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.