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Corn+Soybean Digest

Will Variable Rate Cut N Costs?

A Missouri grower uses aerial imagery to help him cut his nitrogen (N) application by 10% or more on corn following cotton.

John Engram's Sikeston, MO, farm features fields that are among some 100,000 southeastern Missouri acres that are seeing sophisticated variable-rate application through aerial imagery from InTime, Inc.

Based in Cleveland, MS, InTime does most of its business with southern cotton producers and growers in the West. Its goal is to expand farther into the Corn Belt.

It could become a solid tool to combat Asian soybean rust if outbreaks are magnified, says InTime.

The InTime program makes aerial images of fields from digital cameras in airplanes at about 12,000 ft. Images then classify crop biomass and vigor in up to 10 different shades of green and brown. These “scout maps” are accessible to clients through a password at

Engram grows corn, soybeans, cotton and wheat on soil types ranging from sandy loam to gumbo-type clay. “On our sandy loam we usually rotate cotton and corn,” he says. “On our dark soil we go with soybeans and corn, or wheat, soybeans and corn.” To reduce fertilizer costs, he and Intime's Derek Emerine used 2005 aerial imagery to direct soil nitrate sampling on the '06 corn crop.

“We first looked at imagery from the '04 cotton for residual N available for the '05 cotton crop,” says Emerine. “It indicated there was between 40 and 70 lbs. of residual N available. We decided that if this worked on cotton following cotton it should work on corn following cotton as well.”

There was from 20 to 40 lbs. of residual N over about six fields.

“John (Engram) was then able to reduce his sidedress rates and N rates from about 220 lbs. down to about 190. That's about a $10/acre savings, conservatively. John will also receive an image 2-3 weeks before tassel to determine through scouting or tissue analysis if additional N is needed through foliar feeding,” says Emerine.

Engram uses the aerial imagery to map out a prescription for various types of chemical applications. For cotton, he was able to better manage his growth regulator and defoliation program.

“We had a much more even crop with no ‘rank’ growth areas,” he says. “The crop matured earlier and harvested more evenly.”

For corn, he doesn't expect reduced fertilizer applications to impact yields. “I want to try it for two or three years to make a final judgment. But with what we are seeing in fertilizer savings, it's lowering our input costs.”

Engram notes that on soybeans in 2004, aerial field images indicated problems about a week before sudden death syndrome hit the field. “We were able to delay an irrigation that might have worsened the situation,” he says.

Emerine says research in other southern locations shows that aerial imagery may become an effective tool in making sure soybeans infected with Asian rust are properly covered to prevent damage. “It should help produce better coverage on ‘lusher’ fields by applying higher rates in higher biomass areas,” he says.

Costs of the InTime aerial imagery, prescription application rate program vary. There is about a $2/acre charge and a 500-acre minimum in most cases. For more information on InTime, go to

How effective is variable rate herbicide? “Iowa's variation in soil characteristics that influence herbicide rates might be only 10-15%,” says Iowa State University agronomist Bob Hartzler. “A lot of our sprayers will have that much variability from one side of the boom to the other.”

However, in fields with varying weed populations, there could be a place for variable-rate application, says Hartzler. The ability to efficiently generate weed maps has limited the adoption of this practice.

Old-fashioned field scouting may also help growers save on chemical applications, says Andrew Price, USDA-ARS plant physiologist who took part in USDA site-specific application studies in North Carolina and Virginia.

“Growers could save money if herbicides were applied in a site-specific manner, varying herbicides and rates by weed species present within areas of a field and location,” says Price. As few as 10 spot checks per field may be enough to identify fields that would benefit from site-specific herbicide application.

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