Tuesday, March 19, 2019

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

As you may be aware, Iowa State University has recommended MRTN for determining nitrogen needs for corn for a while now. This methodology uses data from numerous field trials to understand how corn responds to nitrogen in both continuous corn and corn-soybean rotations, as well as the price of both corn and nitrogen to determine what nitrogen application rate will, on average, provide the maximum profit per acre. This is the amount of nitrogen that results in just enough yield benefit to pay for itself in the extra yield it produces.
To get an idea of how this recommendation would have fluctuated with time, I looked retrospectively back at the average annual corn price for every year since 2005, along with the average price of anhydrous ammonia to ascertain nitrogen price. This was done for both corn and soybean rotations. The results did show some fluctuation, but in general, for continuous corn rotations, the recommendation was 190 lb N/acre with 140 lb N/acre recommended in a corn-soybean rotation, with a variation of about 5% in this recommendation based on specific crop and fertilizer prices. Another thing to note, there was roughly a 50 lb N/acre difference between the optimum N application in the continuous corn and corn-soybean rotation. While you might think of this due to a soybean credit, we generally call it a rotation effect.
The yield goal method, which is in the in the Iowa Manure Management Plan forms, uses a mass balance approach to estimate how much nitrogen is needed. In the yield goal method, we use the average of the previous five-year’s corn yields plus 10%. This is then multiplied by a factor, 1.2 lb N/bu corn for most of Iowa, to determine nitrogen need. If in a corn-soybean rotation, a soybean credit is also required which is suggested to be 1 lb N/acre per bu soybean/acre up to 50 lb N/acre. If we look at the N-recommendations over the same time frame, we see something interesting. The yield goal method suggested approximately 144 lb N/acre in a corn-soybean rotation and 188 lb N/acre in a continuous corn rotation, but the variation in the recommendation was higher at 20%. More importantly, while the MRTN methodology has remained relatively consistent, with perhaps slightly lower levels starting in 2000 as nitrogen prices increased, the yield goal method has shown the opposite trend, increasing consistently by about 2.5 lb N/acre-yr over this data set. This doesn’t come as a big surprise, yields have consistently shown an increase over this time phase, but what it is slightly more concerning, is that most data today shows optimum N application rate isn’t actually related to yield as suggested in the yield goal method.

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

However, let’s look at and explore this another way. You may or may not be aware, but the amount of nitrogen in a bushel of corn has dropped substantially since the yield goal was first developed. In the late 80’s and early 90’s, it was generally accepted that corn had about 0.8 lb N/bushel (based on the USDA crop nutrient removal tool database) while now it has a bit under 0.6 lb N/bushel, at least based on the best data I seem to be able to find. You may wonder how this could happen – and it really comes down to what we use corn for and what we breed it to do. We want it for the starch or energy, both in animal diets and in making ethanol, so one of the things we’ve seen is larger kernels but with the same size germ, so more starch for the same amount of nitrogen. But the more important part is what does this mean to our nitrogen budgets when using the yield goal method?
Let’s take an example of 200-bushel corn (average of last three years in Iowa), 58-bushel soybean and compare our N budgets for when corn removed 0.8 lb N/bu (old removal estimate) and 0.6 lb N/bu (newer removal estimate) using both the yield goal and the MRTN methods. There are a few things to note; most notable, the yield goal method under high and low N content corn suggests N losses ranging from 30 to 70 pounds, which are in the range typically seen for Iowa soils. The MRTN numbers are substantially tighter budgets with allowable losses of -10 to 30 lb N/acre. This may slightly underestimate nitrogen leaching to put us in the approximate range. This suggests the expected nitrogen efficiency in production with the yield goal method was around 84% which is very similar to where the MRTN prediction of 80% now resides.
Table 1. Partial nitrogen budgets for high and low N content corn using both the yield goal and MRTN methods in corn-soybean rotations.

Yield Goal

0.8 lb N/bu
0.6 lb N/bu
0.8 lb N/bu
0.6 lb N/bu
N applied (lb N/acre)
Estimated N removed with grain (lb N/acre)

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

Tuesday, January 29, 2019

Manure and Nutrient Budgets – How I see it

Recently, an article was published in The Gazette about a new publication on nutrient budgets in Iowa. The article discussed  work published in the journal, Ambio, entitled, “Livestock manure driving stream nitrate.” The paper looked at watersheds in western Iowa and compared the flow weighted nitrate concentration against several watershed parameters including a “nitrogen surplus.” This was a rough nitrogen budget comparing nitrogen additions and removals, various sources of nitrogen application rate, and the percent of the crop land in corn and soybean acres.
While all showed some positive correlation with the flow weighted nitrogen application concentration in stream water, the highest correlations were found with the nitrogen surplus, manure nitrogen application, and then the area portion of the watershed in corn and soybean. Based on this, the authors proposed manure was a key contributor in driving the higher nitrogen losses. However, let’s delve deeper into some of the results and closely examine some of the results.
The first thing to note is a strong relationship exists between the fraction of a watershed alone that is row crop (corn and soybean) and the flow-weighted nitrate concentration. This factor alone explains 65% of the variation in nitrogen content. This is in line with field plot trials conducted at Iowa State University that have generally shown similar trends with nitrogen reductions coming from land use change towards perennial crops or the use of cover crops. A big part of why these practices have nothing to do with an annual nutrient balance, as suggested in this paper, but more temporal dynamics of when nutrients are released from soil organic matter. These cool weather plants uptake nitrogen at times when more traditional row crops (corn and soybean) aren’t actively growing and as a result, can reduce nitrogen losses to waters.
They also constructed a nitrogen surplus budget for the watersheds (comparison of nitrogen applications and removals) and found it did a slightly better job of describing the average flow-weighted nitrogen concentration in the watersheds, describing 80% of the variation, or 15% more than land use alone. One important thing to consider, the nitrogen surplus budget itself is strongly correlated with land use, so these two measures aren’t independent. This isn’t to say nitrogen application rate isn’t important, it is, but to say land use alone may be a bigger component than nitrogen application rate, i.e., the correlation coefficient alone isn’t able to suggest how much of that effect is from land use and how much is from the effect of nitrogen application rate. Despite this, the result is clear, the higher nitrogen application rates will tend to result in higher nitrogen leaching losses, especially when certain thresholds of exceedance are met.
One last thing I found very interesting about the manuscript was manure nitrogen in the watershed was a much stronger predictor than commercial fertilizer. This result needs to be used with some caution, as the correlation efficient can be impacted by the range in date and in the manure nitrogen application rates had a much higher range than the commercial fertilizer application rate. There are several interesting aspects that manure has a stronger relationship. The first of which, is this represents excreted manure and not necessarily available nitrogen applied to the land.
While in most cases we consider nitrogen application to be a single year fertilizer application, with some manure types offering several years of potential fertility benefits due to the organic fraction which can be slow release. Could the higher loss of nitrogen from the high manure watersheds be a result of improved soil health and mineralization of nitrogen from soil organic matter? Potentially, but without good research or understanding of how long-term manure applications (especially of high organic manures like solid cattle manure) impact soil health, it is hard to anticipate how nitrogen needs and losses from the soil would change with much certainty . Alternatively, the improved nitrogen concentration in streams with manures could be indicative of challenges of using manure as a fertilizer; things like timing of application, certainty of the fertility it provides, or even application uniformity can all be issues that make it harder to trust manure fertility and in the right weather conditions could increase losses.
The important point to consider is this doesn’t necessarily mean we have an application rate problem, it means we have to find better ways to focus on the challenges we have with manure and how to overcome them.

Wednesday, December 19, 2018

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?

Saturday, November 17, 2018

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.

Friday, October 19, 2018

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?

Friday, September 21, 2018

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.

Thursday, July 19, 2018

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 hf 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.