Farmers may have to revise burndown strategy

University of Tennessee weed scientist Larry Steckel is rethinking his burndown strategy in light of controlled weed trials in which an application of dicamba herbicide failed to control them.

For the last two decades dicamba has been used extensively as a pre plant burndown on almost all Tennessee cotton and soybean acres, Steckel said.

"It was often used at 8 to 12 oz/A of a Clarity-type product 14 days before planting," he said. "I know it has become tradition to use it in this fashion, but I believe this should be changed for three reasons."

The initial use of dicamba utilized in a burndown was to target horseweed.

"Of course at the time, it was a weed with no effective in-season herbicide option for control. However, that is no longer the case," Steckel said.

Most soybean and cotton acres planted in Tennessee are planted to an Xtend variety and the rest are planted to Enlist varieties. Depending upon which technology, Steckel said horseweed can now be easily removed in crop with a dicamba, 2,4-D or Liberty application.

Second, it is becoming abundantly clear that the decades long use of dicamba plus glyphosate as a burndown has selected for weeds that can survive and indeed thrive in that environment.

Ryegrass, poa, junglerice, barnyardgrass and goosegrass are all now regularly escaping burndown applications of dicamba plus glyphosate.

"I would suggest to go with glyphosate or glyphosate tank-mixed with clethodim to have the best chance for success controlling those grasses before the crop emerges," he said. "Also consider using a residual that has good grass activity."

Steckel says Verdict would a good option here as it has both burndown activity and grass residual. Others like Anthem Max, Dual Magnum and Zidua would also be good choices.

Third, from a financial standpoint Steckel points out that producers should save the money on using dicamba before planting or even at-planting as it will likely hinder grass control.

"In 2019 grasses often, and Palmer amaranth in some fields, escaped the initial POST application of dicamba plus glyphosate. As such there were often second or third POST applications needed to control those escapes," he said.

Money used on dicamba in the early burndown could be put to better use on those follow-up applications, he said.

Steckel said that recent greenhouse screens of Palmer amaranth, junglerice and barnyardgrass plants grown from seed collected from those escapes in 2019 shows that the grasses and Palmer control will be even more of an issue this spring.

University of Arkansas weed scientist Jason Norsworthy told DTN that "further research is confirming that dicamba is in the initial stages of failure against Palmar amaranth". However, they shied away from officially calling the plant resistant to the herbicide.

"It is very apparent that dicamba plus glyphosate have taken a step backwards with respect to consistent control of many grasses, and in some cases Palmer amaranth," Steckel said. "Therefore, starting clean has never been more important."

He noted that gramoxone used right behind the press wheel will be the most important herbicide used on all no-till soybean and cotton acres in 2020.