Nitrogen (N) loss in cornfields has been widespread across large swaths of the Midwest for three straight years, says Peter Scharf, University of Missouri (MU) Extension nutrient management specialist. Between 2008 and 2010, corn growers saw yield losses from too much early rainfall and/or soil-moisture levels that have robbed crops of N applied before or even shortly after planting.

Grower Ted Sander, Moberly, MO, has escaped such yield-loss anxieties with a high-clearance Hagie STS12 sprayer. Its up-front toolbar injects liquid N into corn up through and a little past tasseling – later than traditional sidedress N-equipment.

“We’ve had up to 50-70-bu. corn-yield increases from rescue-N treatments compared to complete preplant N applications with no rescue treatment,” says Sander. He added Ag Leader OptRx crop-sensor technology to his machine in 2010 after trying similar sensors for three previous years in cooperation with the MU on-farm research trials.

“In my area, corn was planted in eight days mid-April, when conditions were warm and dry,” says Sander. “Then the weather turned cold and wet and by the end of May 60-70% of the corn in the area had to be replanted. Guys who replanted but didn’t put on additional N didn’t get good yields. It took the additional N to get the good yields.”

In general, the more stress from N loss, the higher the yield response will be to a rescue-N application, says Scharf. “Last year, in six rescue-N application trials, the average yield response was 34 bu./acre, which is the same average as in all 11 rescue-N trials I’ve been involved in during my career,” he says. “Yet, yield response is also connected to how the corn looks. Last year, the yield response averaged 57 bu./acre in high N-stress areas, 41 bu./acre in medium N-stress areas and 14 bu./acre in low N-stress areas.”

After N rescue treatments, the high N-stress areas never quite catch up in total yield to low or medium N-stress areas, but they do catch up enough to make money on the extra N application, says Scharf.

“To break even on a rescue-N treatment, you need about an 8-10-bu./acre yield response if you’re using an aerial application or a high-clearance applicator with sensors,” says Scharf. “So, rescue-N treatments are definitely worth doing if you have the equipment available and you can see the stress to your crop.”

The crucial element is a machine that can put more N out to the crop if it needs it, emphasizes Scharf. Some of the potential options for rescue-N applications are urea applied aerially or with a dry spinner or dry boom; UAN with pivot irrigation; or a high-clearance sprayer with drop nozzles or injection arms.

“This requires some forethought if you don’t already own the equipment to do it yourself,” says Scharf. Have a plan ready if the weather turns really wet and the soil suffers from nitrate leaching (on well-drained soils) or denitrification (on poorly drained soils), he says. Plan for a rescue-N application before you need it, and establish logistics and contacts ahead of time.

To find N-deficiency symptoms in the field, look for a V-shaped brown or yellow burn up the midrib of lower leaves, advises Scharf. Farmers can also rely on aerial photographs, he adds. “Many airports have a flight instructor who can be hired to take you up. If your corn looks yellow or light green, you need to apply more N.”