Leaf burn caused by top-dressing nitrogen in-season on corn is something we have likely all observed, but is it something we should be worried about? The answer is YES!

Yield loss potential caused by leaf burn increases with crop growth stage. Therefore, the later in the season the top-dress application is made, the greater the potential for yield loss. The reason behind this is that later top-dressing, at the V8-V10 leaf stage or beyond, burns leaves in the upper canopy, the ear leaf and other upper leaves that should be intercepting sunlight at grain fill time. If these upper leaves are damaged, yield loss will occur.
Once average leaf damage on upper leaves, V9—V15, exceeds 6% you start losing yield.
For every 1% increase in leaf damage above the 6% threshold you lose 1% of yield.
Is Top Dressing Urea Safer than 28%UAN?
While it is true that 28%UAN does have more osmolarity (the drying effect causing leaf burn) than Urea, top-dressing with Urea can still cause plenty of damage when applied in-season.
On top of the burn damage, Urea that lands in the whorl of the corn plant is essentially lost because only 7% of that Nitrogen is absorbed, most is volatilized, lost into the air.
Summary:
Yield reduction begins once upper canopy (V9-V15) leaf burn is greater than 6% no matter which product is used for the application.
Urea results in less leaf burn than 28%UAN but is less efficient at getting Nitrogen into the plant due to volatilization.
Resource: Dr. Joshua Nasielski presentation at 2025 Southwest Agriculture Conference
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