The Alchemists Dream: Turning Manure to Gold through Separation and Nutrient Recovery

 Nutrient separation, solid–liquid separation, and manure drying systems are getting more attention across livestock systems, but when do they actually make economic sense? These technologies are often discussed as solutions to manure management challenges. Sometimes they are. Other times, they’re expensive ways to solve problems that don’t really exist, or problems that would be cheaper to address another way.

This article is not meant to be comprehensive, nor is it intended to answer every question about separation and drying systems. Instead, the goal is to provide a realistic look at what actually drives value in these systems by walking through a few example scenarios and comparing manure management before and after treatment. If you’ve wondered whether these systems are a smart investment, or a shiny distraction, this one’s for you.

Why Are These Systems Being Considered in the First Place?

At their core, manure treatment systems are an attempt to solve a logistics problem, not a nutrient problem.

Most manure management challenges come down to some combination of:

·         Too much water relative to nutrients

·         Nutrients concentrated in the wrong place

·         Limited land base near the livestock facility

·         Imbalances between nitrogen and phosphorus needs

·         Increasing hauling distances and application costs

·         Tight application windows and labor constraints

Separation and drying systems don’t create nutrients, and they don’t make regulations go away. What they do is change where nutrients go, how concentrated they are, and how expensive they are to move. Interest in these systems has grown as farms have gotten larger, hauling distances have increased, and nutrient management has become more constrained. In some cases, water reuse or off-farm nutrient export becomes the primary driver rather than fertilizer value alone.

What Do Separation and Drying Systems Actually Do?

Rather than focusing on equipment types, it’s more useful to think about these systems based on outcomes.

Broadly speaking:

·         Solid–liquid separation shifts nutrients unevenly between a liquid fraction and a solid fraction.

·         Nutrient separation systems intentionally concentrate certain nutrients (often phosphorus) into a smaller volume.

·         Drying systems reduce mass and volume by removing water, dramatically changing transport economics.

What matters isn’t the technology, it’s what changes after treatment:

 

·         Total volume that must be hauled

·         Nutrient concentration of each fraction

·         Where each fraction can be applied

·         Application method and timing flexibility

·         Labor, energy, and management requirements

Nearly every system creates tradeoffs. Liquids may become easier to apply nearby, while solids require new handling, storage, or markets. The value only appears if those tradeoffs align with real constraints on the farm.

How Do We Evaluate Whether a System Pays?

Before talking about treatment, we need a baseline.

Step 1: Understand the Untreated Manure System

For any farm, the starting point is:

·         Annual manure volume

·         Typical nutrient content (N, P, K)

·         Average hauling distance

·         Application cost per gallon or ton

·         Effective nutrient value captured by the crop

This baseline represents the true cost and value of manure without additional technology. If untreated manure is already inexpensive to apply to nearby acres with good nutrient utilization, there may be very little economic upside to separation.

Step 2: Compare the Post-Treatment System

After treatment, we look at:

·         New volumes and hauling distances

·         Changes in nutrient distribution

·         New application or storage costs

·         Operating costs of the system itself

·         Any new revenue or avoided cost

The key question isn’t “does it reduce volume,” it’s whether the reduction saves enough money, or creates enough flexibility, to justify the added cost and complexity.

Case Studies: Where the Economics Come From (or Don’t)

The following examples are simplified and illustrative. Assumptions are intentionally transparent, and numbers can be refined. The goal is to highlight drivers, not produce a universal answer.

Case Study 1: Dairy with a Centrifuge Solid–Liquid Separation System

Baseline system:

A 1000-head dairy applies liquid manure to nearby cropland using conventional dragline systems. Hauling distances are moderate, but the farm isn’t making use of phosphorus at field levels have built up.

This farm produces 6.5 million gallons per year and at land application time we’ll have around 81,000 lb N and 70,000 lb P2O5. Manure application will cost about $0.013 per gallon for an annual expense of $86,000 a year. Based on the problem, only the nitrogen is providing fertilizer value, which at $0.54 a pound gives $43,000 in value. This system would be a net negative of around $42,500 per year for the farm.

After separation:

A centrifuge system separates manure into a liquid fraction and a solid fraction. The solid product is phosphorus rich, while nitrogen remains largely in the liquid. Liquids are applied to nearby acres similar to before, while solids are hauled farther to access additional land or sold/transferred off-farm. The total gallons hauled locally decline, but solids require new storage and handling facilities.

This farm still produces 6.5 million gallons per year but because of separation we end up with around 5.4 million gallons per year to land apply and at land application time we’ll have around 77,000 lb N and 24,000 lb P2O5. Manure application will cost about $0.013 per gallon for an annual expense of $73,000 a year. Based on the problem, only the nitrogen is providing fertilizer value, which at $0.54 a pound gives $41,000 in value. However, now we also have a solid manure product that can be hauled to fields where the P is needed. We’d generated about 4600 tons of solid manure that cost about $6.20 a ton to apply for a cost of $29,000 per year, but given its N and P it supplies fertilizer value of $23,000.  This system would be a net negative of around $38,000 for the farm, or save around $5000 per year.

Case Study 2: Dairy with a Livestock Water Recycling System

Baseline system:

A 1000-head dairy just like in the first farm, so we don’t’ need to redo any numbers.

After treatment:

In this case, let’s assume the solid-liquid separation works just like it did before, but the liquid stream is now going to get processed through a membrane system that takes the nitrogen and concentrates it up to 100 lb N/1000 gallons and the remaining liquid water is of discharge quality.

This farm still produces 6.5 million gallons per year but because of separation we end up with around 5.4 million gallons that get run through the membrane system. After treatment in the membrane, we are down to around 1.6 million gallons to land apply and at land application time we’ll have around 157,000 lb N (it is higher because I assumed I’d do something to reduce N loss during storage and it will be 100% available) and 24,000 lb P2O5. Manure application will cost about $0.026 per gallon for an annual expense of $41,000 a year. Based on the problem, only the nitrogen is providing fertilizer value, which at $0.54 a pound gives $84,000 in value. However, now we also have a solid manure product that can be hauled to fields where the P is needed. We’d generated about 4600 tons of solid manure that cost about $6.20 a ton to apply for a cost of $29,000 per year, but given its N and P it supplies fertilizer value of $23,000.  This system would be a net positive of around $40,000 for the farm, or save around $80000 per year. Of course, this would need to pay capital expenses on the equipment, labor and operation expenses to run it, and handle doing something to reduce losses of N from the stored fertilizer product.

Case Study 3: Dairy Farm with a Sedron Technologies Manure Drying System

Baseline system:

A 1000-head dairy just like in the first farm, so we don’t’ need to redo any numbers.

After treatment:

This is a drying system, so rather than wet solids, they’ll be dry. Also, they are doing a different treatment system that should get the nitrogen fraction up to around 7% N content. I’m not going to cover the details this time, hopefully in the future.

This farm still produces 6.5 million gallons that get run through the drying system. After the drying system we’ll be down to about 0.25 million gallons of liquid N fertilizer to land apply and at land application time we’ll have around 157,000 lb N (it is higher because I assumed I’d do something to reduce N loss during storage and it will be 100% available). Liquid fertilizer application will cost about $0.041 per gallon for an annual expense of $11,000 a year. The nitrogen is providing fertilizer value, which at $0.54 a pound gives $84,000 in value. We also have our dried fertilizer product. We’d generated about 2300 tons of solid manure that cost about $7.70 a ton to apply for a cost of $18,000 per year, but given its N and P it supplies fertilizer value of $29,000.  This system would be a net positive of around $80,000 for the farm, or save around $120,000 in manure handling expenses and created value.

The Big Picture

These examples are far from complete, but they do illustrate why we continue to look towards innovative manure treatment systems. Because the dream of turning manure to gold is out there, and this analysis illustrates if we can make the systems cost effective and work there is value to be had. However, doing so will make sure we bring system costs down towards what we are gaining in fertilizer value and potential land application expenses, because while $120,000 sounds like a lot of money if I want this technology to pay back, it means I’ll need to spend less than $1 million dollars on it in start up expenses.

Picking the Right Swath Width: The Overlooked Key to Uniform Dry Manure Application

Load cells, GPS, and rate controllers have changed the game in a good way for solid manure application, there’s still one piece of the puzzle they don’t solve. They don’t tell you how wide your spread pattern 

We treat swath width as if it’s a fixed property of the spreader. But it isn’t. It’s a property of the material, the day, the setup, and the physics of throwing irregular particles into a crosswind. That’s why the single simplest thing you can do to improve manure uniformity, is to pick the right width for the material you’re spreading right now, not what you spread last year.

The Pattern Isn’t a Rectangle, It’s a Curve

When you watch a spinner or beater from behind, it looks like it’s flinging material in a straight band. But if you take a dozen catch pans and run a pattern test, the truth appears fast:

• Every dry manure pattern is a curve; generally heavy in the center, tapering at the edges.

Sometimes that curve is broad and smooth. Other times it looks like a mountain peak with almost no “shoulders” at all. Cattle bedded pack tends to be clumpy. Turkey litter might be fluffy in one load and sticky the next. Layer manure can be powdery or soupy. And because the pattern is a curve, the effective swath width; the width where two adjacent passes overlap enough to even things out, is always narrower than how far the spreader can physically throw material.

 

Figure 1. Example spread patterns, both at the same application rate, but one (in blue) with a perfect, uniform spread pattern and a second (in orange) with a more common spread pattern.

A machine that throws 45 feet often has an effective width of 25–30 feet, but it also depends on the material. That’s why a single “standard width” is fiction.

Why the Same Spreader Behaves Differently Every Day

Think about what determines where a manure particle lands:

• its size

• its density

• its shape

• its moisture content

• the velocity and angle it leaves the spinner or beater

• the wind it encounters

We pretend manure is consistent because that’s convenient. The reality is that even within one pile, all those variables shift. Move to a new farm, where they manage bedding and ventilation differently, and all bets are off. Swath width is not only a machine setting; it’s a material property that changes over time.

You Can’t Outsmart a Bad Width with Technology

GPS lines will keep the tractor straight. Load cells will keep the average tons per acre accurate. Rate controllers will put it together keep the gate or chain feeding consistent amounts. All good things. But none of them can fix a spread pattern that is inherently too narrow or too uneven for the width you’re driving.

Technology can keep the average rate correct. Only the correct swath width keeps the distribution correct. It’s the difference between “I applied 4 tons per acre” and “I applied somewhere between 2 and 6 tons per acre in alternating stripes.” If you’ve ever flown a drone over a dry-manure field and seen stripes, swath width, i.e., application uniformity is a potential culprit.

The Real Question: How Do You Pick the Right Width?

Pattern testing is still the gold standard, followed by fancy math to make it as good as it can be, but here’s the part we often don’t say. You don’t need perfection; it’s manure. What you do need is a sense of the shape and to understand how you are using that manure as part of your fertility program. It’s providing all your nitrogen; you better be pretty uniform. Providing about 2/3 of your nitrogen you are probably hoping everything gets covered, but it doesn’t need to be perfect. Spreading for P and K, uniform enough every plant gets some of the goodness, but trusting your soil to buffer the rest of the nutrient supply.

A simple line of tarps (or catch pans gives you the curve). Once you see the curve, pick a width where the wings from one pass overlap strongly with the wings from the next. You’re trying to turn two curves into a flat line. That’s the entire game.

Looking back at our example in figure 1, if we are using the blue spreader our effective width is easy, 45 feet. If we move over 45 feet every pass, we’ll uniformly cover the field. But what if we are that orange spreader, how far do we move over? In figure 2, I provide the uniformity of the application with different effective swaths. As path width gets smaller, uniformity bets better, but in truth at a path width around 30 to 35 feet the application gets relatively uniform.

Figure 2. Estimated application uniformity when using different effective swath widths. Closer swath widths would require lower application rates with each pass. Despite this we assumed uniformity would be similar width even with lower application rates.

Why You Shouldn’t Use the Same Width for Poultry, Cattle, Compost, or Anything Else

Each material has its own physics:

• Poultry manure tends to be finer, spreads farther but has lighter wings.

• Cattle manure often spreads short and chunky, great center, weak edges.

• Composted materials can throw evenly until you hit a clod.

• If the spread pattern changes, the effective width changes.

The Bottom Line: Swath Width Is Your Cheapest Precision Tool

When we say manage for every plant, sometimes precision agriculture has told us to think about how to manage differently for every plant, but to a large degree the base of that still starts the same. Try to treat every inch of your field the same, then pick that precision equipment and precision fertilizer to manage to that plant’s specific needs. With manure, we aren’t that precision fertilizer, at least not yet, but I’m not sure there is anything better to provide that baseline fertility as long as we are managing it right.

Pigging Out: The Math Behind Blowing Out a Manure Line (AKA What Size Compressor do I Need?)

 When the last gallon of manure has left the pit, or it is getting close to time to switch fields, there’s still a little problem left behind: the line, filled with manure. Whether you’ve got a half-mile of dragline or three miles of mainline, it’s still full of manure. And if you just crack it open and walk away, you’ll quickly find out what a few thousand gallons of “leftover” looks like, and you won’t keep the gallons, and the cash register, flowing.

That’s why applicators finish by “pigging” the line. Pigging involves inserting a device, a foam ball (or bullet) in a launcher upstream of the pipeline and using air pressure to push it (and the manure ahead of it) to the outlet. It’s an efficient way to clear the hose. But behind that satisfying swoosh is a bit of serious fluid dynamics. So, let’s talk about the math behind pigging out your manure line.

The what size compressor do I need was also one of the common questions I saw on the Facebook page, “Manure Kings,” for a while, so I thought it would be fun to see what the math had to say.

How Much Manure Are We Dealing With?

Let’s start small, a 6-inch hose about a mile long. That line alone holds nearly 7,800 gallons of manure. If you move up to a 10-inch mainline stretching three miles, you’re talking about 65,000 gallons of material still sitting there after pumping stops. That’s a lot of fertilizer left to apply and properly place, so time to blow it out. But how much air and pressure does it take?

Doing the Math

There are two questions to answer: how much pressure do I need, and how much air do I need?

When you launch a pig and put air behind it, the pressure you need depends on three things:

·         Friction along the hose wall, and

·         How tight that foam ball fits (the pig seal friction)

·         The elevation change you need to push the manure

To estimate how much pressure you need, you can use the Darcy–Weisbach equation:

                ​

where:

𝑓 = friction factor (~0.02 for turbulent flow),

𝐿 = hose length in feet

𝐷 = hose diameter in feet

𝜌 = manure density ~63 lb/ft³

𝑣 = velocity of the manure that you want to maintain in ft/s (I’m going to aim for 5 ft/s)

Referring back to our examples, I calculate for the 6-inch line, I’d need 36 psi plus whatever the elevation head change requires, and for the 10-inch line, I’d need 65 psi plus whatever the elevation head change requires. If I have the pig moving at 5 ft/s it takes a little under 20 minutes to blow that mile of 6-inch line, but almost an hour to blow the 10-inch line.

Now, we’ll need a little extra pressure to overcome the pig’s drag, but I don’t have a great handle on estimating that. The good news is, as long as we have enough pressure, the manure will move—it’s just a question of how fast.

But how much compressor flow do I need?

Here’s where the flow rate comes in. The compressor doesn’t just need to hit that pressure; it must keep supplying air fast enough to maintain the pressure as the pig advances. If the compressor can’t keep up, the pressure drops, and the pig slows or stops if we end up with no pressure. Sure, it will start again, but we want to have a guess of how long it might take to blow out the line.

The air behind the pig is filling a growing volume of hose as the pig moves forward. The faster the pig moves, the faster the air volume expands, and the faster your compressor has to supply air.

If your 6-inch pig is moving at 5 feet per second, that’s about 60 CFM of hose volume per minute being created behind it. For the 10-inch line, that’s about 163 CFM of hose volume.

Let’s check this against a real-world example: Say I have a compressor with a rated capacity (not an endorsement, but one that had a rated value for me to use) of 185 CFM at 100 PSI. For the 10-inch line, this sounds pretty close, as I said I wanted 163 CFM, but I estimated I only needed 65 psi plus the drag of the pig and elevation change, so let’s call it 80 psi. Given those characteristics, the compressor should work and move everything a little faster than I estimated.

In other words:

·         Pressure gets the pig moving.

·         Flow rate controls how fast it moves.

·         Both determine how long the job takes

Practical Tips Before You Blow

Pigging with air can be deceptively dangerous. You’re storing a lot of energy behind that foam ball.

·         Segment long lines. Don’t try to clear three miles in one go; break it into manageable lengths with valves or disconnect points.

·         Start slow. Ramp up the air gradually while watching a pressure gauge.

·         Be safe, high pressures and air’s want to decompress rapidly make this one of the most dangerous parts of the job.

Cover Crops and Manure Management: Do They Change My Application Rates?

 Cover crops are popping up more these days, and for good reason. They’re great at grabbing leftover nitrogen and keeping it from wandering off, especially if you had to put manure on earlier than you’d like. But in some operations, we might start thinking of them as more than just a cover crop; they can also be chopped for silage to feed ruminants and become a production crop.

So, here’s the question I want to dig into today: Does using a cover crop, or growing one for forage, change my allowable manure application rates in my manure management plan?

The short answer is sometimes. The long answer is it depends:

·         Is your manure plan limited by phosphorus or nitrogen?

·         Are you harvesting that cover crop, or just using it as a cover and terminating it?

If You’re Phosphorus Limited

Here’s where cover crops can really shine. Because they hold the soil in place, they lower erosion estimates in RUSLE2. That can improve your Phosphorus Index score. Sometimes, it may bump you from “P-limited” to “N-limited,” which opens the door for higher manure rates.

And if you’re harvesting that cover crop, say as rye silage, you’re pulling more phosphorus out of the field. More removal = more room in the plan for phosphorus application.

If You’re Nitrogen Limited

If you’re N-limited, the story is a little different.

·         No harvest? Nothing’s leaving the field, so your N removal hasn’t changed. That means your maximum manure N rate stays the same.

·         Harvest the cover crop? Now it counts as a crop, and you can add its removal to your N budget.

What Does Rye Silage Remove?

Cereal rye uptake depends on growth stage at harvest, but a good ballpark for removal is:

·         40-60 lb N/ton dry matter depending on harvest stage

·         15 lb P₂O₅/ton dry matter

·         75 lb K₂O/ton matter

So even a couple tons (2-3 tons/dry matter) of rye silage can swing the numbers in your nutrient plan.

An Example in Action

To put some numbers on this, I ran a simple comparison using RUSLE2, the phosphorus index, and N and P-limited application rates. I assumed a liquid dairy manure testing 25 lb N/1,000 gallons (70% available) and 10 lb P₂O₅/1,000 gallons. I kept corn silage yield at 30 tons/acre (65% moisture) and assumed 2 tons/acre dry matter rye silage when harvested.

I compared three rotations:

·         Corn silage only

·         Corn silage with cereal rye as a cover crop

·         Corn silage with cereal rye harvested as silage

Quick note: these numbers are for illustration only. The specifics at your farm could change the outcomes.

Table 1. Effect of rotation on erosion, Phosphorus Index, and allowable manure application rates.

Rotation	Erosion (tons/acre)	P-Index	Allowable Application Rate (gallon/acre)
Corn Silage	2.7	2.26	12,857
Corn Silage - Rye Cover Crop	2.2	1.97	12,857
Corn Silage - Rye Silage	5.1	3.65	19,714

 

What’s the Take-Home?

Adding rye as a cover crop reduced erosion and dropped the P-Index below 2.0, but it didn’t change how much manure I was allowed to apply in this case. Harvesting rye for silage, though, added nutrient removal to the system. That extra removal let me use more manure, even though erosion ticked from extra field activities.

So, the bottom line is that cover crops helped in phosphorus-limited systems, and if you harvest them, they also change the nitrogen and phosphorus removal math. That’s when your allowable rates in the plan start to shift.

Why Earlier Fall Manure Applications Are More Prone to Nitrogen Loss

When it comes to fall manure application, timing matters. Applying manure too early in the fall creates more risk of nitrogen (N) loss before your crop can use it. Two main factors drive this: temperature and time.

The nitrogen in manure is primarily in two forms: organic N and ammonium N. Both forms are relatively stable when first applied and tend to stay in place if incorporated into the soil. But once in the soil, microbes start working. Under aerobic conditions, microbes convert ammonium into nitrate—a form of nitrogen that is both mobile in the soil and vulnerable to loss through leaching or denitrification.

If manure is applied earlier in the fall, microbes have:

·         Warmer soils, which speed up the conversion of ammonium to nitrate, and

·         More time before crop uptake, which means more opportunity for nitrate to be lost.

Combine those two with fall and spring precipitation moving through the soil, and you’ve created a recipe for nitrogen loss.

Tracking Microbial Activity with μGDD

To make sense of this, researchers have developed a microbial growing degree day (μGDD) index, a simple way to compare how much microbial activity (and therefore nitrogen conversion) might occur depending on when you apply.

Microbial activity roughly doubles with every 18°F increase in soil temperature but drops to zero when soils freeze. Using that relationship, we can calculate μGDD to compare the relative risk of nitrogen conversion across application dates and locations.

The formula looks like this:

Where:

·         T_t is the daily average temperature of the soil

·         T_ref is a reference temperature, which I set at 32°F

·         Q₁₀ is 2, which is essentially saying activity doubles every 18°F temperature increase

This doesn’t tell us how many pounds of N are lost, that takes water movement, and it doesn’t even tell us how much of the nitrogen is converted into nitrate, but it does let us compare the relative risk of N loss at different application dates and across geographic locations.

What the Results Show

When we apply this μGDD approach, a clear pattern emerges:

Figure 1. Relative microbial activity as a function of manure application date. Delaying application until November significantly reduces the potential exposure to microbial degree days and the opportunity for conversion of nitrogen forms to nitrate

Earlier fall applications carry more microbial activity (and therefore risk). Well-timed applications in the fall (approximately November 1 are about double the risk of nitrification as compared to a spring application, but it is really the months of September and October where we start to see the relative risk of nitrification increasing rapidly. Risk does not equal loss, but it does show the opportunity for loss with weather patterns that don’t cooperate and cause wet soil conditions, water movement, and drainage.

The second point I want to make is there isn’t a one-size-fits all recommendation for Iowa. Northern Iowa behaves like “late” southern Iowa. In other words, applying in early October in northern Iowa may carry about the same microbial risk as applying 10–14 days later in southern Iowa. The map shown represents the relative range in microbial degree days a site in southern Iowa would have experienced relative to northern Iowa if they both applied at the same time.

Figure 2. Relative microbial activity as a function of location if applied on November 15^th. Other application dates looked similar. To obtain microbial activity risks similar to northern Iowa, southern Iowa must apply approximately two weeks later.

Putting It in Context

This finding reinforces what we’ve discussed in other articles:

·         Wait until soils are cool (below 50°F) before applying fall manure to slow microbial conversion.

·         Use nitrification inhibitors if you need to apply earlier; they can buy some time, though they won’t prevent all conversion.

·         Consider cover crops to capture nitrate if early application is unavoidable.

At the end of the day, earlier fall applications mean more time for microbes to work and more chance for N loss. The μGDD framework gives us a way to quantify that relative risk and better understand why timing matters. It isn’t the only factor, but it can help identify why the risk changes.

Pre-Sample or Past Average? Choosing the Right Info to Set Your Manure Rate

 When it comes to setting your manure application rate, the golden rule hasn’t changed: test your manure. Every year. But when you’re planning your fall application in August and September, you might find yourself staring at two different numbers: a sample you just pulled from your storage before agitation, and the average of those carefully collected samples from the last few application seasons.

Which one should you trust more?

Let’s talk about what those numbers are really telling you.

The Pre-Sample: A Snapshot with Caveats

A pre-sample pulled late-summer or early fall, from a pit or lagoon that hasn’t been agitated yet, is tempting. It’s fresh, it’s this year, and it feels like it should be the most relevant. But unagitated storages stratify. Solids settle. Nutrients settle with them. That means what you sample near the surface in September might look different than what you actually apply on your field in October when the storages are agitated. How different the manure tests is a function of the manure we are working with, the nutrient we are most interested in, and the distribution of those nutrients between the solid and liquid fraction.

Nitrogen (N)

You might think nitrogen is easy to predict, but it's influenced by:

·         Time in storage (volatilization)

·         Diet shifts (especially protein levels)

·         Storage conditions (temperature, dilution from wash water or rain)

·         In most liquid manures ammonium nitrogen (a dissolved and water-soluble form) makes up about 50-75% of the total nitrogen, the other nitrogen (organic) is attached to solids. Sampling from liquid swine manure where it is 75% of the nitrogen is in the ammonium form, your pre-sample is probably reasonable (within 10% for deep pit storages from swine finishing operations). For dairy manure storages, you might notice a bigger difference of more like 30% in total nitrogen content when agitated.

Phosphorus (P₂O₅)

·         Phosphorus also has a dissolved and particulate bound form, however, in most manure storages greater percentages of phosphorus are particulate bound.

·         A sample drawn before full agitation may substantially underestimate P

·         Settling during pumping may cause uneven P distribution in fields

·         Agitated dairy manures will often test 50% higher than unagitated dairy manure for phosphorus content.

Potassium (K₂O)

·         Potassium is dissolved and mobile in slurry

·         Levels are still affected by dilution or bedding.

·         Generally, pre-samples are within 10% of samples from agitated manure storages

The Running Average: A Stable Forecast

In contrast, a running average of samples collected during past application events, when the manure was agitated and representative of what was applied, gives you a more stable estimate of what’s likely to come out of the tank this year, assuming nothing major has changed in your operation.

This kind of average smooths out year-to-year quirks and captures the manure you actually applied, not just what was floating on top in September. It reflects your real-world nutrient delivery, but doesn’t help you know for certain what was applied this year until after the application was done. If you have an out of barn manure storages, and rainfall amounts differ from year to year, you had a water leak in your barn this year, or made a management change to your manure handling on a change to the diet composition of your livestock the average may no longer be representative of what is in your storage this year.

Which Should You Use? Pre-sample or Running Average

We looked at data from six swine farms (finishing and gestation farrowing farms) with data series ranging from two to six years, with multiple samples (2 to 12) collected each year throughout land application. All sites used in-barn manure storages. Across all sites, phosphorus was the nutrient most likely to be misestimated using a single sample in any year, due to how tightly P is linked to manure solids. If solids settle or are not evenly agitated, a sample might not reflect the full picture.

In contrast, nitrogen and potassium, which are more often in dissolved forms, were similarly estimated whether using a single sample or a prior-year average, though no advantage was found to using a single sample from the current year as compared to running average nitrogen content to estimate the manures nitrogen content.

In work I’ve done at Nashua we routinely take pre-application samples from an unagitated manure storage and compared a single pre-sample to the manure results at the time of application. At this farm, we routinely test 20% higher for total nitrogen content at pre-sampling compared to what is obtained at the time of application. However, at this farm our pre-sample is generally collected before manure application occurs from the farm, while our as applied sample generally comes from manure applied after commercial manure applicators have been to the facility and emptied the pit. Steve Hoff suggested ammonia emissions during manure agitation were 4.5x higher during agitation than prior to agitation. While generally ammonia emissions are low from a deep pit barn (about 26 lb/day from a 1200 head barn); however, that means on the day of agitation this is 117 lb NH3 and it stays elevated for a short period (we’ll assume a week) after at about 40 lb N/day. These elevated emissions resulting from the agitation would result in about a 10% change, so not the 20% we saw, but similar in magnitude and a unique situation how we are operating at this farm.

Where does this leave us?

If you are trying to apply all your nitrogen with the manure, a pre-sample becomes a must. You need an estimate to set your rate. However, what I’ve started to do is just the pre-sample and compare to samples tanking during application and adjust accordingly. So, when my Nashua Iowa pre-sample comes back at 70 lb N/1000 gallons and I’ve historically, and consistently, seen 20% lower at the time of application, I adjust to 56 lb N/1000 gallons and roll with it.

It also means that if you're making decisions about application rates in advance of agitation, using your running average makes sense. It’s your best estimate of what you’re likely to apply, and it avoids the pitfall of making decisions based on unrepresentative samples, and as long you know the barns management is similar to previous years, it makes sense.

That doesn’t mean a pre-sample is useless, far from it. If you’ve changed diets, added water, or seen other operational changes, or are getting manure from a barn you for which you don’t know how this year’s management compared to previous, pulling a pre-sample can be a valuable early signal. Just use it as a flag to adjust expectations, not as the final say.

With that said, often times manure isn’t our only form of nitrogen. If this is the case and we are applying to be short on nitrogen, sampling during manure application and using those samples to know how much is applied is the best of both words. As long as we don’t exceed the amount of nitrogen, we want it informs us of how to adjust our commercial nitrogen application that will happen later.

How Many Samples Should You Collect?

While this question sounds vastly different than the one we asked earlier about collecting a manure sample, in many respects it is the same style of question. Again, it is about the value of information gained, in this case from every additional manure sample. The place to start is by understanding how variable samples are, but in this case, not from farm-to-farm but within a manure application event at a single farm.

Similar to what we saw earlier, the variation in manure samples is proportional to the average concentration of the manure, with higher sample concentrations having more variation. Generally, at the coefficient of variation I typically see for manures we additional samples to help hone into the correct amount of nitrogen supplied was worth around $3 an acre, however, even at this price a manure samples every 40-acres would pay for itself. To put this in perspective, this is approximately every 100,000 gallons of manure or three or four manure samples from a 1200-head barn. All this to say, we could be collecting more manure samples than we are in most cases to better understand variation while we are applying. Information is power, and this is a case were accessing that information will help us better understand variation or if there is a trend, like increasing nitrogen as we move to the bottom of the manure storage. Figure 1 and table 1 show the trends in variation of nutrient content while emptying a single storage and estimate the value additional samples offer.

Figure 1: Variation, as denoted by standard deviation, during a manure removal event as compared to the average nutrient concentration of the manure.

Table 1: Estimated value from additional manure samples that help you fine tune your manure application rate. While not as valuable these results still suggest that collecting additional samples provides enough benefit to get one average every 40 acres covered.

Tuning Up Your Manure Storage: Mid-Summer Maintenance That Pays Off

While summer is flying by, now is the time to tune up your storage, not just your equipment.

Waiting until September to check your pit, pond, or tank can leave you scrambling. A quick mid-summer storage review can help avoid headaches later and get you ahead of both environmental and logistic risks.

1. Review How Full You Are

Like most years, rainfall in Iowa has been variable throughout the state, but many of our livestock producing areas have seen substantially more rainfall than average, with some pushing 8-inches of rainfall above normal through this point in the year. While a few portions of Iowa had to deal with abnormally high rainfall additions to outdoor manure, the dry summer and fall helped alleviate some of the stress of full storages come application season. While it is too soon to know what the rest of this summer and fall will bring, reviewing how full your manure storage is and assessing to your storage needs to make it to manure application season this fall, it is critical to ensure storage success. Figure 1 provides a map of Iowa rainfall through July 18th as compared to normal, indicating that some locations are trending about 8.5 inches more than normal since January.

To help put in perspective, what this amount of water of rain means, if you had a 150-foot diameter circular manure storage (the same as the ISU dairy) and received 8.5 inches of direct rainfall more than normal it would add 93,600 gallons more water than normal to the manure. This is approximately equivalent to the manure produced over the year by 13 dairy cows.

Figure 1. Comparison of January 1 to July 18th precipitation as compared to normal, indicating that some areas of Iowa have seen much higher rainfall than average through this period of the year.

2. Project Your Fall Application Window

If you are in a situation where your manure storage volume might be tight, start communicating with your custom applicator or cooperating landowners now. If you’re looking at needing an early application, it’s also time to review nitrogen stabilization strategies or soil nitrate retention tools. Cover crops are especially effective at capturing early applied nitrogen.

3. Inspect Safety Features & Clean Up Around the Storage

Manure often isn’t the first thing on our mind as we are busy with spring field work and then again with fall harvest, but with summer hopefully it gives some time to think about maintenance of the manure storage. You see it every day, but have you really looked deeply at it to see how it is working?

• Check fences for wear and areas that need repair.
•  Look over or add signs around access points to ensure safety around open storages or pits.
•  If you have a push ramp, make sure it is in good repair and will stop you from rolling into the pit as you are pushing in manure.
• Check over agitation and pump-out ports so they secured and in good repair so when it comes time to move them in the fall you can quickly get them out of the way.
•  Clear weeds and brush to improve visibility and reduce pest risk. Check for signs of erosion, cracking, or damage around embankments or pit walls.
• Evaluate roads and paths to the manure storage to ensure equipment can access critical areas and movement of mud to roadways will be minimized
• Review your emergency response plan; make sure items are up to date and you are prepared for pumping season with critical contact numbers.

Figure 2. Clearing away brush and debris and keeping the area around the manure tank mowed allows easier assessment of storage conditions and risks.

Bottom Line:

Just like a planter check in February saves stress in April, a manure storage tune-up now pays off in smoother, safer application this fall. You’ll avoid overflow risks, reduce emergency pumping costs, and give yourself time to plan smarter.