Dryland corn farmers looking for a way to protect water quality and optimize nitrogen (N) applications can now find help through a variety of commercial precision sensing technologies. However, much of this new equipment is still being perfected. Although it shows great promise, an economic payback isn't always guaranteed.

For example, three years of on-farm trials in Missouri comparing sidedress N applications to corn using a standard N rate to variable rates from using in-field sensors provided mixed results, says Ken Sudduth, USDA Agricultural Research Service (ARS) ag engineer, based in Columbia. The research trials were a collaborative effort by the USDA-ARS, the University of Missouri and MFA Inc. The researchers collected data from both NTech Industries' GreenSeeker optical sensors and Holland Scientific Inc.'s Crop Circle Plant Canopy Sensor.

“Both sensors performed similarly, in a general sense,” says Sudduth. “It appears that they were both varying N appropriately and would call for high N applications in spots that looked more N stressed and low N applications in spots that looked less N stressed.”

Both in-field sensors did well in directing appropriate levels of N to the crop, agrees Peter Scharf, a University of Missouri nutrient management specialist. However, the profit levels achieved from using the equipment were a bit disappointing, he adds.

“Overall, we lost 2.6 bu./acre of yield by using the sensors, compared to the producer rate, but we saved an average of 31 lbs. of N per acre,” he says. “Using period-average prices for both N and corn, we achieved about a $6.50/acre advantage from the sensors, not counting application costs and technology costs. However, with the recent increases in corn price and declines in N price, much of that advantage has now evaporated.”

Still, state and federal environmental incentives or regulations could also increase the technology's value.

“Right now, the sensor technology isn't a big winner economically, but it's probably a winner on the environmental side,” he says. “In Missouri, the Environmental Quality Incentives Program (EQIP) pays qualifying farmers $20/acre to do this. So if you have the EQIP payments, this gives you a pretty cost-effective way to prevent N from escaping from corn fields.”

Corn growers in other states might profit more from the sensor technology than corn growers in Missouri, adds Scharf. “As you go farther north in the Corn Belt, there might be better opportunities for N savings than in Missouri, because of higher organic matter soils that exist there,” he says

Yet, further west and south, researchers at Oklahoma State University have seen better economic results from using optical sensors to improve N management for corn than in Missouri.

“Our on-farm research and demonstrations in wheat and corn have shown that a farmer can save at least $10-20/acre by using sensors to manage N applications — and the potential economic benefits would be more in corn than in wheat,” says Hailin Zhang, Oklahoma State University soil scientist. “So this technology could pay for itself in one year's time, especially for a large-acreage farmer.”

Relatively few Oklahoma farmers have yet to buy the optical sensors, says Zhang, who says the cost to purchase a handheld sensor would approach $3,500. He adds, however, that crop consultants and Extension agents have used the sensors for several years as a service to farmers.

“The sensors have also helped to prevent over fertilization,” says Zhang. “This year, we had a drought and the sensor predicted that less N was needed in some fields.”

High-clearance sprayers may be needed for some farmers who would like to use in-field sensors to better manage N applications in corn, points out Jerry Mulliken, owner of JM Crop Consulting, Nickerson, NE.

“We don't have a good way to handle N applications above waist high, except with a high-clearance sprayer,” explains Mulliken. “If the crop is irrigated, we can apply additional N through the pivot, but variable-rate applications won't work that way.”

The timing of N applications can be crucial to optimal yields for corn, he adds. “The problem with corn is that its main N uptake period comes after you're able to drive through it with an ordinary sprayer,” he says. “Also, when corn is short, it's difficult for GreenSeeker to pick out the plant from crop residue in the background.”

In addition to working with in-field sensors, Mulliken also offers an aerial imaging service to his farmer clients. “We do true-color infrared images and geo-reference them,” he says. “I work primarily on irrigated fields, and the big thing that it focuses on is N in corn.”

Using remote sensing together with in-field sensors has its advantages, says Mulliken. “We need remote sensing as an early warning system and to possibly refine our yield maps with aerial imaging,” he explains. “A sprayer can calculate the N rate according to the sensor, but it's got to be able to cover the ground fast enough to make it commercially viable. An aerial imagery base map would help speed things up.”

The aerial imagery system establishes in-field reference strips applied with high N rates early in the season, says Mulliken. “We use variable-rate controllers to apply a strip with 50 lbs. extra N in a band across the rows,” he says. “If the N next to this strip is deficient, you'll see a difference from the air. So, it helps to establish an index to calibrate the field.”

Not all corn varieties reflect the same amount of light, and varying soil types will also have an effect on the corn plant's N uptake, says Mulliken. “So, the strip should go across all varieties and soil types,” he advises. “We apply 110 lbs. N per acre preplant, except in the bands, where we apply 160 lbs. N per acre.”

Additional applications would be fairly small, about 20-30 lbs. at a time. He says, “The biggest thing is getting the preplant application so that it is not excessive,” says Mulliken. “If the whole field shows no N stress, you can pretty much bet that you over-applied. We'd like to have the plant show some stress right around tasseling time and then add a little more N after that.”

Combining sensor data from both remote and in-field technology will likely improve the results from variable-rate N applications in corn, agrees Jim Schepers, USDA-ARS soil scientist, Lincoln, NE.

As early as this spring, ARS will come out with a simple geographical information system (GIS) that can merge with sensor data as equipment moves through the field, says Schepers. “What will likely happen is that the industry will incorporate aspects of each technology,” he says. “We'll use yield history, soil color and in-field sensors to variably apply N.”

The cost to a farmer might be $5-10/acre for a fertilizer dealer to provide the service and to pay for the gadgetry, predicts Schepers. “However, finding consultants that have enough expertise to do this is another matter,” he adds. “It's really hard to find people trained in GIS and remote sensing technology to do this successfully.”