Weed Control Minus Dicamba

ROCKVILLE, Md. (DTN) -- The stunning ruling from a federal appeals court, vacating the registrations of three dicamba herbicides designed for use on Xtend crops, has forced scientists and farmers to confront an alarming question that has lurked on the edges of the industry for three years.

What does Xtend weed control look like in a world without dicamba?

Technically, the court order removes XtendiMax, Engenia and FeXapan for use in Xtend crops. So far, Syngenta's Tavium appears unaffected by the ruling. Nonetheless, that takes a huge bite out of Xtend crop growers' postemergence weed control arsenal, weed scientists told DTN.

For growers who have not yet planted soybeans, University of Arkansas weed scientist Jason Norsworthy recommends seriously considering a switch to another herbicide-tolerant platform, such as Enlist E3, LibertyLink or LL GT27, particularly if you are battling waterhemp or Palmer amaranth with resistance to Group 14 (PPO-inhibitors) and Group 15 (very long chain fatty acid inhibitors) herbicides.

If you've already planted Xtend soybeans, one legal postemergence dicamba herbicide remains -- Tavium -- but remember the product carries a spray cutoff growth stage of V4 and it cannot be used in double-crop soybeans, cautioned University of Illinois weed scientist Aaron Hager. Tavium is also legal to use over the top of Xtend cotton fields.

Otherwise, postemergence options in Xtend soybean fields boil down to adding three classes of herbicides: Group 2 (ALS herbicides), Group 14 (PPO-inhibitors) and Group 15 (very long chain fatty acid inhibitors -- for soil residual control) to glyphosate tank mixes.

For waterhemp and Palmer amaranth control, Xtend soybean growers could add various Group 14 PPO-inhibitors, such as lactofen (Cobra), fomesafen (Flexstar) and sodium salt of acifluorfen (Ultra Blazer) to glyphosate in tank mixes, and perhaps a Group 15 residual herbicide, said University of Missouri Extension weed scientist Kevin Bradley.

Keep in mind, however, that PPO-resistant waterhemp and Palmer amaranth is an increasingly widespread problem in both the Midwest and South.

"Xtend soybeans have no good options in the Midsouth as Palmer amaranth is essentially all PPO-resistant," concluded University of Tennessee weed scientist Larry Steckel.

If you're fortunate enough to have giant ragweed that isn't resistant to Group 2 (ALS) herbicides, adding herbicides such as Classic (chlorimuron) or FirstRate (cloransulam) can help, added Purdue University Extension weed scientist Bill Johnson.

For marestail populations in Xtend soybeans, Ohio State University weed scientist Mark Loux and Penn State weed scientist Dwight Lingenfelter said recommendations are limited, given the common resistance profile of this weed in their region.

"Once dicamba is out, we have no postemergence options (in Xtend crops) for control, since marestail is all glyphosate- and ALS- resistant," Loux warned.

"We possibly could recommend a 2x rate (1.5 lbs. acid equivalent) of glyphosate or a couple of in-crop applications to stunt them in hopes the beans can canopy," Lingenfelter added. "This will not control [marestail] but might suppress them somewhat if they are sprayed early enough."

Xtend cotton growers may find themselves in a better situation, as those cotton varieties can also tolerate in-season applications of glufosinate, Arkansas' Norsworthy noted. Residual Group 15 herbicides can also help, but keep in mind that some growers in Arkansas and Tennessee are seeing Group 15 resistance in Palmer amaranth, he added.

In the weeks ahead, the legal status of the three vacated dicamba herbicides could continue to change, depending on how EPA responds to the court order. Follow our coverage on those developments here: https://www.dtnpf.com/….

For more information on weed control options if dicamba is not available:

https://extension.entm.purdue.edu/…

http://bulletin.ipm.illinois.edu/…

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

Follow her on Twitter @Emily_Unglesbee

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