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How late is too late to sidedress N?

How
and when to apply Nitrogen to your cornfield to maximize yield and
profitability are already complicated decisions to make.  But add the volatile weather into the
mix, and it is downright perplexing. 
Unfortunately, every acre may have different N needs, but research at
Purdue University in the Department of Agronomy has suggested there are some critical
periods in every crop to do your sidedressing.

Wet
field conditions can severely delay plans for sidedress applications.  Peter Scharf, University of Missouri
agronomist, said the last few years’ higher-than-average precipitation has been
raising concerns about soil health. 

“My
rule of thumb is that more than 16 inches of rain from April through June—or
more than a foot in May and June—will lead to N deficiency problems in a
substantial number of cornfields,” he said. 

Most
of the Corn Belt is already facing these circumstances, leading to questions as
to whether forced later-applied N will impact yield and profit. 

The
answer is yes, according to results from a 2010 13-acre field-scale experiment
at Purdue—but it can be offset.  

During
the experimental planting, all rotational corn plots received an initial 24 lbs
N/acre as starter fertilizer.

The
emerging corn received 28% urea-ammonium nitrate, sidedressed at either growth
stage V7 or V15 at 0, 40, 80, 120, 160 and 200 lbs actual N/acre.  Like is common on many farms, the
later-applied N was intended for growth stage V12, but prolonged due to weather
and an equipment malfunction.   Application at V7 was done with a
traditional knife injection tool bar, whereas a high clearance sprayer with a
mounted coulter-injection toolbar was used in the V15 application.

 

A
difference in response to N among conditions was not evident until four weeks
after silking. It was documented that N applied at V15 had increased
aboveground biomass accumulation compared to the starter-only control.   

By
growth stage R2, the number of ovules was relatively equal between the V15 and
starter-only groups.  However,
potential kernel numbers for both of these groups were approximately 9% less
than the V7 condition. 

Supporting
the notion that it is rare to see yield loss due to N stress when applications
are done between V6 and V8 is Fabian Fernandez, University of Illinois
Extension specialist in soil fertility and plant nutrition.  “Little N is needed by the crop during
early vegetative stages to about the fifth leaf development stage.  The largest portion of the total N
taken up by corn occurs during the eighth leaf to VT development stages.  Nitrogen uptake is mostly done shortly
after pollination,” he said. 

This
trend continued into the corn’s physiological maturity.  The number of harvestable kernels was
greatest in the V7 sidedress condition, with the V15 group following with 6%
less than this, but 28% more than the starter-only control.  Kernel weights were nearly identical
for the V7 and V15 sidedress conditions. 

After
harvesting, the V15 plots yielded 100 bu/acre more than the starter-only
control.  V15 was only 13 bu/acre
less than the traditional V7 sidedress group, with the difference being
explained in kernel number per ear at harvest.

These
results may ease worries: corn can recover from significant N deficiency stress
with sidedress applications of N fertilizer as late as growth stage V13 to
V15.  Yield will likely not be as
high in later-applied N circumstances, but the loss will not be as extensive as
if N was not applied at all. 
Previous recommendations from Purdue research suggest in extremely
saturated soils and ponding fields which root damage is likely, no more than 60
lbs N/acre should be applied late during the vegetative period (Brouder and
Mengel, 2003).

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