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.
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.
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