Field tests conducted by John Deere agronomists in Brazil suggest that growers should increase gallons-per-acre and spray at higher pressures to insure effective Asian soybean rust (ASR) treatment.

The tests measured the effects of speed, gallonage, pressure, nozzle type, and other variables on coverage of the plant — particularly at the middle and bottom of the canopy where infections initiate.

“The tests verified that penetrating the mature canopy and reaching lower leaves is difficult,” says Jeff Barnes, John Deere crop systems specialist, “But higher carrier volume and pressure can make a difference. Best results were obtained by spraying 15 gallons-per-acre or more at pressures of 70 psi or greater.”

Field speed results

Field speed, however, had little effect on coverage. In fact, the percent of lower leaves covered at 15 mph versus 5 to 8 mph were nearly identical. Because many experts suggest fungicides must be applied within two to three days after the first infection to protect yield, growers may want to consider increasing field speed to cover acres quickly once ASR is identified.

“Because of the threat of ASR many different recommendations have been made for sprayer and equipment adjustments,” says Don Borgman, John Deere manager of market planning. “Some suggestions just didn't add up, so we decided to invest research dollars and go to Brazil to gather real data. Farmers have too much at stake and need reliable information to support their decisions in dealing with ASR.”

The tests were conducted in the Brazilian state of Matto Grasso do Sol. Special spray droplet test cards, Krome Kote, were placed at low-, medium-, and upper-canopy heights. After each sprayer pass, the cards were collected and scanned using DropletScan, a proprietary software program that measures percent coverage and spray deposition rate (GPA).

“We had a lot of theoretical models to work from, but we wanted to verify them in a real-world environment,” Barnes explains. “And that meant going into the field. Overall, we collected and scanned nearly 2,500 of these cards. Together, they gave us a pretty good idea of how certain variables affect plant spray coverage, and how a grower might use this information to fight ASR.”

More water is better

One factor was made perfectly clear by the testing: When it comes to ASR, spraying with more water is definitely better than less. As a rule of thumb, 15 gallons-per-acre is the minimum growers should consider.

“At the bottom of the canopy, the higher GPAs provided significantly more coverage,” says Mike Miller, John Deere product planner.

“In fact, with the other variables being equal, coverage at the bottom of the canopy using 20 GPA was more than 17 times greater than using 5 GPA. At the middle and top of the canopy, the difference was even greater, but when it comes to rust, you're more concerned with the amount of fungicide that you can get to the lower part of the plant.”

The John Deere team tested SprayMaster tips with different published pressures: low-pressure (40 psi), medium-pressure (60 psi), and high-pressure (70 psi and higher), plus TwinCaps that hold two standard spray tips at 30-degree angles (front- and rear-facing).

“When comparing the spray coverage at the bottom of the plant, there really wasn't much difference between the tips,” Barnes says. “At the top of the canopy, the higher-pressure tips provided more coverage.

“The real difference showed up at the middle canopy heights,” Barnes continues. “Tips that can maintain about a 350 micron droplet size and still operate at 70 psi or higher provided significantly better coverage in the middle of the soybean plant, where the fungicide is needed. For instance, the best overall mid-canopy coverage was provided by TwinCaps equipped with two ultra low-drift nozzles.”

Maintaining a consistent droplet size of approximately 350 microns is critical for effective application. Droplets that are too fine may drift easily and blow over a dense canopy. Larger droplets are less likely to drift, but the fewer number of droplets produced will reduce the chance of fungicide reaching the bottom third of the plant.

“We found that regular flat fan nozzles and hollow cone tips create finer droplets that just aren't able to penetrate as well to middle and lower sections of the soybean plant,” Miller says.

Theoretical studies suggest that reducing field speed in curative applications will reduce boom movement, lateral drop movement, and provide better coverage. However, the actual field tests in Brazil suggest there's no significant degradation in coverage at faster field speeds — even as fast as 18 mph.

“The coverage at 8 mph was less than 10-percent better than the coverage at 18 mph,” Barnes explains. “Compared to the positive benefits of finishing fields faster in a very narrow application window, a 10-percent improvement in coverage is negligible at best.”

Vital considerations

But field and crop conditions should still dictate field speed, and boom stability must be a vital consideration.

“Older sprayers with older-style suspension systems will definitely work,” Borgman says. “But you may have to slow down significantly to insure coverage. If you're running fast enough that your booms bounce occasionally, coverage to the critical middle and lower sections of the plant will be compromised and you must slow down. Our tests at 18 mph were done with state-of-the-art boom and chassis suspension technology. We wouldn't recommend those kinds of speeds with many older sprayers.

“For older sprayers that can't run as fast without boom bounce, another alternative is to spray 24/7 when the application window opens,” Borgman explains. “Fungicide companies say there's no difference in efficacy at night versus daytime. Auto guidance can be a benefit to reduce skips and overlaps and for spraying at night.”

For more information on Asian soybean rust and recommended sprayer settings, check the Web site at www.JohnDeereAg.com or visit your local John Deere Spray Center dealer.