Syngenta agronomists warn that last year’s most notorious weed, Palmer amaranth, will continue to hold its place as the No. 1 weed to monitor in 2017. Not only has the weed continued to spread northward to new states, it also is now showing resistance to multiple herbicide modes of action.
Palmer amaranth has earned its title as one of the most threatening weeds because it has shown the ability to reduce soybean yield by up to 79 percent and reduce corn yield up to 91 percent. Found for the first time in Minnesota this fall, Palmer amaranth hasn’t spread to North Dakota yet, and researchers with the North Dakota State University Extension urge landowners to keep it that way. This comes a year after South Dakota reported its first detection of Palmer amaranth in 2015.
The weed’s spread is accelerated by its ability to produce nearly half a million seeds that are relatively small and travel easily, according to Purdue University Extension.
“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,” said 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,” said 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,” said 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 pre-emergence weed control of Palmer amaranth with long-lasting residual.
“When resistant Palmer amaranth came along, we started using Boundary herbicide as a pre-emergence herbicide and have been doing so for the last three years because of its really good Palmer amaranth and annual grass control,” said Trey Koger, a grower in Belzoni, Mississippi.
“Boundary helps us go into the season clean, and Flexstar® GT 3.5 herbicide helps us control the grass and broadleaf weed species,” Koger added.
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,” said 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.”