New Dicamba Herbicides Require Care

Dicamba tolerance is finally here, as Monsanto’s Roundup Ready 2 Xtend soybeans will enter fields this spring. But with this powerful new herbicide system comes a push for additional training and careful application.

Farmers Co-op Society agronomist Ben Van Beek, a certified crop advisor based in Sanborn, Iowa, said the ability to “start clean with a good burndown” using a spray-and-plant or plant-and-spray application seems to be the most appealing aspect for farmers.

Applying dicamba herbicide at planting requires either Monsanto’s XtendiMax with Vapor Grip or BASF’s Engenia herbicides. Using older dicamba formulations at planting, like Banvel, violates the label.

Some farmers have been taking a wait-and-see approach, while others have been willing to try dicamba-tolerant beans on more acres, Van Beek said.

“It’s another tool in the toolbox,” he said.

Bob Hartzler, Iowa State Extension weed specialist, said Iowa farmers need to consider the risks associated with dicamba-tolerant soybeans when determining where they fit in their program.

“It’s my view there is greater risk with this product (dicamba) than with other materials available,” Hartzler said.

Finding the right fit

Pre-emergence applications of dicamba in no-till soybeans can provide control of marestail and giant ragweed with less risk of off-target injury than post-emergence applications.

Growers may also be interested in using dicamba-resistant soybeans due to increased problems with herbicide-resistant waterhemp, but post-emergence applications will be needed for waterhemp due to its prolonged emergence, Hartzler explained.

Dicamba provides an alternative to Group 14 herbicides (Cobra, Reflex, etc.) and Liberty, and unlike these herbicides, dicamba is systemic, meaning the plant absorbs the chemical and moves it throughout the plant, he said.

Label requirements for dicamba-tolerant soybeans specify dicamba must be used on weeds less than 4 inches tall.

“Timing of application is really critical,” Hartzler said, noting dicamba will not have the same broad application window as glyphosate. “It’s going to need to be used in an integrated program with pre-emergent herbicides.”

Drift and volatility

Given the effectiveness of dicamba herbicides against broadleaf plants, the importance of monitoring drift and volatility for successful application cannot be overstated.

To meet label requirements, Bill Johnson, a professor of weed science at Purdue University, said applicators are required to use approved nozzles and pressures when spraying. These are intended to create larger droplets to limit drift, the movement of spray particles by wind from sprayer to target.

A required buffer of 110 feet downwind to sensitive areas is also included in the label.

How this buffer requirement is defined has been subject to some interpretation, but it’s clear the requirement will give increased importance to records of wind speed and direction, Johnson said.

Volatility, on the other hand, is the movement of the gaseous form of the herbicide after it has been deposited on its intended target as a liquid. The new dicamba formulations have been shown to have lower volatility — hence the label requirement against using older dicambas at planting.

But Johnson said some plants, such as grapes, tomatoes, non-tolerant soybeans and peach trees, are so sensitive that “it doesn’t take many molecules to affect symptoms.”

For this reason, it will be especially important for anyone applying to know what their neighbors are growing.

“For example, you may not know that somebody is putting in a grape orchard or putting in tomatoes for the farmers market,” Johnson said.

Understanding temperature inversions is also key to managing drift and volatility.

If an inversion is traced back to the date of application, it’s a violation of the label and the applicator could be subject to fines or other punishment, Johnson explained.

These are not new issues for applicators, but “emphasis on label is something that is relatively recent,” Johnson said.

Van Beek agreed, “Seems like they are putting more teeth into what they have (on label).”

Applicators will need to check the label before applying for detailed directions such as the triple rinse procedure to avoid tank contamination and tank additive specifications, he said.

VanBeek, who farms himself in addition to his agronomy work, said, “We want to makesure we keep this technology — and not lose it. We want to make sure we can utilize this for years to come.”

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