The most notorious weed, Palmer amaranth, will be the number one threat to growers this year, according to Syngenta agronomists. 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, in a recent release.
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.
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.
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. In corn, Syngenta has helped manage resistance threats with premix products like Acuron and Acuron Flexi herbicides, which contain multiple, effective modes of action.
“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.”