Variable-rate sidedress can work
By Steve Fairchild

What some may call a "water war" is a battle that has been fought since the late 1980's over the Missouri River.

If you’ve planted a nitrogen-dependent crop recently, you know that efficient use of the nutrient has huge effects on profit. And when nitrogen efficiency is the goal, sidedressing has long been a proven method. Now, scientists are implementing the benefit of sidedress timing with the ability to change application rates on the go. The technique is driven by a coming of age for reflective light technology.

The nitrogen efficiency conundrum is as old as modern soil science. Nitrogen is a boon to corn but can get lost or tied up on the way to the plant. Fall application is handy because farmers have time on their hands, but fall-applied nitrogen is prone to leaching and volatilization. Spring applied is more efficient, but weather can push farmers come application time and some nutrient can still be lost.

Harlan Palm, a research assistant professor of plant science at the University of Missouri, said that with the advent of new technology that captures crop canopy reflectance, on-the-go, variable-rate sidedressing can help solve long-standing questions about getting the right amount of nitrogen on at the right time.

“What we’re doing is collecting light reflectance data under certain wave lengths,” he said. “There is the near infrared, which we cannot see, and also we collect information in the visible spectrum.” As light is bounced off plants, a sensor reads the reflectance, which provides information about biomass and corn nitrogen needs for the computer to digest. The sensors delineate levels of green and signal the variable-rate applicator to apply more or less nitrogen depending on the need of the plant—less for deep green plants that obviously have sufficient nitrogen, more for yellow plants that are nitrogen starved.

Through a combination of rates in 30-pound increments, the computer can choose eight levels of application, from zero pounds per acre to 210 pounds per acre.

Palm said that growers are aware of the complicated nature of the nitrogen cycle. He added that, as far as getting nitrogen to the plant, bad things like leaching and volatization generally happen in fall and winter and good things happen when the growing season kicks in.

“The amount of these losses and gains change from one year to the next depending on soils, temperatures, moisture and etc.,” said Palm. “That’s the dynamic nature of what we’re dealing with. It’s dynamic in time as well in space, and by space I mean the within-field variability. Up till now, farmers haven’t had a way to deal with this variability.”

No way to deal with it once the corn is up, that is. The traditional way to choose application rates has been to project a yield goal and back figure the amount of nitrogen needed to reach it. In that formula, leaching and volatilization would be considered. More recently, intensive soil testing has provided field maps that provide a snapshot of soil nutrient levels. GPS grid soil sampling in combination with matching yield maps has been quickly adopted by farmers who figure they can more efficiently meet a crop’s nutrient needs. But the process is time consuming and expensive. And it’s just a snapshot in time.

Electronic eyes

On their test rig, the researchers have used a couple light sensors—GreenSeeker by Crop NTech and Crop Circle by Holland Scientific. The sensors are mounted on metal framing that extends in front of the applicator, directly above a row of corn. As they pass over the plants, the active-light sources emit rapid bursts of light. An accompanying sensor reads the reflected light, sending the information back to the cab. An on-board computer programmed with algorithms developed by the research team determines the condition of the corn. That information is relayed to the tank pumps which deliver the liquid fertilizer between the rows. The research applicator is set up to fertilize six rows. For each row, three droplines extend from the boom. The nozzles on each line deliver fertilizer at 0, 30, 60 or 90 pounds per acre, allowing for eight separate rates.

A real-time read

To figure a corn field’s optimal rates, the researchers apply a full-rate of nitrogen to a strip of corn just after emergence. Upon entering the field, the applicator will run down that strip to calibrate its instruments. Information from that well-fertilized strip is used as the basis for the varying rates in the rest of the field. A solution of UAN with Agrotain is being used at the research level, although a couple of the project’s farmer cooperators are using sensors for sidedressed anhydrous ammonia.

While delivering variable-rate sidedress nitrogen has been successful through the project, Palm said there are agronomic/cultural practices to consider in proceeding.

“There is the chance that if ‘too much’ nitrogen is applied pre-emerge, green corn could give a false read as to the nitrogen need for the rest of the season,” he said.

And that spurs some of the other questions the research team will address as they push forward: How late can we apply N and still optimize yield potential? Can a small nitrogen-rich reference area serve a large, variable field? Will the practice work without a reference area within each field?

Though results from last year’s trials were marred by drought, the team is on the way to answering these questions. Palm said that it is his goal to keep the research practical and applicable. He said that by measure of willingness among farmers to cooperate in the trials, there is real interest.

While offering the caveat that it was a field less affected by last year’s drought, Palm showed numbers that might explain farmer interest. Compared to the costs and return associated with a corn field fertilized at a constant nitrogen rate of 180 pounds per acre, variable rate sidedressing paid. With 60 pounds of nitrogen at emergence and the rest delivered at variable-rate sidedress (to total 131 pounds), the variable-rate-at-sidedress treatment netted some $16 more per acre.

“It’s a combination of things that will make it attractive to producers,” said Palm. “Consider EQIP and CSP incentives for variable-rate nitrogen application. And there is the possibility for savings on nitrogen, or at least placing it where and when it is most needed. And there can be environmental benefits,” he said.

Palm, who spent 29 years with DuPont before taking his post at MU seems genuinely excited about the possibilities for variable-rate sidedressing. “It looks like it will work,” he said.

The project is sponsored by the NRCS (Conservation Innovation Grant). High-tech portions are led by MU-based ARS scientists and engineers. MFA is a collaborator, providing in-kind donations for personnel time and equipment.