Dicamba Review

ROCKVILLE, Md. (DTN) -- In a late, delayed spring like this one, growers may have to rely on postemergence herbicide applications more than usual to control weeds.

If dicamba-tolerant soybeans or cotton are in your acreage mix, in-season dicamba applications can help -- but take care. The new dicamba herbicides (XtendiMax, FeXapan, Engenia and Tavium) are restricted-use pesticides that come with a host of label restrictions, and a growing amount of research on how they move off-target.

Here are five important things to keep in mind when spraying these chemicals in 2019.

1. CHECK FOR INVERSIONS

All four dicamba labels ban spraying when a temperature inversion is underway. Inversions occur when a cool, stable air mass is trapped near the ground and can suspend particles within it -- such as herbicides -- until the air eventually warms and disperses.

Thanks to four years of research by University of Missouri scientists, we have a much better understanding of when inversions occur and how they affect pesticide applications. Here are the main takeaways from their research:

-- Inversions are occurring frequently -- as often as 60% of all evenings in June and July of 2018. They develop most often in the evening hours, even before sunset, the point at which new dicamba labels limit spraying.

-- Inversions can trap dicamba in the air and increase off-target movement. Missouri scientists detected three times more dicamba following an application made during an inversion compared to an on-label application.

-- Your field's geography matters -- tree lines and buildings can block wind and cause inversions to form earlier than the surrounding area.

-- Use mobile spray apps with caution. Weather-based spraying apps can help show potential application windows, but they cannot predict inversions or confirm boom-level conditions for a particular field. Keep in mind that some apps use weather data taken at heights higher than an inversion can be detected. Examine your actual environment for inversion warning signs at the time of application for a final spray decision. Look for calm air, clear skies, smoke or dust hanging near the ground and sound distortions. See more details here: https://www.dtnpf.com/….

See the full summary of the University of Missouri's temperature inversion research here: https://ipm.missouri.edu/….

(Editor's Note: DTN is one of several companies offering weather-based tools to help schedule spray applications. MyDTN subscription customers have access to the DTN Spray Outlook tool and app. Details on those tools are here: https://www.dtn.com/….)

2. CHECK YOUR SURROUNDINGS

Many plants and trees are extremely sensitive to tiny doses of dicamba. For two consecutive years, farmers, homeowners and specialty crop growers in the Midwest and South have filed complaints about dicamba injury to a wide variety of row crops, trees, horticultural crops, ornamental plants and even bees.

Ultimately, the applicator bears final responsibility and liability for any off-target movement of dicamba, so knowing what crops and plants surround a field before you make an application is critical, University of Tennessee Extension weed scientist Larry Steckel said.

Some fields may simply not be a good fit for dicamba-tolerant crops and dicamba applications, he said. "There are some fields, especially in more urbanized regions, where you just shouldn't be relying on spraying dicamba because the probability that you can do it safely isn't there."

Applicators can use sensitive crop registries such as FieldWatch to help check surrounding areas, but they can't rely exclusively on it, as some sensitive crops and plants may not be registered there. See here: https://fieldwatch.com/….

Remember that the labels require a 57-foot omnidirectional buffer in counties with certain endangered species. Check yours here: https://www.epa.gov/….They also require a 110-foot or 220-foot downwind buffer, depending on application rate, and ban dicamba applications altogether when the wind is blowing toward a sensitive crop.

3. CONSIDER LEAVING GLYPHOSATE OUT OF THE TANK MIX

Weed scientists have discovered that when the pH of a dicamba tank mix is lowered below 5.0, dicamba volatility and off-target movement increases significantly.

The dicamba labels instruct applicators to check with experts such as Extension agents to make sure their chosen tank mix ingredients don't lower the pH too far. But, in reality, the data is not yet there for scientists to make these recommendations on the wide range of tank mix options available, Steckel said.

Even ingredients as basic as water can affect a tank mix pH. "When we started looking at this research, we tested wells across Tennessee and the starting pH of the water was all over the place, from 8.0 down to 4.0, so that makes a difference in your tank mix from the get-go," Steckel noted.

However, university weed scientists have confirmed that one popular tank mix ingredient -- glyphosate -- consistently lowers the pH in a dicamba tank mix. In one of the most recent studies, University of Tennessee scientists found that adding Roundup to both Engenia and XtendiMax lowered the pH below 5.0, thus increasing the risk of volatility. "The research we've got from a number of states is clearly showing we are picking up more dicamba leaving treated surfaces with glyphosate in the tank mix than without it," Steckel said. "If you're in sight of sensitive crops, I'd advise not putting it in."

Other known culprits are ammonium sulfate (AMS) and urea ammonium nitrate (UAN), which are banned from use with dicamba by the federal labels. The University of Missouri has also done research showing that low soil pH can also increase dicamba volatility. See a summary here: https://ipm.missouri.edu/….

See the Tennessee research here: http://news.utcrops.com/….

4. WATCH THAT WIND SPEED

The dicamba labels limit applications to wind speeds between 3 and 10 miles per hour. At zero to 3 mph, the air is still enough that an inversion is possible. At 10 mph and above, off-target physical particle movement is possible.

Threading that 7-mph needle is difficult. Even university and company scientists have found themselves unable to apply dicamba on-label to their research fields, due to wind speed constrictions. (See here: https://www.dtnpf.com/….)

Purdue University weed scientists have found that applicators had only 47 hours in June 2018 to apply dicamba legally in northwestern Indiana, largely because of the wind speed restrictions: https://extension.entm.purdue.edu/….

Applicators should consider using hand-held anemometers to take wind speeds at boom-height before an application. They are also required to record and document those wind speeds by the federal labels.

5. KNOW YOUR STATE'S RULES

A number of states have opted to use 24(c) special local needs labels to add additional restrictions to the federal dicamba labels. Those restrictions range from additional state training requirements to firm cutoff dates for dicamba use.

These more restrictive state regulations take precedence over the federal label, so applicators need to know and follow them. Check with your state department of agriculture and see more details on the most significant state restrictions on dicamba here: https://www.dtnpf.com/….

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

Follow her on Twitter @Emily_Unglesbee

(PS/ES/AG)

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