On-Farm Nitrogen Calibration In Irrigated Corn
ALEXANDRIA, LA.
Nitrogen (N) is a critical component of corn
production and represents a substantial
portion of a corn producer’s fertility budget.
Nitrogen costs have remained stable in the recent
past but many recall two and three years
ago when N prices were well above historical averages.
Corn producers strive to maximize farm
profits by optimizing yields with judicious use of
fertilizer inputs, including nitrogen. Growers
face a dilemma about how much nitrogen to
apply each year due to the lack of a reliable N
test that can accurately predict crop N needs for
a particular growing season. As a result, many
producers will apply an amount of N that will
ensure optimum yields regardless of weather
and growing conditions. Many producers follow
Extension recommendations for corn nitrogen
inputs, yet some are concerned that these scientific
recommendations are based on trials that
may not represent their particular farm conditions
or practices. On-farm nitrogen calibration
is one method producers can use to more accurately
quantify the optimal nitrogen needs of
their particular crop and location.
A calibration trial was established on a farm
located in Tensas Parish in the Louisiana Delta
on center pivot-irrigated land with a predominantly
silty clay loam texture. The corn hybrid
Pioneer 31D59 with a population of approximately
30,000 live plants per acre was planted
on March 5 and harvested on August 12, 2010.
Soil tests prior to nitrogen fertilizer application
indicated that all phosphorus, potassium, minor
and micronutrients were adequate for optimal
potential yields. Treatments were established as
12-row, field-length strips in a repeating order
of 190, 220 and 250 pounds of nitrogen per acre
and replicated three times. The field was examined
in three zones – east, central and west – for
purposes of evaluating potential soil differences
within the trial. Nitrogen was sidedress-applied
as a liquid urea-ammonium nitrate solution,
knifed-in at crop stage V4. Parameters measured
were pre-plant soil total nitrogen, crop
yield, post harvest corn stalk nitrate content,
and post harvest soil residual nitrate-nitrogen.
Potential differences in crop yield between treatments
and location within the field were examined
by comparing the standard deviation of the
means.
Soil total nitrogen prior to the N sidedress-application
was consistent across zones at 0.12%.
Crop yields differed numerically by N rate, but
differences within treatments were greater than
differences between treatments (Figure 1). The
mean corn grain yield on the 190 lbs. N/acre
treatment was 170.2 bushels per acre. The yield
on the treatment with 220 lbs. N/acre was almost
identical at 172.3 bushels per acre. The
mean yield for the 250 lbs. N/acre treatment
was numerically the lowest at 168.6 bushels per
acre. Stalk nitrate samples did not differentiate
greatly between N rates* although the high N
rate treatment had the fewest number of samples
below 250 ppm (Table 1). Post-harvest soil
residual nitrate-nitrogen levels did not differ
greatly between N rates at 41.3, 41.8 and 44.7
lbs of nitrate-N per acre for the 190, 220, and
250 pounds of N applied, respectively. These levels
demonstrated that a substantial portion of
nitrogen was available in the nitrate form at the
end of the season, and yet had not been utilized
by the crop. These data indicate in aggregate
that for this growing season, the optimal nitrogen
rate appeared to be 220 pounds of N per
acre, although the lower rate of 190 pounds of N
per acre was similar in total yield, as well as
stalk nitrate levels and soil residual nitrate nitrogen
levels. Two years of data are preferred before
producers decide the optimal N rate for a
corn crop for their farm location. Δ
Figure 1. Corn yields for irrigated corn in the
Louisiana Delta at 190, 220 and 250 pounds of
nitrogen per acre applied at V4. Error bars indicate
the standard deviation of the means.
Table 1. Number of stalk nitrate samples
measured that were below 250 ppm (low), between
250 and 700 ppm (marginal), between
700 and 2000 ppm (optimum), and above 2000
ppm (excess).
*Blackmer, A.M., and A.P. Mallarino. 1996.
Cornstalk testing to evaluate nitrogen management.
Iowa State University Extension Publication
PM 1584.