I’ve previously talked about nitrogen rate selection, using
either the yield goal method or maximum return to nitrogen, and what that may
mean from both a production and nutrient use standpoint. This time we are going
to do something similar, but will look at a different aspect of it, the impact
of when the nitrogen gets applied and how that may impact where it ends up.
The nitrogen cycle is complex; there is a lot going on and
it is highly weather dependent – temperature, soil moisture, rainfall, and biology
of plants and microbes. So at best, this is an incomplete nitrogen budget as not
all sources of nitrogen are going to be accounted for as no measurement of soil
nitrogen mineralization was made. Similarly, all the places nitrogen could end
up aren’t measured, such as the amount of ammonia lost to volatilization, or
nitrogen that ends up as N2O or N2, or the amount
accumulated in soil organic matter. At best, this is a partial budget that
looks at the amount of nitrogen ending up in tile water and in the crop.
So we are going to take a look at four treatments: 1) Spring UAN (corn-soybean rotation, chisel
plow and field cultivate, N rate at 150 lb/acre to corn phase), 2) Early Fall
Manure (corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as
liquid swine manure in early to Mid-October), 3) Early Fall Manure with Cover
Crop (corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as
liquid swine manure in early to Mid-October), and 4) Late Fall Manure
(corn-soybean rotation, no till, N rate at 150 lb/acre to corn phase as liquid
swine manure in early to Mid-November).
As a first step, let’s take a look at average corn yield for
these treatments between 2016 through 2018. On this figure, the first thing
that stands out is the nitrogen application timing played a big role in the
actually yield, with spring applied UAN out yielding late fall applied manure
by around 35 bushels per acre and late fall manure out yielding early fall
applied manure by around 40 bushels per acre on average. No difference in yield
was seen between the early applied manure with and without cover crop (the
cover crop in this case was cereal rye).
Figure 1. Average yield data for 2016 through 2018 crop years for corn
in corn-soybean rotation with differing fertilization treatments (EFM – Early
to mid-October manure application, EFM+CC- Early to mid-October manure
application and a cereal rye cover crop, LMF- Early to mid-November manure
application, UAN – spring UAN fertilizer application). All plots received 150
lb N/acre.
A second way to think of this
data as what percent of the maximum yield was obtained and what this means for
nitrogen utilization efficiency of the fertilizer source. One way to think about
and visualize this data is as a function of where we fall on a typical yield
response curve. While this curve looks different from year to year, I’m going
to use the state average data yield response curve to look at and interpret
what this means. The blue diamond shows the spring UAN application and suggests
that it would have achieved 99% (or a little better than) of maximum yield. The
late fall manure achieved about 84% of maximum yield and would have been
equivalent to about 70 pounds of spring applied nitrogen fertilizer, while the
early fall manures achieved about 67% of maximum yield and was similar in value
to approximately 10 lb N/acre fertilizer application. I’ve marked these two
points on the curve in Figure 2 with red dots.
Figure 2. Looking at a typical yield response curve to understand the
effectiveness of manure fertilizer in this study. The blue diamond represents
spring UAN, the red circles represent Late Fall Manure and Early Fall Manure
applications respectively.
Looking at the next part, what
did this mean for nitrate concentrations in the tile drainage water? In many
ways, the results tended to mirror what we saw from the yield numbers, with one
notable exception. Places where yield was higher tended to have lower nitrate
concentration. The exception to this was the cover crop plots, where despite
having lower yield, nitrate concentrations in the tile drainage remained low.
The other thing of note was, in general, early fall and late fall manure showed
more variability from year to year, indicating it doesn’t always increase loss,
as much as it increases the chance of loss.
Figure 3.
Average nitrate-nitrogen concentrations in tile drainage water for 2016 through
2018 crop years for corn in corn-soybean rotation with differing fertilization
treatments (EFM – Early to mid-October manure application, EFM+CC – Early to
mid-October manure application and a cereal rye cover crop, LMF – Early to
mid-November manure application, UAN – spring UAN fertilizer application). All
plots received 150 lb N/acre.
