How late is too late to sidedress N?

How late is too late to sidedress N?

By: Audrey Kittrell 06/17/2011 @ 9:59am

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).