New soybean varieties resistant to weedkillers 2,4-D and dicamba are expected to be available to Missouri producers next year, pending regulatory review.

The new varieties offer more options for soybean producers contending with the growing number of weeds resistant to glyphosate and other herbicides. But there’s a downside.

Use of these herbicides likely will increase the risk of drift and tank contamination, says Mandy Bish, senior research specialist at the University of Missouri.

“Spray tank contamination, especially with dicamba, is going to be a huge concern,” Bish says.

Farmers spraying dicamba or 2,4-D should thoroughly wash spray tanks before using them to treat fields with crops that aren’t resistant to those herbicides.

When cleaning spray tanks, the third time is the charm, she told visitors at a recent MU Extension pest management field day at MU’s Bradford Research Center in Columbia.

A research team including Bish and MU Extension weed scientist Kevin Bradley found that a triple rinse—water, then ammonia, then water again—minimizes the risk of injuring crops that aren’t resistant to dicamba and 2,4-D, she said. Single- and double-rinse cleanings reduced yields.

Check operator manuals and the herbicide labels to learn how to properly rinse and clean sprayers after use, Bish says. Some machinery dealers suggest using commercial tank-cleaning products.

Pay special attention to boom lines in sag areas, she says. Work your way down the boom, checking for plugged nozzles. The last nozzle on the boom is where buildup is worst.

Flush screens carefully. Even small amounts of residue can contaminate lines.

In addition to tank contamination, another concern is the risk of herbicides drifting from their intended targets, Bish says.

The Environmental Protection Agency says up to 70 million pounds of pesticide are wasted to drift each year.

Both 2,4-D and dicamba can cause significant economic loss for ornamental or commercial crop growers. “Good neighbors follow good practices,” Bish says.

She advises producers to check wind speed and direction before spraying. MU’s weather stations report real-time wind speeds to help figure drift risk. Go to agebb.missouri.edu/weather/stations for weather data.

In general, drift risk lessens when herbicides are applied at wind speeds from 3 to 10 mph. But sometimes what appears to be a good day to spray, due to low wind, is possibly the worst, Bish says.

Temperature inversions, when air near the ground is cooler than the air above, can happen when winds are less than 2-3 mph.

Spray particles hang in the air instead of reaching plants. Dew can prevent spray from being absorbed. “Dicamba and 2,4-D are more volatile than many other herbicides and more prone to move off-target due to temperature inversions,” Bish says.

Dew or frost, horizontal smoke patterns, ground fog in low-lying areas and clear skies in the evening indicate possible inversion.

Pesticide applicators and beekeepers can view www.driftwatch.org, an online registry of the locations of sensitive crops.

Using color-coded flags is another method to alert growers to the type of soybean being grown in a field. Each type of soybean is associated with a different color flag.

Be aware of nearby land when spraying, Bish says. Grapes for Missouri’s $1.6 billion wine industry are especially susceptible to 2,4-D damage in mid-April when grapes break bud.

To learn more about the potential impact of the off-target movement of 2,4-D and dicamba herbicides, go to weedscience.missouri.edu/extension/pdf/synthetic%20auxins2.pdf.

For more information about controlling drift, the MU Extension publication “Controlling Drift of Crop Protection Materials” (G1886) is available for download at extension.missouri.edu/p/G1886.