It is no secret that Palmer amaranth, commonly known as pigweed, is one of the most troublesome of resistant weeds.

In 2016, Syngenta agronomists called it the "most notorious" weed. They are now warning that pigweed will continue to hold its place as the No. 1 weed to watch in 2017.

Palmer amaranth is not only spreading northward to new states, but it is also showing resistance to more herbicides and more modes of action.

In soybean fields, it is one of the most threatening weeds because it has been shown to reduce yields up to 79%. In cornfields, the threat is even more decimating — up to 91% of yield can be lost.

Resistant Palmer amaranth was found for the first time in South Dakota in 2015 and in Minnesota last fall. It has not yet shown up in North Dakota, and researchers at North Dakota State University Extension are urging landowners to fight to keep it away.

According to researchers at Purdue University Extension, the spread of the weed is accelerated by its ability to produce nearly a half million seeds per plant — seeds that are tiny and can be blown long distances.

Populations of Palmer amaranth resistant to glyphosate were first documented in Kansas five years ago. At that time, these populations were limited in range to isolated areas of south-central Kansas. Glyphosate-resistant Palmer amaranth has gradually expanded in the last several years, and it now appears to be increasing rapidly, especially through the central part of the state.

Other states have similar problems.

“Palmer amaranth is quickly moving across a larger geography than we’ve seen with any other resistant weed. The movement is occurring through equipment, feed, seed and even waterfowl,” says Kevin Bradley, associate professor at the University of Missouri.

States are now also reporting the first confirmed cases of multi-herbicide-resistant Palmer amaranth. This October, the University of Missouri identified a population of Palmer amaranth with resistance to both glyphosate and PPO-inhibitors. The more a mode of action is applied, the more easily Palmer amaranth adapts before quickly spreading herbicide-resistant genes. The combination of its ability to develop resistance, its aggressive and competitive growth, and its extended emergence period makes Palmer amaranth especially difficult to control.

“You’ve got to have a certain amount of fear to attack resistant weeds. I had to see up close and personal the damage that Palmer amaranth can do,” says Tim Hambrick, agriculture Extension and NC Cooperative Extension agent. “That was a turning point for me in understanding the damage resistant weeds can cause and the time and effort needed to fix those fields over several years.”

To avoid spreading Palmer amaranth to nearby fields and other states, growers can regularly mow ditches, waterways and field borders; they should also meticulously clean machinery such as combines.

To prevent or delay resistant Palmer amaranth in fields, growers can adopt an integrated weed management program that includes both a comprehensive herbicide portfolio and complementary cultural practices such as crop rotation.

“Diversity is the key in trying to maintain the sustainability of the herbicides we have available, so they remain effective for the future,” says Don Porter, Syngenta herbicide technical lead. “We must be good stewards of the chemistries and learn from our experiences — and mistakes — over the past 20-plus years.”

Syngenta offers growers an effective weed control program in soybeans that starts with BroadAxe XC or Boundary 6.5 EC herbicides for preemergence weed control of Palmer amaranth with long-lasting residual.

“When resistant Palmer amaranth came along, we started using Boundary herbicide as a preemergence herbicide and have been doing so for the last three years because of its really good Palmer amaranth and annual grass control,” says Trey Koger, a grower in Belzoni, Miss.

“Boundary helps us go into the season clean, and Flexstar GT 3.5 herbicide helps us control the grass and broadleaf weed species,” Koger adds.

In corn, Syngenta has helped manage resistance threats with premix products like Acuron and Acuron Flexi herbicides, which contain multiple, effective modes of action. 

“We want to stay ahead of Palmer amaranth, knowing that we already have other pigweed species we’re struggling with,” says Paul McGuire, a grower in Urbana, Ohio.

“We decided to try Acuron this year because we wanted a product with longer-lasting residual control. With Acuron, we were able to start clean ahead of planting and then depend on its strong residual to keep even our toughest acres clean.”

To learn more about weed resistance management tools in soybeans and corn, visit syngentaus.com.

Source: Syngenta