Several Southern producers and researchers are hoping to reduce nitrogen costs on corn and increase harvest efficiency in cotton this coming season with variable-rate applications based on on-the-go sensing technology.

In corn, Courtland, Ala., cotton producer Larkin Martin’s on-the-go technology features six N-Tech Greenseeker sensors, each a little smaller than a tissue box, mounted on the toolbar of a side-dress rig, pulled by a tractor. As the implement moves through the field, the sensors read plant vigor, and subsequent application rates are varied according to a predetermined formula.

“As the sensors go across the field, they receive six readings of plant vigor and then combine that into an average number,” Martin says. “Rates are chosen based on the relative vigor of the plant beneath it.”

While Martin has cut her nitrogen costs significantly with the practice, it’s far from perfect. “We tried the side-dress nitrogen on corn for two years and messed up two different ways. But it’s where I have seen the most potential for savings. And that was before nitrogen doubled in price. I think the interest in these sensors could increase because of the price pressures on nitrogen.”

The sensors can be removed from the corn side-dress rig and mounted on cotton sprayers spaced out over a 90-foot boom for Pix and defoliation applications.

On the benefits of variable-rate applications of Pix, Martin says, “Prior to variable-rate, you had to decide on an average rate for the entire field which put too much on a lot of areas and not enough where you really needed it. So yes, there are savings in chemistry, but even if there weren’t, it is a better use of the chemistry over the field. That translates into harvest efficiencies and overall better yields.”

In cotton, Martin has to do some scouting in advance of programming the variable-rate sprayer. Early in the season, she uses a handheld Greenseeker sensor “about the size of a weedeater which is held over a representative plant at the height of the boom.”

From this, Martin establishes a numerical rating that helps her determine Pix rates on cotton. “For example, whenever the sensors see a 7, you program your rig to put out a certain rate.

“One of the problems at mid-season and even at defoliation, is that once the rows have lapped and there’s a universe of green out there, the human eye has a tough time determining where strong and weak parts are in the field.”

One way to resolve this is to put the sensors in mapping mode during a previous pass through the field to generate a geo-referenced map of relative vigor. “And that usually doesn’t change during the year. Your strong spots are probably going to stay strong. It does take some forethought to attach the sensors when you’re putting out a herbicide to get a map to scout.”

On-the-go technology does not necessarily require a GPS, notes Martin. “But if you want to record the information and know what you did, (or map the plant vigor in the field) you have to have the GPS receiver there. You need good as-applied data which you can follow up with yield maps to see if there are correlations.”

Martin says the sensors work well in the often-cloudy Southeast. “You don’t need a third party provider to acquire an image and you’re not weather dependent.”

Glendora, Miss., cotton producer Mike Sturdivant, Jr., says on-the-go sensors “are a complement” to a service offered by InTime Inc., of Cleveland Miss., in which aerial images of field are used to map plant vigor for variable-rate applications.

He says InTime imagery is more useful when fields are too wet to enter with a ground rig for an on-the-go trip. On the other hand, on-the-go sensing is his choice when there is too much cloud cover to effectively shoot aerial imagery.

Sturdivant says that imagery from InTime Inc., and maps of plant vigor generated by the sensors, “are almost identical.”

Sturdivant saves money on variable-rate applications of Pix, “but if I do a good enough job on it, I don’t see quite the benefit from variable-rate defoliation. But, hopefully, I’ll have a fairly uniform canopy.”

Like Martin, Sturdivant uses a hand-held Greenseeker to determine differences in plant growth or vigor in a field, and then develops prescriptions based on the readings.

Sturdivant plans to test the on-the-go sensors for variable-rate nitrogen applications in cotton and corn this summer. “We’re going to try it on about 150 acres of corn and we’re going to try it on cotton too.”

Jac Varco, professor, plant and soil sciences, Mississippi State University, is developing a model for variable-rate application of nitrogen in cotton using a different brand of sensor, the Yara N-Sensor. The technology, from Norway, “has some promise for getting a good idea of the growth status and possibly the health of the plant in terms of nitrogen nutrition.”

But the idea is the same no matter what the brand. “Any excess nitrogen you put out could be hurting you. And you can never forget about the environmental aspect. You don’t want to be polluting, and if you are, you’re wasting money.”

According to Robert Mehrle, owner of Agricultural Information Management, which distributes Greenseeker technology in Mississippi, Louisiana and parts of Arkansas, a big benefit of the on-the-go sensors is that each sensor emits its own light source, “so it can be used in cloudy weather when airplanes can’t get good pictures, and it can run at night.

“Plus the grower owns it. There are no monthly or recurring fees. It’s a one-time investment. The grower, the consultant and the farm manager all work together to make it work and they’re in control.

Martin, a long-time proponent of precision agriculture, had this advice for farmers considering variable-rate technology.

“None of this comes without effort. You’re going to have to learn new things to use precision ag. It’s not going to be the same old routine. Different farms will find different fits depending on the time available for office work and the service available in their area. It’s not a one size fits all.”

e-mail: erobinson@farmpress.com