By
khardi j Mukuyu
If it seems like Palmer amaranth weeds are more plentiful this year, you’re not imagining things – particularly if your fields are in the western Corn Belt.
Herbicide-resistant Palmer amaranth (or pigweed, as it’s often called) is moving from the Mid-South into Missouri, Oklahoma, Kansas, and Nebraska, where the University of Nebraska-Lincoln sponsored a Palmer amaranth Resistance Field Day July 14.
Take it from someone who knows:
Once pigweeds get established, it is mighty tough to get rid of them.
“This weed is the most troublesome weed in the U.S.,” says Jason Norsworthy, professor and endowed chair of weed science at the University of Arkansas and keynote speaker at a Palmer amaranth field day near Shickley, Nebraska, July 14. “Once it gets established, it spreads like wildfire.”
Norsworthy should know.
Pigweeds resistant to HPPD-inhibitors, ALS-inhibitors, glyphosate, and atrazine sweep across the state. Small wonder – the weed can grow up to 3 inches a day, reach 6 to 8 feet in height, and each female plant can produce more than 1 million seeds, which quickly overpowers the crop.
"Palmer amaranth does not play around,” Norsworthy adds. “If you don’t stay on top of it, it will put you out of business, and in a hurry. It is the perfect weed.”
Jason Norsworthy
Norsworthy says resistance to chemical weed control occurs due to use of the same means of weed control year after year.
For instance, when Roundup Ready soybeans were introduced in 1996, chemical weed control was easy.
Over time, however, farmers tended to use only glyphosate (the active ingredient in Roundup) on weeds. Meanwhile, the Roundup Ready gene was inserted in other crops, including corn and cotton.
Before long, weeds adapted to the herbicide. To date, glyphosate-resistant Palmer amaranth has been found in 30 states.
What Does Resistance Look Like?
Norsworthy reckons if herbicide-tolerant pigweeds aren’t yet present in your fields, they soon will be. Here’s how to know resistance is developing:
Escapes. If a herbicide controls all the weeds in the field but one or two weeds are clean (and should not be) . . . you might have resistance.
Growing patches of green. If a small patch of weeds has been treated and is still alive, and the patch gets bigger over time . . . you might have resistance.
If you have Palmer amaranth in the field, and some live plants are right next to dead plants after treatment . . . you might have resistance.
When this happens, you have to try other means of controlling the weed.
What Do You Do?
Stay on top of it, Norsworthy explains. It is imperative that before planting, fields be weed-free. A good burndown herbicide is absolutely essential. Or, use tillage implements to get the field clean. After planting, scout the field religiously. In fact, he urges growers to make notes on where problem areas occur the year prior, so you can be sure to scout the areas with a pigweed problem.
“Next, apply the recommended rate at the correct size," the weed scientist says. The only chance to kill pigweeds during the cash-crop growing season is to get ’em before they get 3 inches tall. Herbicide resistance develops when growers use a rate less than label, and the following years some of those plants survive. “Timing is critical, and no reduced rates,” he warns.
Growers need to understand the phrase multiple effective modes of action. Chemistry diversity is essential, and farmers must know which products work on Palmer amaranth.
Finally, exploit the crop’s biological weakness. Pigweeds are less apt to thrive when growers use narrow rows, as opposed to 30-inch rows. Rotate crops.
Adding cover crops can help prevent pigweeds from emerging in the spring. Turning over the soil with deep tillage can prevent pigweed seeds from growing. (The seeds will sprout when shallow tillage occurs.) “Moldboard plow and then add a cover crop,” Norsworthy suggests.
“That strategy shows a 93% reduction in pigweed emergence compared to no-tillage.”
Norsworthy says growers need to be diligent at preventing seed production. Mow roadsides, and prevent seed from being spread from field to field (or farm to farm, in the case of used equipment trading).
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