Off-target herbicide movement has been on growers' radars since dicamba injury began making headlines during the 2017 growing season. In the last couple years, much of the focus surrounding off-target movement has been on volatilization.

Recently, however, more attention has been given to what applicators can do to mitigate drift. Research shows particle drift, and sprayer and tank cleaning are also important, along with cleaning transport tanks.

At the recent Weed School at the University of Nebraska's Eastern Nebraska Research and Extension Center, Bob Klein, Nebraska Extension’s Western crops specialist, highlighted what works and what doesn't work when managing dicamba drift.

"The biggest thing in managing spray particle drift, of course, comes back to wind and wind speed and wind direction. That's No. 1 in managing particle drift," Klein said. "We always say spray in wind speeds between 3 to 10 miles per hour in a direction away from susceptible vegetation or crops. As you double wind speed, you get seven times more particle drift 90 feet from the sprayer."

It's also important to check the wind speed and direction at boom height at the start and end of spraying — and every time the sprayer is reloaded. Klein suggested using a WeatherFlow meter, which works with smartphones, records wind speed and direction directly to a smartphone, and costs about $90.

There's been plenty of discussion on wind speed and particle size, but Klein noted it's also a good idea to increase application rates when using a larger particle size — usually at least 15 to 20 gallons per acre to improve coverage with newer dicamba formulations.

However, sprayer boom height has largely gone ignored until recently, but it's the second-most important factor in managing drift, Klein said.

"The new labels on dicamba products require 24 inches of maximum boom height above target pest or crop canopy," he said. "We used to think particle size was more important, but boom height is the second-biggest factor in particle drift. If you double boom height, from 18 to 36 inches, we'll get 3.5 times more spray particle drift at a distance of 90 feet from the sprayer."

"It's a real challenge to maintain the correct height with larger booms — some 120 feet or more — unless you have an automatic height controller. They do a pretty good job," Klein said. "However, boom height controllers don't work very well once you get faster than 14 miles per hour. They can't react fast enough." Of course, applicators have to comply with labels on maximum application speeds.

Since drift has become a hot-button subject in recent years, several companies have advertised different drift reduction agents, or drift retardants. However, Klein notes, not all reduction agents are created equal. Some retardants work well, but it's important to use the right retardant with the right nozzle. In many cases, it takes a different retardant to work with air induction nozzles.

"In a lot of the newer nozzles, including air induction nozzles, there is a pre-orifice where the spray solution is measured, and a final orifice that makes the spray pattern," Klein said. "You can see what a nozzle is doing by comparing the pre-orifice size to the size of the final orifice. If there are big differences in sizes between the orifices, you'll have a much bigger spray particle."

Using a nozzle with a bigger difference in size between the pre- and final orifices means there's a bigger particle size, and in some cases, there may not be a need for a drift reduction agent.

And when matched up with the right nozzle, the right reduction agent can be beneficial.

"Good drift retardants matched with the spray nozzle that works with the retardant eliminate a lot of smaller particles," Klein said. "We have eliminated 7% to 8% of really small particles. In many case, the drift retardant will pay for itself, and you'll increase activity 7% to 8% because you're using a particle size that gives you more control."

To learn more, see Nebraska Extension's 2018 Weed, Disease and Insect Management Guide, which includes a section on managing herbicide drift. The 2019 guide will be available in early January.