Battle secrets for dealing with glyphosate resistance

In a roomful of weed-weary farmers, Marshall Hay preached to the unconverted.

The Kansas State University agronomy graduate student told of how he walks countless fields in the spring where producers failed in their burndown because they were using only glyphosate. For a weed scientist to see farmers make the same mistakes is frustrating.

“Folks, I have a message for you,” he said. “The solution to our herbicide resistance issues is not going to come from a jug. Because, guess what, there are not any new jugs to be had. It’s over.”

The battle to control crop-choking weeds is being fought across almost every farm field across the nation. Fast-growing weeds like kochia and palmer amaranth—or pigweed—are defying even multiple douses of top-selling herbicide Roundup whose primary ingredient is glyphosate.

There are several other herbicides that aren’t as effective today, Hay added, including HPPD and atrazine. For now, producers must look at other options, which includes using integrated strategies to manage weeds.

Pigweed 101

Pigweeds are hardy plants, Hay said, giving the crowd a pigweed 101 lesson.

The pigweed is native to the southwestern United States. It is well adapted to dry conditions.

Pigweeds have a rapid growth rate that peaks in the summer. “When sorghum is shutting down because it is over 100 degrees, pigweed is ramping up,” he said. Palmer amaranth doesn’t reach its maximum level of photosynthesis until 120 degrees Fahrenheit. 

Each pigweed has its own genetic makeup. “Just like we have our own DNA, the same thing goes with pigweeds. They have different characteristics and different strengths,” Hay said. “I can’t think of any weeds more competitive than pigweed.”

He stressed three key takeaways to farmers: How to use cultural practices such as cover crops and row spacing to manage weeds; how herbicide resistance works; and how to develop a management plan regarding herbicide use and weed control. 

Don’t believe everything you hear

While widespread glyphosate resistance is the norm, no herbicide is safe from resistance.

Herbicide resistance, Hay said, results from random genetic mutations. Thus, that weed will survive an application of glyphosate.

Do that a few times and the only ones left in the field are the resistant weeds.

But don’t believe everything you read on Twitter or hear from the neighbor—high pH, drought stress and dust aren’t causing resistance.

“A lot of producers might have a misnomer that herbicides create herbicide resistance,” Hay said. “It’s actually the exact opposite. We create the herbicide resistance by using that herbicide inappropriately. Herbicides don’t cause mutations. Herbicide resistance is always linked to a genetic trait.”

Meanwhile, low dose selection can happen, but for most of the state’s cropping systems, it isn’t at the forefront of the issue.

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“It is more about using that repeated selection year after year—to select for those biotypes that have the genes that can confer that resistance genetically,” he said.

Finding a solution

He said farmers should develop a herbicide plan that includes multiple, effective sites of action.

For instance, tank mixing is better than rotating, he said. “We know it is more effective if you can put two effective herbicides together,” he said, adding farmers should scout their fields routinely.

Hay also researched different weed management strategies at three separate locations last year. The study included three different row spacings—30-inch, 15-inch and 7.5-inch rows. Some of the studies incorporated cover crops.

However, planting narrow rows isn’t the solution, either, he said.

“We have to have a herbicide out there to control the weeds,” he said.

Amy Bickel can be reached at 620-860-9433 or [email protected].