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Meet the $50 per acre weed

Preemergence residuals need to be the backbone of soybean herbicide programs to manage waterhemp, says Purdue’s Bill Johnson

One thing you can say about 2019—it kept weed scientists like Bill Johnson busy. The Purdue University Extension weed scientist’s days were filled with questions about how to work around weather-caused disruptions in burndown and preemergence residual herbicide plans. In some cases, farmers planted first and then concentrated on terminating weeds or cover crops. 
“We were playing catchup all season long,” said Johnson to those attending this month’s Top Farmer Conference at Purdue University. 
This year poses some unique weed management challenges, given all the prevented planted acres that occurred in 2019. Some farmers dealt with weed challenges on these acres with cover crops or tillage. 
“The big surprise will come next year for folks who did not do anything on those acres,” he says.  “Waterhemp and ragweed that went to seed will be major headaches.”

Change in Thinking
Postemergence treatments often garner the lion’s share of attention when it comes to herbicides—the Roundups (glyphosate) , Libertys (glufosinate) , Enlists (2,4-D choline), and the Xtends (dicamba) of the weed control world.
Johnson and other weed scientists are trying to shift the attention to the heavy lifters of the herbicide galaxy—preemergence residual herbicides. 
“The backbones (of managing weeds) are residual herbicides. The postemergence herbicides should be for cleaning up the weeds that residuals do not get,” says Johnson. These programs aren’t cheap. 
“We have to get used to waterhemp being a $50 per acre weed, “says Johnson. “You just can’t rely just on postemergence treatments. You have to be of the mindset to have overlapping residuals.” 
In waterhemp’s case, tank mixes containing Valor, Authority or Sharpen (Group 14 herbicides) can serve as a base residual. Then, farmers can buy products or tank mix their own to gain a second herbicide site of action, whether it is Sencor (Group 5) or one of the group 15 herbicides (acetochlor or metolachlor), Johnson says. He then advises coming back with an early postemergence herbicide layered with a Group 15 herbicide. 

Weed management on prevent plant acres may be slightly tweaked, but Johnson says a full rate of a preemergence residual herbicide still needs to serve as the foundation. 

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