Dow responds to 2,4-D resistant waterhemp

An isolated incidence of waterhemp resistant to 2,4-D herbicide in Nebraska has been known since last year; it wasn’t a sudden discovery Aug. 14, although some media coverage seemed to make it sound like the cat had just been let out of the bag.

An evaluation by the University of Nebraska-Lincoln (UNL) supported by Dow AgroSciences has been ongoing. A news release from the Weed Science Society of America (WSSA) about the situation this week was a little sensational as the society promoted its current issue of Weed Science where the weed resistance was detailed.

The article and news release from WSSA fell into supporting the camp of activists trying to derail registration of a new formulation of 2,4-D herbicide (2,4-D choline with Colex-D technology) and the Enlist Weed Control System for corn with traits to make it resistant to the new 2,4-D choline, glyphosate and FOP grass herbicides.

The Nebraska waterhemp population resistant to 2,4-D developed in a grass seed production field that “received sequential treatments of 2,4-D up to twice a year for approximately 15 years—an extreme and unusual use pattern,” the university and Dow AgroSciences reported.

Resistance to “auxin” herbicides, such as 2,4-D and dicamba, has been relatively rare even though the products have been around for about 60 years. The UNL evaluation released by Dow AgroSciences contends there is the need for multiple herbicides with different modes of action be used over the years in any field to restrict the potential for weeds to become tolerant to a herbicide.

“Dow AgroSciences and Nebraska researchers are working with the grower to contain and control this waterhemp population. Fortunately, Nebraska researchers have confirmed this biotype is not resistant to glyphosate, nor is it likely that it is resistant to several other herbicides. A program approach including multiple modes of action along with altered cultural practices should make for a low likelihood of this biotype spreading,” Dow AgroSciences explained in a white paper providing background information.

Dow AgroScience’s conclusion noted, “This evaluation reinforces the need for rotating chemistries and using multiple modes of action. Technology that offers those attributes, such as the Enlist Weed Control System, will help farmers address weed changes like the one occurring with glyphosate as well as this one that occurred in Nebraska.”

Bullet points provided by Dow AgroSciences about the situation are below:

• An evaluation by the University of Nebraska involves an isolated incidence of auxin-resistant waterhemp in a grass seed production field—not a row crop setting.

• Resistance to auxinic herbicides, like 2,4-D and dicamba, is rare despite six decades of use.

• The herbicide use pattern was unusual and extreme, and put extensive pressure on this mode of action over many years.

• While the evaluation showed neither 2,4-D nor dicamba provided commercially acceptable control of this biotype, 2,4-D consistently outperformed dicamba on control of susceptible waterhemp.

• Nebraska researchers confirmed this biotype of waterhemp is not resistant to glyphosate, which means Enlist Duo herbicide [2,4-D choline and glyphosate], when commercialized, would control it.

• Trial results show that soil applications of Sonic herbicide in soybeans followed by post applications of 2,4-D + glyphosate provide 98 – 99 percent control of the glyphosate-resistant or susceptible biotypes that make up virtually all of the waterhemp infestations in row crops today.

• The Enlist Weed Control System will control herbicide-resistant waterhemp by offering multiple modes of action and will be stewarded to maintain long-term effectiveness.