Oklahoma Cooperative Extension Service
PSS-2186
Pre-Plant Nitrogen Response
in Irrigated Corn
April 2019
Jason Warren
Associate Professor, Soil and Water Conservation
Oklahoma Cooperative Extension Fact Sheets
are also available on our website at:
facts.okstate.edu
Brian Arnall
Associate Professor, Precision Nutrient Management
Sumit Sharma
Post-doc
Saleh Taghvaeian
Assistant Extension Specialist, Water Resources
Cameron Murley
Senior Station Superintendent
Introduction
A 2018 study was initiated at the McCaull Research and
Demonstration farm in the Oklahoma Panhandle to evaluate
corn yield response to pre-plant applied nitrogen with different
irrigation rates based on evapotranspiration (ET) replacement. The irrigation rates were 1 inch (80 percent ET), 1.25
inches (100 percent ET), and 1.5 inches (120 percent ET)
approximately every five days. The resulting yield data was
used to provide estimates of economic returns as a function
of gross revenue and variable costs. This large, field scale
project provides valuable insight into the benefits of optimizing N and irrigation management to improve profitability and
sustainability of irrigated corn production in the Panhandle.
Field Study Description
In 2018 a large scale field trial was initiated at the McCaull Research and Demonstration Farm located in northern
Texas County, OK near Elkhart KS. The study was conducted
on a 125-acre pivot where pre-plant nitrogen was applied at
rates of 100, 150, 200, 250 pounds nitrogen per acre-1 as
anhydrous ammonia plus 100 pounds of a blended fertilizer
containing 12 percent nitrogen, 40 percent P2O5, 10 percent
sulfur and 1 percent zinc. These treatments were applied in
three replicated strips around the pivot between wheel tracks
4 and 7 (Figure 1). A 16-row commercial strip till applicator,
with 20-inch row spacing was used to apply these treatments
March 23. This allowed for the five nitrogen rates to be located
between pivot tracks. The zero anhydrous ammonia check
strips received 100 pounds per acre-1 of the blended fertilizer
and were located towards the center of the pivot instead of
within the replicated area to allow for more replicated fertilizer
nitrogen treatments within the main study for a higher resolu-
Figure 1. Orientation of pre-plant fertilizer strips, replicated
3 times between pivot wheel tracks. The zero anhydrous
ammonia check strips are in lime green near center of pivot.
tion assessment of nitrogen response at the top of the yield
curve. Irrigation was applied at 1-inch, 1.25-inch and 1.5-inch
application rates such that the 1.25-inch treatment served to
replace 100 percent crop ET with a minimum return interval
of 4.9 days to simulate 3.8, 4.8 and 5.8 gallons per minute
acre-1 irrigation capacities. The amount of irrigation water applied was manipulated for each treatment by adjusting pivot
speed. These irrigation treatments were applied in 18-degree
slices of the pivot overtop of the nitrogen rate strips to provide
four replicates of each irrigation treatment (Figure 2).
Results
The cumulative in season rainfall at this location was
12.4 inches, which is above the typical average of 10 inches
during this time of year in the Panhandle region. Figure 3
shows the cumulative ET as estimated using the Mesonet
irrigation planner, as well as the cumulative rainfall + irriga-
Division of Agricultural Sciences and Natural Resources
•
Oklahoma State University
Figure 4. Corn grain yield response to pre-plant anhydrous
ammonia applications under three irrigation regimes at
McCaull R&D farm in 2018.
Figure 2. Placement of the 1” (red), 1.25” (yellow), and
1.5” (green) irrigation treatments using pivot telemetry
and speed control.
Figure 3.The cumulative ET as estimated from the Mesonet
irrigation planner (Eva station) and the cumulative rainfall
+ irrigation resulting from each irrigation treatment.
tion applied with each treatment. The final irrigation event
was applied September 2. The 1.25-inch irrigation treatment
was successful in replacing crop ET as estimated from the
Mesonet. The 1-inch, 1.25-inch and 1.5-inch irrigation treatments resulted in 17, 21 and 24.5 inches of irrigation during
the growing season, respecively.
Figure 4 shows the nitrogen response curves for corn yield
produced under the three irrigation regimes. The data shows
no difference in yield response among the irrigation treatments. Yields in the fertilized strips ranged from 210 bushels
per acre-1 with 100 pounds nitrogen acre-1 to 245 bushels per
acre-1 with 300 pounds nitrogen per acre-1. The check strip
which received no nitrogen except for 12 pounds nitrogen per
acre-1 in the blended fertilizer produced an average yield of
168 bushels per acre-1.
The data show that optimum yield could be achieved
with the 17 inches of irrigation applied in the 1-inch treatment,
which supplied approximately 80 percent of crop ET. The
ET reported at the Eva station for this corn crop was below
the 15-year average ET reported at the Goodwell station (40
inches). This, combined with in-season rainfall, which was 2.4
inches above average, can further explain the relatively low
amount of irrigation required to optimize yield. Furthermore,
many research findings in the regions have shown that 75 to
85 percent of ET replacement is sufficient to optimize irrigated
corn yields.
Figure 5 shows the revenue based on $4 corn price minus
the variable costs. The data show that there was very little or
no return on investment for nitrogen fertilizer at nitrogen rates
above 200 pounds nitrogen per acre-1. Furthermore, it shows
that the 1-inch treatment, which supplied 80 percent of crop
ET provided for optimum revenue over variable costs with
$300 revenue-variable cost, whereas the 1.25-inch treatment
provided $260 revenue-variable cost per acre. It is important
to note that this research was limited because only the speed
of the pivot could be changed to alter the irrigation applied.
This is why the different depths were used. A similar financial
outcome could have most likely been achieved by reducing
the frequency of the 1.25-inch application so it provided for
80 percent of ET replacement.
Figure 5. Revenue (based on $4/bu corn price) minus variable costs as a function pre-plant anhydrous ammonia
applications under 3 irrigation regimes at McCaull R&D
farm in 2018.
PSS-2186-2
It is interesting to note that the 1.5-inch treatment did not
decrease yield response to nitrogen in the fertilized treatments.
This suggests that in this one year of data, the irrigation applied
in excess of ET did not cause sufficient leaching to decrease
availability of the pre-plant applied anhydrous ammonia.
Summary
Corn yield response was unaffected by the amount of
irrigation water applied in this study. In fact, yield was unresponsive to irrigation treatments imposed because the 1-inch
treatment provided irrigation capacity sufficient to supply 80
percent of ET which was sufficient to optimize grain yield.
The excess water supplied by the 1.5-inch (120 percent of
ET) irrigation treatment apparently did not cause detrimental
leaching of the pre-plant anhydrous nitrogen applied in this
study. Economic returns were optimized with the 1-inch irrigation treatment receiving 200 pounds nitrogen acre-1; first
because of reduced pumping cost; and secondly because
the added revenue resulting from the yield maximized with
300 pounds nitrogen per acre-1 was insufficient to pay for the
additional nitrogen and harvest costs associated with this
fertilizer treatment.
Acknowledgements
Thanks to Daren Buck for his support of our efforts at
the McCaull Research and Demonstration Farm. He and his
personnel not only helped by fertilizing, planting and harvesting
these plots, but also provided a great deal of guidance in the
management plan for this field scale project that supported
its success.
PSS-2186-3
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