New choices are available for herbicide-tolerant postemergence weed control for soybeans in 2021. As you reevaluate your options, what things should you consider?

Bill Johnson, a Purdue University Extension weed control specialist, and Jeff Nagel, an agronomist with Ceres Solutions based near Lafayette, Ind., share their thoughts. Read more from them in Explore options for herbicide-tolerant soybeans in 2021 and Custom-build a soybean postemergence system. Both suggest you shouldn’t forget about future weed resistance when making decisions about which herbicide-tolerant systems to use.

“There are reasons to worry about future weed resistance,” Johnson says. “If we continue down the path of heavy reliance on post herbicides and multiple applications of post herbicides, we will end up with the same result as we had with other herbicides. These include the ACC class, ALS herbicides and glyphosate.

“Look for resistance within three to seven years of introduction of the trait. You heard it here first.”

Nagel agrees. “Glyphosate made weed control too easy, and we forgot about implementing sound weed management principles, which led to resistant weeds,” he says. “Waterhemp and Palmer amaranth should be on every farmer’s radar. Minimizing weed seed production is imperative, and we can’t rely on post applications only.”

Nagel notes that resistance has now been documented in waterhemp to seven different site-of-action herbicides, including dicamba and 2,4-D.

“Essentially, we have three relatively effective products left to control waterhemp and Palmer amaranth postemergence: dicamba, 2,4-D and glufosinate,” Nagel says. “While we don’t have documented waterhemp resistance yet to dicamba and 2,4-D in Indiana, we’ve seen variable results even when everything has been done right.”

He adds that Liberty rates today are higher than just a few years ago. He believes it’s just a matter of time before growers could face widespread resistance to all three of these herbicides.

“That’s particularly true if we don’t implement sound weed management programs,” he says. “Those include many things but begin with using residual herbicides as a foundation for weed control.”

Examples

Nagel uses hypothetical examples to illustrate how growers could inadvertently encourage future weed resistance.

“Suppose farmer Joe tills in the fall and makes another tillage pass in the spring and starts with a clean seedbed,” Nagel begins. “He plants beans in mid-April and doesn’t apply a soil-applied residual herbicide. When soybeans are about V3 and some waterhemp has emerged, he post applies Liberty, Roundup and a residual Group 15 herbicide, such as Dual, Warrant or Zidua, on LibertyLink GT27 soybeans.”

The step Joe omitted was no residual herbicide at or near planting. The only mode of action controlling emerged waterhemp is Liberty. “This approach will select out for waterhemp plants that are more tolerant/resistant to Liberty over time,” Nagel says.

In a second example, farmer Bob no-tills soybeans but didn’t get his fall-applied herbicide application made in 2019. He planted Roundup Ready 2 Xtend soybeans in 2020 and applied a labeled dicamba herbicide and glyphosate in April, Nagel says.

It controlled winter annuals, including overwintered marestail. To save money, Bob skipped spraying in fall 2020 for winter weeds and plans for the same burndown program in 2021 since it worked so well.

Nagel says the only mode-of-action herbicide controlling the overwintered marestail is dicamba. Again, this will lead to selecting for marestail that is more tolerant/resistant to dicamba over time. “If the scenario sounds familiar, it’s exactly what happened to glyphosate,” Nagel cautions.