Dicamba herbicides were registered for 2021 by the Environmental Protection Agency, but not without key changes to federal labels. In addition, Indiana imposed an earlier cutoff date on dicamba herbicide application.

The Indiana Pesticide Review Board moved to classify dicamba as “highly volatile” under state law, and impose June 20 as the cutoff for spraying of most dicamba products.

Good weed control

“Assuming soybeans are planted on time, the primary time to apply dicamba herbicides on dicamba-tolerant soybeans would be early June anyway,” notes Bill Johnson, Purdue University Extension weed control specialist. “These products are effective on many weeds, but they’re most effective if sprayed while weeds are small. That’s usually well before the June 20 cutoff date.”

Johnson suggests paying close attention to the label for each herbicide regarding weed size. For many weeds, maximum weed height is 4 to 5 inches, essentially the height of a soda can. Traditionally, many people have waited until weeds are taller to make the postemergence application.

He notes that it’s extremely important to get more realistic about weed size, do more scouting to find those weeds earlier and make applications based upon labeled heights. Control on certain weeds for these products slips once weeds are above labeled size.

One reason many growers likely gravitated to XtendFlex soybeans for 2021 was to pick up the ability to spray glufosinate, the active ingredient in Liberty, later than they could apply dicamba herbicides.

“There is also a limit on how far into the season you can spray Liberty,” Johnson notes. “It’s defined in the label based on growth stage of soybeans, not a date on the calendar.”

If you’re applying Liberty, remember that it is a contact herbicide, not a systemic herbicide like dicamba, Johnson cautions. That means good coverage is essential for control. It typically requires larger spray volumes than you would choose for dicamba soybean herbicides.

Volatility reduction agents

New federal labels for the dicamba soybean herbicides require adding a VRA to the spray mix. Johnson notes that these compounds adjust the pH of the spray solution.

Once the pH of the spray solution drops to 5 or below, becoming very acidic, the compound added to dicamba to help reduce volatility begins to separate out, he explains. Once that happens, there is less protection against the dicamba moving off-target.

These VRA compounds may be sold in various brand names. Check with your retailer to see which products meet the label requirement to add a VRA compound.

There are three dicamba herbicides labeled for dicamba-tolerant soybeans for 2021. They include XtendiMax from Bayer, Engenia from BASF and Taviuum from Syngenta. Corteva Agriscience did not choose to reregister Fexapan, Johnson notes.

Be sure to check labels online before heading to the field to spray. Checking labels within a specified time is one of the requirements for applying the product.