Dicamba herbicide drift damage to soybeans

Dicamba herbicide drift damage to soybeans.

Arkansas considers newly registered dicamba formulation

The federal Environmental Protection Agency has registered a new dicamba formulation it says “is specifically designed to have lower volatility” when sprayed over cotton and soybeans genetically modified to tolerate the herbicide.

The Arkansas State Plant Board, which regulates use of pesticides in Arkansas, will host a public comment hearing session for the new dicamba formulation on Nov. 21 in Little Rock.

Xtendimax with Vapor Grip Technology, a Monsanto product, is intended for use by spraying over the top of Xtend cotton and soybeans. Although the crop seeds were available and planted throughout the Mid-South this year, the EPA hadn’t registered the Xtend dicamba formulation until Nov. 9, meaning it should not have been used to help manage weeds in 2016.

This past growing season, the Arkansas State Plant Board received 28 official complaints related to dicamba drift injury in soybeans. According to various reports, Missouri’s Department of Agriculture received multiple dicamba-related complaints this summer, while Tennessee was also investigating dicamba-related complaints. Last month, the EPA’s criminal investigation division, served warrants in four Missouri counties, investigating complaints about off-label use.

At issue is volatility — the tendency under certain conditions for a substance to vaporize. When vaporization happens in a farm field, winds or warmth can cause the substance to drift, or move away from, the intended application area.

Objective testing for best management practices

The Arkansas Division of Agriculture is not a regulatory agency, but it has a major role in objective development of best management practices for Arkansas growers. In that role, scientists regularly conduct multi-year trials of crop varieties, pesticides and cultural practices to provide the best science-based recommendations.

This year, 56 of the 217 soybean varieties in full-season testing were Xtend varieties with eight Xtend varieties among the 14 in early-planted tests. Of the 35 cotton varieties tested this year, 23 were Xtend varieties.

Bob Scott, Extension weed scientist for the University of Arkansas System Division of Agriculture, said that although the Xtend soybean varieties have done as well as other varieties in research test plots in Arkansas, agronomists’ knowledge was still limited because there is only one season’s worth of data.

“Growers have a lot of questions about these varieties,” Scott said. “Are they going to yield? Are they any good? We have very limited information to tell the growers on pests, disease resistance.

“We do have weed control recommendations in place for pigweed. Dicamba’s not the best pigweed material we’ve ever looked at, but it can control pigweed if you use it right.”

Scott said Monsanto had not released the new Xtendimax with VaporGrip technology to the Division of Agriculture for volatility testing.

Bill Robertson, Extension cotton agronomist for the Division of Agriculture, said that even without using dicamba, the Xtend cotton varieties fared well last year. He said two-thirds of Arkansas’ cotton acres were planted in Xtend, with the crop showing good yield and fiber.

Self-defense

Scott said growers have told him they may begin to grow Xtend crops simply in reaction to their neighbors’ decisions to grow them.

“A lot of growers are very concerned when they find out their neighbor is growing Xtend beans,” he said. “They’re telling us that they’re looking at planting Xtend beans this year, whether they plan to use dicamba or not. They’re growing them in self defense, to avoid drift damage from their neighbors.”

Several years ago, the Division of Agriculture held “PigPosium,” a symposium designed to educate growers and consultants on the problem of glyphosate-resistant pigweed, and best practices for controlling it. Jason Norsworthy, professor of crop, soil and environmental science for the Division of Agriculture, said the next PigPosium is set for Feb. 28 in Forrest City, Ark.

Norsworthy said speakers and experts will provide attendees with updated information on PPO-resistant pigweed and its spread throughout the Mid-South. Weed control measures, application technology and a new software package will be discussed, he said.

Potential issues

Tom Barber, professor of weed science for the Division of Agriculture, said further study is needed to enable him and others to better advise growers. Many weed scientists concur that the EPA’s action is a step in the right direction, but there are still many questions to be answered, which is typical of any herbicide being introduced.

“I think there are issues with buffers to sensitive areas that will make it difficult for growers to make an application in the field without leaving an area out, because looking at the language on the label, there will be areas in most fields that could easily be classified as a sensitive area,” Barber said. “It’s going to be difficult for a farmer to manage pigweed with the Xtend technology, especially in soybeans with this label, and not be off-label in their application.”

Barber said the label registered Nov. 9 by the EPA is actually a supplemental label to an existing master label, which means, among other things, that it comes with a two-year expiration term that has the possibility for a three-year extension once that two-year term expires.

“That way, they can go back after two years and see if some adjustments need to be made,” he said.

Barber said the label also doesn’t allow for tank mixes, which he said could negatively affect the herbicide’s effectiveness.

“We know, as weed scientists, that we need multiple modes of action in the field — residual herbicides in conjunction with Xtendimax, for example, to do a good job of resistant weed management,” Barber said.

“It’s unfortunate that we won’t be able to put those in there as a tank mix,” he said. “We’ll have to make a separate application for those residual herbicides.”

TAGS: Cotton
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