As you may recall, last week we talked about a bit about
manure in relation to precision farming. We said there are really three aspects
to precision farming, measuring something, making a decision based on that
thing we measured, and then implementing our decision. Today we are going to
look at these concepts in terms of manure management, specifically in terms of
manure sampling and testing.

To start with we need to understand a little bit about
manure. One of the most common questions asked about manure “what is the
fertilizer value of manure.” This question usually means, “How much nitrogen is
in the manure?” or stated a slightly different way, “how much manure should I
apply to fertilize my crop?” This question doesn’t have an easy answer, because
unlike commercial fertilizer (anhydrous ammonia, MAP, DAP, or urea) we might
usually purchase, manure doesn’t come with a guaranteed composition. Instead,
there can be substantial variation in nutrient content from one-farm to the
next, year-to-year on the same farm, and to a lesser extent the manure that we
are pumping out during a single land application event. This variation makes “book
values,” i.e., estimated nutrient contents available in Midwest Plan Services
or state extension materials a good starting point for planning purposes, but
means that we really need to measure the nutrient content of our own manure to
understand what is in it.

One way to think about this is based on probability. This is
demonstrated for a swine manure from a deep-pit manure storage in figure 1
below. What you see is that the manure has the greatest probability, or the
highest chance of having at a nitrogen content of 7 kg/1000 L (or right around
58 lbs. of Nitrogen per 1000 gallons of manure). However, what’s harder to tell
is that there is something like a 20% chance that swine manure might have more
than 9 kg N/L (75 lbs./1000 gallons) or a 20% chance that swine manure will have
less than 5 kg N/L (41 lbs./1000 gallons). This means if we were basing our
manure application off “typical” or average swine manure and trying to applying
150 lbs. N/acre, there is a 20% chance we apply more than 190 lbs. N/acre and a
20% chance we apply less than 110 lbs. of N per acre. So what does this mean to
us? Well, it implies there is a pretty good chance we won’t be applying the
amount of nitrogen we think we are. This is costing us lost revenue from either
not fully utilizing our N (if we end up applying more nitrogen than we need) or
from nitrogen limitation in our crop growth (if we are applying less N than we
think we are).

Figure 1. Probability or a swine manure from a deep-pit manure
storage having different nitrogen contents.

Based on this I think we have an interesting question we can
ask ourselves, and that is, what is the return on investment of manure sampling.
That is, how much value does manure sampling add to our operation? In
determining the value of the manure test, it is important to understand how a
farmer can use the information gained from the test results, i.e., how having
this information alters the farmer’s nutrient management and affects the farm
profit. This is a complex topic, as almost limitless possibilities exist, but
to get a general idea I’m going to make a few assumptions and approximation
about common cropping and manure management systems in the Midwestern United
States. This may mean the results don’t give you an exact value for your
operation, but they give you a pretty good starting point.

In this evaluation, I assumed that the manure application
method would be either injection or immediate incorporation to maximize N
utilization. Additionally, I assumed that best management practices for manure
application timing were followed; as a result, the yield response to available
N (defined here as the sum of ammonia N and organic N expected to mineralize in
the first growing season) would be the same as the yield response to mineral N
fertilizer. Finally, we limited crop rotation choices to continuous corn and
corn-soybean rotations, as these represent the dominant rotations in the upper
Midwestern U.S. The impacts on the value of the manure test of N-limited or P-limited
application, as well as when sampling or testing was conducted, were handled by
evaluating all cases. Finally, the basis of this effort was that farms intend
to use their manure resources to support crop production. In cases where
farmers have insufficient land to use all their manure resources, they can only
extract the value of the manure test if they can find buyers for the manure
nutrients.

The value of the manure test was calculated by estimating
the profit that would have been made if the manure was assumed to have a “typical”
nutrient composition and then to compare this to the profit generated if the
actual nutrient composition was known. The profit graphs for these steps are
shown in figures 2, 3, and 4. As a note, I performed this analysis earlier this
spring when corn price was $5 per bushel.

Figure 2. Probability of different profits due to different
nitrogen contents of manure assuming manure standard rates.

Figure 3. Expected profit from applying manure of a known
composition at the maximum return to nitrogen (rate changed to account for
measured nutrient content).

Figure 4. Expected value of the manure test, based on graph
3 minus graph 2 (adjusted based for sampled manure nutrient content case minus
assumed nutrient content case).

To put these results in perspective I used the model to
evaluate what manure sampling would be worth for a 1000-head capacity swine
finishing farm using a deep-pit manure storage. On average, the facility
generated 4 L of manure per head per day. This farm has collected and tested
manure samples every year for the last five years. The first four years of
manure sample values were 0.84%, 0.72%, 0.98%, and 0.62% N, with an average and
standard deviation of 0.79% ±0.16% N. The N content for the current year was 0.92% N.
If no sample was tested, this operation assumed that the manure had an
available N content of 0.79%, the average of the previously collected samples.
Using pre-application sampling and assuming that manure application was N
limited, the value of the manure test would be $30.96 ha

^{-1}. Assuming that the manure sample is representative of all the manure from this building, the overall value of the sample was $1,759 (the farm would have applied manure to 56.8 ha). This represents a good return on investment, as the approximate cost of obtaining this information would be $50 for manure testing, $50 for shipping the manure to the testing lab, and $100 for the farmer’s time to collect, label, and ship the sample, giving a return of almost 9:1. If manure application was P limited and manure was sampled during application, the estimated value would be $14.20 ha^{-1}. In this case, the manure was applied to 112 ha, so the actual value of the test would be $1,589
The same analysis was performed for other manure types and
estimated values of manure testing are shown in table 1.Overall, the results
indicated that sampling and testing the manure provided enough economic value
that it would pay for itself.

Table 1. Estimated value of the manure test for different
manure type and crop rotations.