Dicamba Details - 2

ROCKVILLE, Md. (DTN) -- The roll-out of new dicamba herbicides last year came with a complex new vocabulary. Terms like "volatility" and "inversion" swirled around and confused many.

Yet at least one term -- spray particle drift -- was familiar to farmers, easy to understand and, most important, mostly under an applicator's control.

So what exactly is particle drift and why does it occur? We turned to University of Nebraska weed scientist Greg Kruger and University of Kentucky weed scientist Travis Legleiter to help unpack the most common causes of how it can cause injury symptoms in the field.

In its simplest definition, physical particle drift is the movement of pesticides off-target in the form of very fine particles, Kruger explained. Kruger believes particle drift stemmed from three major drivers in 2017: Wind conditions, improper boom heights and spray droplet problems.

First up, Mother Nature.

WIND AND TEMPERATURE INVERSIONS

"Wind speed and wind direction are by far and away the No. 1 most important cause of physical particles moving off target," Kruger said.

That's what makes the buffer zone language on the labels for Engenia and FeXapan and XtendiMax dicamba herbicides so key, he explained. "If there is something sensitive downwind, don't spray," he said. "Just don't risk it. When the wind moves away from a sensitive area, then you can consider it."

Remember that "sensitive areas" means crops that are sensitive to dicamba, such as non-dicamba-tolerant beans and tomatoes, and also areas likely to house threatened or endangered species, bodies of water and residential areas. Keep in mind that the new dicamba labels for 2018 also limit spraying to wind speeds between 3 and 10 mph.

A trickier environmental phenomenon affecting particle drift is the temperature inversion, Kruger added. During an inversion, particles become suspended in cool air masses near the surface of the earth, allowing them to move off target and deposit when the inversion ends.

BOOM HEIGHT

The new dicamba herbicides require applicators to position their spray booms no more than 24 inches above the crop canopy.

"I contend that this is absolutely critical," Kruger said. "Some say it's not reasonable, but I believe that if you can't meet this requirement, you shouldn't be using that product."

The 24-inch boom height restriction is tough for growers with rolling topographies or large acreages that tempt them to push their sprayer speeds, Legleiter noted. The new labels limit sprayer speeds to 15 mph.

"My suggestion to these guys is slow down, which I know is easier said than done, and have sensors on the boom that help regulate their height," he said.

"Both options are not cheap, but I just have to encourage them to keep their booms as low as they can," he added. "The longer those droplets spend in the air, the more likely they are to move off site."

DROPLET SIZE

"To me, this is the one we have the most control over," Kruger said. "The bigger the droplet, the faster it falls."

The new dicamba herbicides require extremely coarse or ultra-coarse droplet sizes.

Each dicamba label links to a list of the approved nozzles, but for some applicators, confusion still reigned here last year, Kruger noted. Check the labels carefully and consider reaching out to the manufacturers, some of which are distributing label-compliant nozzles to applicators in 2018.

See the approved nozzles for Engenia at: http://agro.basf.us/… and XtendiMax at: http://www.xtendimaxapplicationrequirements.com/…

Selecting the right nozzle is only part of the equation, Legleiter warned. Applicators must also obey the spray pressure requirements listed for each approved nozzle.

Resist the temptation to push spray pressures up to account for a higher sprayer speed, and don't confuse higher pressure with faster falling droplets, he said.

"There is a misperception out there that higher pressures are going to get droplets to the target faster, but that's not true at all," Legleiter said. "What's happening is that as you are increasing pressure, you're reducing droplet size, so it actually falls slower."

See the XtendiMax label here: http://www.cdms.net/…

See the Engenia label here: http://www.cdms.net/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/ES/AG)

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