Putting dicamba to the test

Written by Allison Jenkins on .

ON OCT. 31, THE EPA MADE its much-anticipated announcement that dicamba registration will be extended for over-the-top use on cotton and soybean plants genetically engineered to resist the product.

The approval is for two years, and the EPA will consider the issue again in 2020. In its news release, EPA Acting Administrator Andrew Wheeler said the registration extension was made because dicamba has proven to be a valuable weed-control tool for America’s farmers.

The decision is welcomed by MFA Director of Agronomy Jason Weirich, who agreed that growers need the dicamba-tolerant technology in the Roundup Ready Xtend crop system as a weapon against the increasing number of herbicide-resistant weeds.

“We need dicamba technology to combat weed resistance, but we have to be good stewards,” he said. “Protecting the technology for the future is important, not only from an economic standpoint but also an environmental standpoint.”

This past year, Xtend seeds, which are tolerant to both dicamba and glyphosate, were planted on some 40 million acres, representing nearly half of all soybeans and cotton in the United States. Bayer, which now owns Monsanto, expects that total to grow to 60 million acres in 2019.

In the two seasons that Xtend technology has been commercially available, growers have seen success in both weed control and yield response, Weirich said. However, there have also been widespread complaints that dicamba herbicides drifted and harmed non-tolerant crops.

To help mitigate those issues, EPA imposed additional restrictions on dicamba spraying. Starting in 2019, only certified pesticide applicators will be allowed to spray the chemical, and applications must end 45 days after planting soybeans and 60 days after planting cotton.

MFA placed its own stringent protocols on the sale and use of dicamba during the last growing season to minimize the risk of damaging non-targeted plants. A network of “sentinel plots” across MFA’s service territory allowed crop scouts to track soybean growth and provide timely information to applicators about crop progress. MFA applicators were alerted to stop spraying when the majority of soybeans in their area reached the R1 reproductive stage, when dicamba injury can do the most harm to non-target plants.

That protocol will continue in 2019, Weirich said.

“We consider our system to be a success,” he said. “In 2018, we didn’t have nearly as many complaints as we had in 2017 and actually sprayed more acres of Xtend soybeans.”

Weirich and his team of agronomists are also conducting their own research to help understand more about what causes off-target dicamba movement. Trials at MFA Incorporated’s Training Camp site in Boonville, Mo., this past summer helped shed some light on best practices in applying dicamba products, Weirich said.

“When we set up these plots, we had just come out of 2017 season, where we saw a significant amount of off-target movement, and we were looking for reasons why,” Weirich explained. “This trial specifically looked at different tank mixes and their role in dicamba volatility. There’s been a lot of talk about how ammonia sulfate, different salts in glyphosates and other issues could cause off-target movement.”

To encourage conditions for dicamba volatility, Weirich and the MFA agronomy team used 15-by-5-foot plastic-covered hoop houses placed over soybeans at the V3-V4 stage in mid-June. A flat of soil taken from the site was filled to capacity with moisture and treated with either straight dicamba or a dicamba mix. The flats of soil were sprayed more than 50 yards away from the plot so there was no risk of contamination.

The trial included 14 different treatments. Each flat was placed underneath a hoop house over two rows of soybeans and left for 24 hours.

“The temperature got up to 97 that day,” Weirich said. “It was hot and humid. This trial was designed to make the environmental conditions conducive to volatility.”

Six of the 14 flats were treated with straight dicamba: Clarity, which is not labeled for over-the-top application on soybeans, and two approved formulations, Engenia and XtendiMax. Each was applied in both a 1⁄2-pound and 1-pound rate.

“Clarity showed visible damage to the ends of the 15-foot tunnel. It’s a non-improved formulation, and it definitely showed increased volatility,” Weirich said. “With Engenia and XtendiMax, movement was very minimal, no more than 1 foot away from the flat. We couldn’t tell much of a difference between the two products.”

Other lessons learned from the trial, Weirich said, were the detrimental effects of ammonium sulfate (AMS), commonly used as a water conditioner in glyphosate tank mixes. In the Training Camp plots, a full rate of AMS with both XtendiMax and Engenia showed significant damage compared to applications of straight dicamba.

“We saw severe cupping and stacked nodes a good 5 to 7 feet outside the tunnels on both ends,” Weirich said. “We also tested a 1⁄10 rate of AMS, and also had significant damage. In fact, visually, I couldn’t tell the difference between the full rate and 1⁄10 rate.”

The take-home message, he continued, is to meticulously clean out application equipment before spraying an Xtend field, even if the sprayer is moving from a Roundup Ready field to a Roundup Ready Xtend field.

“What this tells us is that a very small amount of AMS is detrimental to Xtendimax or Eugenia,” Weirich said.

Another section of the trial compared different formulations of glyphosate with varying salt content, which has also been suspected in dicamba volatility. One plot looked an application of Xtendimax with Roundup PowerMax and another tested Xtendimax with Durango, a glyphosate product from Dow AgroSciences that is not labeled for application with dicamba.

With the XtendiMax-PowerMax mix, there was movement to the end of the tunnels with increased damage near the flats compared to the straight dicamba product at the same rate. With Durango, however, there was significantly increased injury.

“There are some best practices out there to minimize volatility, and at MFA, we’ve taken the stance that we will use PowerMax, based on the research and data out there,” Weirich said. “This trial shows us there is a big difference in the various salt formulations of glyphosate and the effect on volatility.”

Weirich acknowledged that this trial was a one-year demonstration with the main goal of educating MFA employees about the risks associated with off-target dicamba movement and the importance of proper tank cleanout. He said MFA plans to continue the study next summer.

“Overlapping residuals are still key for the future of dicamba or any technology coming down the pipeline,” he said. “At MFA, we’re doing everything we can to manage this technology by training our applicators to know the risks and follow all federal and state guidelines. We need this technology, and we want to make sure we are good stewards.”

For more details on the dicamba movement trials at MFA’s Training Camp, visit mfa.ag/DicambaResearch.

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