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
|
MRTN
|
|||
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)
|
190
|
190
|
150
|
150
|
Estimated N removed with grain (lb N/acre)
|
160
|
120
|
160
|
120
|
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.
