areas of a pen. Getting a handle on this variability is critical to make variable rate application a success.

A study by ISU on variable rate manure application showed it wasn’t unusual for fields to span four to five soil test phosphorus classes within a field. By varying the manure application rate across the field they were able to positively impact yield similar to what was obtained with a fixed application rate, not apply manure to areas that were optimal or high in soil test P and in so doing, save manure to be utilized in other areas. Moreover, the variable application rate decreased soil test variability, reducing soil P in areas that were high (and as a result more susceptible to phosphorus loss) and increasing phosphorus content in areas that were low, and the phosphorus in the soil positively impacted future crops.The variable rate allowed for better manure management by conserving manure where it wasn’t needed and placing it where more value could be obtained while also reducing the risk of phosphorus delivery from the field to water resources.

Variable Rate Manure Application

With the very idea of the topic, questions begin to emerge:  what, why, and how to use the technology. We will try to walk through a few of these questions to address what we do and don’t know. With today’s technology, including things like GPS location guidance, flow controllers, or weigh scales on manure spreaders it is possible to make maps of how many gallons per acre are applied.

In terms of solid manure, where decisions are often made based on phosphorus management, grid sampling can be used to determine current soil phosphorus levels. A map is generated of how much phosphorus we want to add to hit a certain level and this prescription map used to determine manure application rates on the go. Some current equipment even has the capability of using these prescription maps on-the-go to change the rate as you move through the field. This is effectively how variable-rate of application other commercial fertilizers has been done for a while, but there are some additional challenges with manures.

However, the question when using manure as the fertilizer source substantially increases these questions. Things like how accurately do we know the manure nutrient content, how variable is the nutrient content during application, how accurately can you hit rate, how uniform is the application, how good is the application method, and we are left with questions about if we can control these variables accurately enough to make variable rate application pay . If we try to extend this to nitrogen, it can get even more complicated as we now need to consider additional factors such as the quality of injection/incorporation throughout the field and its impact on ammonia volatilization and the variation in nitrogen mineralization and variability. This is to say, getting a firm grasp on these details would be the first step towards working towards a variable-rate manure application.

In terms of variable rate nitrogen application with manure, the first step would be determining what we parameter we want to vary nitrogen application rate based on.  Some ideas that have been proposed, include previous year’s yield maps, soil type, or soil organic carbon levels. Two weeks from now we will take a closer look at each of these potential methods, why they may be considered, and science available behind how well it works.