Finish strong with a late-season nitrogen application that may help corn yields

A wet, cold spring means many farmers, caught in the flurry of corn planting, were not able to apply enough nitrogen (N) to maximize yield before the crop was planted. A late-season application of N may give the crop an extra push to finish the season strong, says Dorivar Ruiz Diaz, Extension soil fertility specialist at Kansas State University (KSU).

When corn is in the midst of the reproductive stage (silking to dough stage, depending on geography), it has accumulated about 70% of the total nitrogen it needs, according to Purdue University research. The remaining 30% of its N needs will come from a combination of corn stem and leaf tissue and new N.

Yet as the 2022 season races to the finish, the question remains: Can a late-season application of nitrogen help farmers maximize yield?

Research from Ruiz Diaz and his KSU colleagues shows that nitrogen applied at VT-R1 can bump yield in a scenario where nitrogen is short.

Over three years, the group researched the issue in four variations of N application:

Post-planting applications of nitrogen featured UAN, dripped between the rows.

Across eight sites, the three-year average yield gain was 9 bushels per acre with the late-split application of N compared to all preplant.

However, Ruiz Diaz reports a caveat: The experiments were conducted in areas where the risk of nitrogen loss applied preplant was high.

That premise matches our opening scenario: If farmers didn't apply enough nitrogen on their corn preplant, they can apply nitrogen late to capitalize on promising growing conditions.

"I think we can have situations where dryland fields were fertilized for 120- to 130-bushel-per-acre corn and are now expecting closer to 180 bushels per acre. There likely isn't enough nitrogen to reach that yield," Ruiz Diaz says. "In Kansas, when we have good years, maybe we end up a bit short of nitrogen. I see this one situation where late-season nitrogen can pay, as long as we get the rain to incorporate it."

The specialist says R1, the silking stage, is about as late as one should apply supplemental nitrogen.

"At R1 we have the number of kernels formed [pollination], but after this stage all we have is kernel weight. The chance of yield increased is much more limited then, as all you can do is increase kernel weight," he says.

With very late applications — or those after R1 — nitrogen in the grain may increase, meaning the plant is still taking up nitrogen, but the added nitrogen occurs too late to actually add bushels.

How to apply late N? Ideally, farmers can use high-clearance applicators to apply nitrogen, says John Sawyer, soil fertility specialist at Iowa State University. If those aren't available, some agronomic companies apply UAN with an airplane as a last resort.

"While all product/ method/timing options may not be ideal, not getting N applied is a much greater concern," Sawyer says.