If you’ve scratched your head when you read about inconsistent results from planned late-season nitrogen applications on corn with a high-clearance sprayer, you’re not alone. Agronomists have also wondered why adding some extra N to corn at the V12 stage or later that had nitrogen applied earlier provides a yield boost sometimes but not at other times. 

The results in two on-farm field trials conducted by Bob Nielsen, Purdue University Extension corn specialist, and Jim Camberato, Purdue Extension soils fertility specialist, are typical, Camberato says. In one replicated trial in Shelby County, Ind., a program that included putting the final shot of N on around V12 with high-clearance rigs yielded significantly more, although the yield increase was small. All the plots in this trial received the same amount of total conventional nitrogen over the course of the season.

Yet in a very similar on-farm trial near Peru, Ind., there was no difference in yield based upon when the N was applied, Camberato says. These observations are in line with what others have reported across the Corn Belt — sometimes the planned late-season N application yields more and sometimes it doesn’t.

“There are even situations where applying the last shot of N late could be detrimental if the soil is very dry, it stays dry, and the roots can’t take up the late-applied N in time,” Camberato says.  

Rescue N treatments
Contrast those observations to what Camberato and Nielsen have observed in fields where the crop was obviously short on nitrogen early, and a rescue treatment was made using high-clearance rigs at V12 or even later. Those responses are often dramatic if the corn is healthy other than being short on nitrogen.

“We’ve seen incredible results in some of these cases,” Camberato says. “In one case the corn was yellow, and an application was made at V15, which is basically at tasseling. We’re talking applying relatively large amounts of nitrogen here. It’s usually a case where either nitrogen couldn’t be applied on time due to wet weather, or where nitrogen applied earlier was lost due to wet soils. It’s clearly a rescue situation, not something that was planned ahead of time.

“In this field, yield jumped from 115 bushels per acre to around 235 bushels per acre. We tried it again in a similar situation, making the application at V12. In that case the application upped yield 100 bushels per acre,” Camberato says.

The increase may not always be quite so dramatic, but yield increases of 50 to 60 bushels per acre in these situations are common, and obviously cost-effective, he notes.

“We believe it’s possible because corn hybrids typically take up two-thirds of their nitrogen during grain fill,” he says. “If the plant is healthy otherwise, you can apply the N late and weather is favorable afterwards, these kinds of increases are possible.”

So why doesn’t this late application show up in planned programs? “It’s likely because you have already applied the bulk of what the crop needs,” Camberato says. “If the nitrogen is still there, then adding more doesn’t necessarily help much.”