Wet weather has left many corn fields short of nitrogen, either because applied fertilizer was lost from wet soils or because wet soils prevented application. Our experience implies that rescue nitrogen applications to corn fields deficient in nitrogen typically more than pay for themselves.
In the last issue of the IPM newsletter Peter Scharf documented fields that have lost 40 to 180 lbs N/acre due to wet conditions. Many of these fields would benefit from supplemental nitrogen if you have a way to get it applied.
Quite a bit of research has been done on delayed N applications for corn in Missouri, by us, Bill Wiebold, Kelly Nelson, and Gene Stevens. Application timing has been as late as silking in a few cases. In many of these fields the corn plants were severely nitrogen deficient when we applied nitrogen. In those experiments we found that:
If you can get the needed N to the corn by growth stage V11, you have a good chance to make full yield, especially if some fertilizer N was previously applied. This conclusion is based on 37 N timing experiments. Most of these experiments were conducted in producer fields. Corn is typically over 60 percent of its final height with 11 leaves fully emerged at V11.
We've had about half a dozen experiments in which the first nitrogen application was delayed until the V12 to V16 range. They averaged only 3 percent reduction in yield potential.
In 3 experiments where the first nitrogen application was delayed until silking the average reduction in yield potential was 15 percent. Yield increase due to nitrogen still exceeded 35 bushels per acre in all three experiments where nitrogen application was delayed until silking.
Any type of nitrogen fertilizer, broadcast at rates that can correct N deficiency, will cause some leaf burn. Thus the ideal way to apply late rescue nitrogen is between the rows below the canopy using high-clearance applicators.
In many cases broadcast applications are all that is available for late fertilizer applications. Data from eight Missouri experiments indicated that urea was the safest N source to broadcast on tall corn (3- and 4-foot corn heights were the latest timings included in these experiments) and gave the best yields. Broadcast ammonium nitrate gave more leaf burn and lower yields than urea when corn was 3 feet or taller, but gave better yields than urea when corn was 2 feet or shorter. This was probably due to volatile N loss from urea in the shorter corn. When urea was applied to corn 2 feet tall or shorter, coating the urea with Agrotain volatilization inhibitor gave a profitable yield response by preventing N loss.
In summary, rescue applications of nitrogen are recommended late into the growing season, certainly through silking. Profitability of a rescue application depends much more on deficiency level than on timing. In significantly N-deficient corn, rescue applications are likely to be profitable up to two weeks after silking. Apply the fertilizer between the rows below the canopy if possible. If broadcast applications are necessary, use urea if corn is taller than 3 feet. If corn is 2 feet or shorter, use ammonium nitrate or urea coated with Agrotain. Do not broadcast UAN solution on corn.
Any source of nitrogen is safe for rescue nitrogen applications on corn when it is applied as part of a fertigation program through irrigation equipment.
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REVISED: April 6, 2012