Manure management plans are a tool used by both the farm to
make sure they are getting the most from their manure and by society to ensure
manures are managed in a way that is appropriate and only allows acceptable
risk to environmental quality. I think we can all agree these are good things
we want to occur but like most things, the devil is in the details. How do we,
as a society and individual farmers, work to select the right nutrient
application rates to balance both fertility decisions and impacts on yields and
economics, with the potential environmental consequence?
In most manure plans, the yield goal method is used to
define the maximum allowable nitrogen application rate. This approach is based
on a mass balance approach where we are trying to match nitrogen application
rates to removal and loss rates. In this sort of system, there is a fair amount
of uncertainty in understanding where nitrogen ends up and how it moves. Changes
in corn genetics have impacted how the existing factors in this equation may
interact. Below I’ve constructed a partial nitrogen budget (estimate N
application using the yield goal method minus the amount removed in grain) with
two different grain nitrogen content. The old budget uses the 0.8 lb N/bu of
grain which is what is listed on the USDA crop nutrient removal tool, while the
newer partial budget uses 0.6 lb/N bu of grain which is just a bit higher than
what newer research on corn nitrogen suggests is occurring. I’ve also marked
two vertical lines; the gray line represents Iowa’s corn yield in 1995 (state
average) while the black line represents Iowa’s corn yield in 2018.
What should we be taking away from this information? It
isn’t that the yield goal method is archaic, but rather, as corn genetics have
changed we need to be thoughtful about how it impacts the nitrogen use
coefficients listed for different crops within the document. It is possible, at
some point, these will need to be updated and deciding when, how, and what is
the right factor is critical to providing a realistic yield estimate. While
there is a lot to the nitrogen cycle and it can get a bit confusing, there are
two things to note in this figure. The first is the two lines diverge from each
other at yields get higher and the second is over the last 25 years the yields
have greatly increased.
So what does this mean at the 1995 yield level? The old
partial N budget application approximately matched the removal in corn grain.
If we look to current yields, even at the old corn N removal rate, we were
putting on around 25 lb N/acre more nitrogen than would be removed in the
grain, but as grain nitrogen content has come down, this amounts to about 40 lb
N/acre.
So what does this really mean? While the yield goal method
is based on a mass balance approach for nitrogen, many of the factors in mass
balance are hard to predict. Thinking about how our agricultural systems have
changed over time and what this means for rate selection, is critical for
making an informed decision. Moreover, this is one of the reasons Iowa State
has switched to recommending the maximum return to nitrogen approach for rate
selection.
Figure 1. Partial nitrogen budgets (input – grain N
removal) for old (corn with 0.8 lb N/bu) and new (corn with 0.6 lb N/bu)
estimates. The vertical gray line represents corn yield in 1995, while the
vertical black line represents corn yield in 2018. Representation for a corn
soybean rotation with a soybean credit (rotation effect) of 50 lb N/acre)
