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.

Why Manure Sampling Is Still Your Most Valuable Tool

I spend a lot of time talking about managing manure as a resource, and while manure sampling has become very popular in Iowa, with more than 90% of farmers reporting they take at least one manure sample a year. But here’s the hard truth: no other management change gives you more immediate, practical value than a good manure sample. It’s not about checking a box. It’s about unlocking real dollars in nutrient value and reducing risk.

The Value of Information

There’s a concept economists use called the value of information. In simple terms, it means that better decisions are made when you have better data. If you’re applying manure without sampling, you’re basing application rates on assumptions, and those assumptions often cost you.

Let’s say your manure is actually 40 pounds of nitrogen per 1,000 gallons, but you guessed 32. If you’re applying 5,000 gallons per acre, that’s a 40-pound-per-acre miss. In this case, it’s 40 lb N/acre more than you wanted, and that means you wasted about $19 worth of N fertilizer value. In a paper we wrote a few years back, we walked through how just one manure sample per year, interpreted correctly, could return $8–28 per acre if applying at a nitrogen limited rate and $1-20 per acre when applying at a phosphorus limited rate, depending on nutrient prices and application rates. Now scale that across a 1,000-acre operation. Sampling starts to look like a no-brainer.

The Manure Database: Variability Is Real

You might think your barn is like your neighbor’s, or that one pit looks like another. But ManureDB, a Manure Composition Database says otherwise. Lately it feels like I’ve been looking at the database a lot to determine what an “average” or “typical” manure looks like and to determine if we can determine differences in nutrient content between different types of manure. However, here we want to do something different, we are more interested in how variable the manure samples are within a single system. This is because manure types that are consistent might be able to be estimated with a book value with reasonable accuracy, while samples that are more variable need to be sampled more frequently to ensure a representative sample.

This same concept can be continued to determine how often you should be manure sampling or how many samples you should collect during manure application. If samples are consistent, fewer samples are necessary, if nutrient concentrations vary substantially from the start of the manure application event to the end, either with a changing trend with time or with high variation, more samples are necessary. While this second question is useful, we will focus on the first, how valuable is sampling in general in this article, and then on the second question of variability of nutrient content during manure application a little later when we explore when to use a running average of samples or when samples from the current year are preferred.

How Variable is Manure Between Farms

Manure variability can be expressed in multiple ways, but the two most prominent are either the standard deviation or the coefficient of deviation. The coefficient of variation is the standard deviation divided by the average, while the standard deviation is a measure of the dispersion of the data relative to its mean. While both these concepts are useful, there is another concept called mean deviation that quantifies the average distance between each data point and the mean. For normal distributions, the mean deviation is about 80% of the standard deviation.  Figure 1 shows the relationship between the average nitrogen concentration of a manure type and the standard deviation of the nitrogen concentration in the manure type; effectively as average concentrations get bigger, so does the variation.

Figure 1. Relationship between the average N content and the standard deviation of that manure types nitrogen concentration.

For our purposes we will use the mean deviation (both positive and negative), which is 80% of the standard deviation, and the mean value averaged together to estimate the potential value of a manure sample. While this makes the math much quicker and easier, it isn’t a perfect answer as the value of the sample is a non-linear function due to the yield response being non-linear, but it gives a quick approximation. 

Table 1 provides an estimate of the value of knowing the true nutrient content of the manure, i.e., from sampling adequately to estimate the true mean of the manure being applied by different animal manure storage types, storage types, along with the coefficient of variation and the value of a manure test in the continuous corn and corn-soybean rotation. Manures that have higher coefficients of variation (essentially the standard deviation divided by the average) get more benefit from sampling because our guess about what would be in the manure if we didn't have a sample, is worse.

Table 1. Estimated value of knowing the nutrient content of the manure for different rotations (CC – Corn following corn, CS- Corn following soybean) and livestock/manure storage combinations.

Make It Count

When you know your manure’s nutrient content:

• You match rates to crop needs
• You reduce commercial fertilizer inputs
• You track nutrient balances
• You have proof of value for the manure user

Doing these is the difference between managing manure as a waste and managing it as a fertilizer. Now, more than ever, it is critical that those of us using manure that we document the value of manure and prove we are using it to the best of our ability.

Bottom Line:

The cost of guessing is higher than the cost of a test. A manure sample might be your most valuable tool this year, and every year. It pays for itself, protects your crops, and helps you manage manure smarter. The time and effort of getting a manure sample you can trust and using those results pays for itself in the first 2 to 5 acres.

Understanding Nitrogen Availability from Liquid Swine Manure: Why Ratios, Timing, and State Guidance Matter

 Liquid swine manure is one of the most nitrogen-rich manures used in Midwestern cropping systems, but not all nitrogen in manure is equally available to plants. Understanding how nitrogen behaves after application is critical to making the most of this resource — and to minimizing its environmental footprint. In this article, we’ll dig into why liquid swine manure tends to be highly plant-available, what makes Iowa’s and Minnesota’s availability recommendations different, and how those differences matter (or don’t) depending on your crop rotation and nutrient strategy.

Forms of Nitrogen in Liquid Swine Manure

Most of the nitrogen in liquid swine manure exists in two forms:

• Ammonium (NH₄⁺): This is readily plant-available and behaves similarly to commercial fertilizer nitrogen.
• Organic Nitrogen: Found in proteins, cells, and other organic residues — this N must mineralize to become plant-available.

One of the key indicators of nitrogen availability is the NH₄⁺: Total N ratio. In liquid swine manure, this ratio often ranges from 70–85% for typical manure storages, though exceptions can happen. High NH₄⁺:TN ratios signal a greater portion of nitrogen is immediately available for plant uptake, making it more predictable and efficient as a fertilizer source. Using information from ManureDB, Iowa liquid swine manures average 75% ammoniacal nitrogen while Minnesota averages 74%, so basically no difference.

The Role of C:N Ratio in Predicting Organic N Behavior

When it comes to the organic portion of nitrogen, carbon-to-nitrogen (C:N) ratio is critical. Organic nitrogen in manure with a low C:N ratio (<15:1) tends to mineralize, releasing nitrogen over time. High C:N ratios (>30:1), in contrast, promote immobilization, where soil microbes tie up nitrogen to digest the carbon.

Liquid swine manure tends to have low C:N ratios in its organic fraction — meaning it contributes more nitrogen than it "costs" microbes to break it down. This is another reason why liquid swine manure is considered a high-availability material.

Iowa vs. Minnesota: Availability Recommendations Differ in Meaning

A key point of confusion arises when comparing Iowa and Minnesota nitrogen availability estimates:

• Iowa calculates first-year availability of manure N as a function of mineralization only and has a second factor for volatilization losses. In other words, Iowa breaks these two processes apart, accounting for losses (like ammonia volatilization) separately from how much of the total nitrogen becomes plant-available. In so doing, the range that Iowa provides isn’t dependent on the application method selected.
• Minnesota, on the other hand, includes volatilization loss within its availability values. That means a Minnesota “availability” percentage is often lower, but also that it is a function of the application method used.

Current Iowa guidelines in PMR 1003, Using Manure Nutrients for Crop Production, suggest swine manures in Iowa are between 90-100% first year available. The second-year availability would then be the remaining fraction that wasn’t claimed in year 1, i.e., 0-10% of the N applied.

In Minnesota they have a publication “Manure Management in Minnesota” that is used for estimating manure nitrogen availability, and I’ve often heard it quoted that it says that Year 1 swine manure is 80% available. I wanted to look closer at their table and clear this up, they effectively have a few different things going on. They show a different availability with different application methods, such as surface broadcast with no incorporation, surface broadcast with incorporation in less than 12 hours, and injection (which they have two categories, one for sweep and one for knife). These categories were selected because they are the closes parallels between Iowa’s direct injection, immediate incorporation, and no incorporation application methods discussed in PMR 1003.

The first thing to notice, is that to make the availability of Minnesota’s publication similar to that of Iowa’s 90 to 100%, we should be summing what they say is available in year 1 and their losses, to get a number comparable to what we suggest, that is 85% as compared to 90-100%. To me, when comparing this, the bigger difference is how different we are on estimating volatilization losses (which their footnote also suggests includes dentification, presumably denitrification above what would have occurred with commercial fertilizers).

Table 1. Suggested year 1, year 2, and gaseous nitrogen losses (volatilization and denitrification) from liquid swine manure in Minnesota.

Table 2. Estimated year 1, year 2, and gaseous nitrogen losses (volatilization) from liquid swine manure in Iowa.

Bottom line: The 90-100% availability in Iowa is accounting differently than the 80% often cited for Minnesota, though they seem to apply 85% first year availability.

When Does This Matters

In a continuous corn rotations using manure every year, this difference makes almost no impact on what you’d apply, because year-to-year mineralization overlaps and the resulting N carryover.

Let’s take a look at this to see it illustrated. Say I have a swine manure that tests 60 lb N/1000 gallons. I’m going to inject and have minimal volatilization losses (let’s say 2%) and I want to put on 190 lb N per acre. I’m going to compare what rate I’d use if I was assuming 100% available based on Iowa guidance or 85% available based on Minnesota guidance.

Table 3. Estimated impact of using Iowa and Minnesota availability recommendations on manure application rates in continuous corn rotations receiving manure annually (estimates are based on the average median field and yield response curve for Story County, Iowa).

So, in this case, because of how the math works out on the losses, Minnesota’s availability would suggest a slight (10 gallon per acre) lower rate than the Iowa assumption of 100% available.

But in manure-to-soybean systems or when rotating manure use so that it isn’t applied every year, the results can be quite different. We’ll rework the problem above, but in this case say the target N demand is 146 lb N/acre (if you are curious about where the N demands I’m using come from, they are the Story County median rates for corn following corn and corn following soy). In this case, the N carry over to a corn crop, occurring in the 3rd year after manure application in both cases is about 20% higher than when using assuming 100% availability.

Table 4. Estimated impact of using Iowa and Minnesota availability recommendations on manure application rates in corn soybean rotations receiving manure (estimates are based on the average median field and yield response curve for Story County, Iowa).

This isn’t to say one number is right and one is wrong, but rather to illustrate when the assumption matters and the magnitude of impact it can have. In case you are wondering, the N-FACT suggested impact of only getting 85% availability when you assumed 100% is about 22 pounds of nitrogen, or 3.5 bu/acre on average (the year makes a difference). If you went the other way, and assumed 85% available and got 100% it would be worth that would have added 26 lb N/acre and resulted in about 1 more bushel of corn per acre.

A second version of this is how much environmental impact would this have. While that is a tough and nuanced question, I’m going to simplify and say the field when fertilized at 146 lb N/acre yields 200 bu per acre, so when we planned on 100% availability and only got 85% availability, we went from a “nitrogen use efficiency” of 0.73 lb N/bu to 0.63 lb N/bu. On the other hand, when we assumed 85% availability and got 100%, we went from 0.73 lb N/bu to 0.86 lb N/bu. While these calculations don’t tell us everything, we need to know to estimate losses, they give an estimate about the slack or potential for loss.

Timing and Temperature Impacts

Availability isn’t just about the manure it’s about what happens after application. Nitrogen mineralization depends on:

• C:N ratio (low = more mineralization)
• Soil moisture
• Microbial community activity
• Temperature (higher = faster microbial activity)

I wanted to comment only briefly on manure characteristics and spend most of our time on temperature effects, but the manure database suggests on average 75% of swine manure nitrogen is in the ammonium form for both Iowa and Minnesota farms. That is to get to the 85% available Minnesota is assuming 40% of the organic nitrogen is mineralizing during the first growing season, while Iowa is suggesting 60-100% of the organic nitrogen will mineralize during the 1st growing season.

With that said, let’s focus on a different characteristic temperature. Because microbial activity doubles for every 18°F increase in temperature, timing matters. A fall-applied manure on October 1 in southern Iowa will experience more microbial activity compared to a November 15 application in northern Iowa. As we move further north, to Minnesota, this difference grows even greater. 

We’ve developed a microbial growing degree day (μGDD) scale to help compare how much microbial activity, and thus potential manure organic N conversion, might occur depending on when you apply. This scale accounts for how microbial activity speeds up with warmer temperatures (doubling every 18°F) but shuts off below freezing. It doesn’t tell us what percent of the N becomes available, but gives us a method to estimate mineralization potential in different locations and different application timings and compare them to each other.

Where:

t is a day count with t=1 as the of manure application

Tt is the 4-inch soil temperature that day in degrees F

Tref is a reference temperature to benchmark activity against and for our purposes here set to 32ºF

It should be noted

that the absolute value of µGDD has no meaning and this term is only use as a measure of relative microbial activity compared to another µGDD.

Thankfully, Iowa has invested in a soil temperature monitoring network (4-inch depth) where you can get soil temperature data. Currently there are 26 stations in the network. We assumed a November 15th manure application and evaluate Microbial Degree Days though June 15th. From the northern part of the state, to the southeastern edge of Iowa, there was a 20% difference in microbial degree days (Figure 1). While I didn’t continue this analysis into Minnesota (there temperature network appears to be at a 6-inch depth) given the spatial change over distance, it appears that there would be about 10% less microbial degree days in southern Minnesota than northern Iowa. So, if you were wondering about why there is an assumed difference in nitrogen mineralization, this would be a big part of it.

I did want to look at this one more way, how would our microbial growth degree days vary as a function of the date the manure is applied. Not surprisingly, manure that is applied earlier is exposed to more microbial growth degree days (Figure 2). But what I wanted to point out is, if you applying in fall vs spring there was about a 40% difference in microbial activity the manure was exposed to, which is bigger than the spatial differences in Iowa at a single application date and that most of the estimates behind swine manure nitrogen availability are based on what is available assuming a spring manure application (because we want to know about availably and not losses of N related to application timing) .

 

Figure 1. Relative microbial activity as estimated based on microbial growth degree days from Northern Iowa to Southern Iowa for manure applied on November 15th until June 15th.

 

Figure 2. Relative microbial activity as estimated based on cumulative microbial growth degree days from the manure application date through June 15th.

Conclusion

Liquid swine manure is an excellent fertilizer, offering high N availability, especially as it has a high NH₄⁺:TN ratio and a low organic C:N ratio. Understanding how Iowa and Minnesota calculate nitrogen availability can help farmers and advisors use those numbers appropriately. Additionally, understanding regional differences in microbial activity (as proxied through 4-inch depth soil temperatures and microbial activity growing degree days) provides insight into why availability estimates may be lower in Minnesota than in Iowa.