Resistant Pigweed Explodes in Midwest Fields – DTN

A sea of Illinois soybeans surrounds Aaron Hager, but he only has eyes for the waves of Palmer amaranth seedlings stretching before him.

“Southern growers call it the red tide,” said Hager, a University of Illinois weed scientist. “Look across the field early in the season and you can see the swaths of red-tinted weed seedlings. It’s something I hoped I’d never see in central Illinois.”

But here it is and in volumes that resemble college students on a Florida beach during spring break. “I wouldn’t even ask a grad student to count how many are here,” Hager said, trying to find some levity regarding this particular field near Kankakee (about 60 miles south of Chicago). Similar Palmer reports are coming from around the Midwest. Weed scientists in states such as Kentucky, Iowa, Indiana, Michigan, Missouri, Minnesota and Nebraska have all issued requests for farmers to be watchful and report Palmer amaranth discoveries to help track the weed.

Purdue University weed scientist Bill Johnson said Palmer amaranth has spread to nearly every corner of Indiana. “We know of populations in at least 20 counties in this state. It would be hard to drive more than a few counties without finding some,” he told DTN.

A plant born of the desert, Palmer amaranth (pigweed) first transplanted itself in the South and quickly changed Southern agriculture by overcoming popular herbicides. Capable of fast growth and multiple generations, small patches of the weed entangle fields within a few short years. Rotation of chemistry, crops and new practices are helping Southern farmers battle back, but weed control costs have also skyrocketed. Hager fears the same could be in store for Midwest growers if they take the weed for granted.

Hundreds of small plastic flags also dot the Kankakee field Hager is studying — an indication of the many herbicide trials in action. “A custom applicator alerted us to a small population in this field last year,” he said. “We’re not sure how it got here, but we can see how quickly it has exploded.

“We have a lot of information on what happened in the South and management practices they’ve put into practice, but we really have no frame of reference for how it will behave in the Midwest. So far, it looks like Palmer really likes its move north,” he said.

Johnson got serious about keeping an eye on the weed this spring and placed a trail camera at an experimental site to take pictures of the weeds every five minutes. The camera has been positioned to take pictures of two plots, one with a residual herbicide and one without, from planting through mid-July.

“The first couple of weeks did not provide many great pictures as the Palmer amaranth slowly emerged in the unseasonably cold and rainy weather of mid-May. The week of May 30 through June 6, however, provided warm temperatures and timely rain events that promoted rapid growth of Palmer amaranth from small seedlings to 4- to 6-inch plants,” Johnson said. “In a week’s time the Palmer has gone from a very manageable weed to a weed that may only be marginally controlled with our available postemergence herbicides. Given another week, or rather a few days, the Palmer will likely be at an unmanageable growth stage.”

Johnson estimated Palmer began growing 1 to 2 inches per day now once temperatures warmed up. “We have plants putting seed heads on already,” he said. “These plants could mature and drop seed in July. There’s not a lot of dormancy in the [Palmer] seed, so we could see a second generation. We’re just not sure yet.”

Trials so far have shown the merit of preemerge residual herbicides. However, Johnson said, unfortunately the weather didn’t always cooperate and growers experienced more herbicide burn (on soybeans) than preferred. “I’m really worried that will turn farmers off of these products for next year,” he said. “They can’t afford that.”

The weed scientists are also worried growers may wait — or may have waited — too long for postemergence herbicide applications this spring and summer. “See these bigger pigweeds [Palmer amaranth],” Hager said, pointing to the biggest siblings in the weed patches. “The temptation is to wait for the rest of these babies to grow up so they can spray once to avoid multiple herbicide applications. That is exactly what you don’t want to do.”

Good Palmer amaranth control is achieved when the plant is no more than 3 to 4 inches tall, Hager said. “Growers think they know how big that is, but are surprised when they actually measure,” he added.

Johnson said those that have missed the spray window shouldn’t just let the weed go. “Just realize you may not get the control you want if the weed is larger than 4 inches,” he said. “Make sure to use full rates and add a residual to the tank.

“Not every Palmer amaranth weed population is resistant to glyphosate,” he added. “But you might as well assume it is in soybeans given the heavy reliance we’ve had on those [Roundup Ready] programs,” he said.

Ignoring the problem is the worst thing you can do, Hager added. “I don’t want to say the sky is falling. I think we can stay on top of this thing, but let it go and you could be looking at $60- to $90-per-acre herbicide costs to get it cleaned up.

“I think every Southern grower would tell you they wish they’d spent the money upfront to keep Palmer under control,” he said.

What do you do if the pigweed gets away from you? “Pray,” said Johnson. “Then get out the hoe.”

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