Dicamba-resistant crops offer weed management options

Frances Gould, Bogren, Richard C.  |  10/24/2014 12:03:31 AM

AgCenter engineer Randy Price examines a spray boom on a plot sprayer he uses to test and evaluate different types of nozzles. (Photo by Rick Bogren)

With the availability of the dicamba-resistant crop technology a year or more from commercial use, AgCenter weed scientists have been investigating ways to best use the technology in Louisiana crops.

Although the availability of dicamba-resistant crops will provide alternative weed management options, the risk of off-target movement of herbicide to sensitive crops is of concern, said AgCenter weed scientist Jim Griffin.

Glyphosate has been and still is an effective herbicide. Its overuse, however, has resulted in glyphosate-resistant weeds, which is a major problem in adjoining states, Griffin said.

Glyphosate-resistant johnsongrass and Palmer amaranth populations have been documented in Louisiana. "We are hopeful that new technologies on the horizon will allow us to better manage resistant weeds as well as other weeds that are less sensitive to currently used programs," he said.

Dicamba-resistant soybeans will be marketed as the Roundup Ready Xtend Crop System and will be tolerant to both glyphosate and dicamba. It looks like it may be 2016 before varieties are commercially available, Griffin said.

The challenge with this new technology is that recommended soybean varieties are extremely sensitive to dicamba. Griffin’s research has shown that one-thousandth of the use rate of dicamba resulted in a 3 percent yield loss in soybeans not resistant to dicamba. And one tenth of the use rate reduced yield more than 50 percent.

"It’s a little like Brylcreem," Griffin said, referring to a men’s hair product heavily promoted in the ’50s and ’60s. "A little dab’ll do ya."

To put this in perspective, soybeans are more sensitive to dicamba than grasses are to glyphosate. So the concern for crop injury due to off-target drift will be of particular importance when dicamba is applied adjacent to sensitive soybeans. Horticultural crops and ornamentals are also of concern, Griffin said.

Sensitive plants also run the risk of exposure through spray tank contamination. It will be imperative that proper sprayer cleanout procedures be followed. "We want to help growers prepare for what to expect," Griffin said.

Although the new technology has not yet been approved yet by USDA or EPA, the labels for the dicamba products to be used in conjunction with the technology are expected to be more restrictive than other labels in the past, Griffin said.

He expects the label to include prohibitions on aerial application, requirements for buffer zones around the treated crop, specifications as to nozzle type and limitations on the height of the spray boom above the crop canopy. "

At this time, we have not seen any labels," Griffin said. "And they won’t come out until EPA approves them." But they will be restrictive with the goal being to keep the herbicide on target.

"We have had a lot of years to look at this," Griffin said. "We are prepared to address any problems that may occur. Right now it is important that we educate consultants and growers on what to expect and how to best use the technology to aid in controlling weeds."

As a result, the AgCenter has formed a herbicide technology task force to provide critical information to Louisiana farmers before the new products hit the market.

Composed of weed scientists, commodity specialists and others in the AgCenter, the Louisiana Department of Agriculture and Forestry Pesticide Division and representatives from the Louisiana Agricultural Consultants Association, the task force will focus on education about the new herbicides, herbicide drift and weed resistance, said AgCenter pesticide safety education coordinator Kim Pope, who’s co-leading the group with AgCenter weed scientist Daniel Stephenson.

"Each chemical company has made a commitment to provide background information on their chemicals, including the herbicide-resistance issue and how their products will fit into a production program," Pope said.

In addition to learning about the chemicals, producers will have the opportunity to learn about new spray nozzles that reduce drift, said AgCenter engineer Randy Price.

"We want spraying systems that produce droplets, not ‘driftable fines,’" Price said. Worn-out nozzle orifices shatter droplets and create lightweight droplets that could be carried across a field. Research has shown that these droplets can be carried 50 to 60 feet from the target area in normal winds.

With sprayer booms requiring as many as 60 to 70 nozzles for proper application, choosing the proper nozzle is critical, Price said. And boom height should be within 20 inches of the crop surface to minimize drift potential.

"Nozzle pressure is also critical to droplet size, and growers should operate nozzles in the recommended range to help eliminate drift," he said.

Managing drift with these products is important and will require growers to include buffer zones between target crops and sensitive areas, Pope said. "This is important because Louisiana has so many different crops that are grown next to each other." Rick Bogren

This article was published in the 2014 Louisiana Soybean & Grain Research & Promotion Board Report.

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