Resistant Weeds Continue Their March Across the Landscape

Driving across many parts of the Midwest this past summer, crop field watchers were likely to find an unexpected sight among the tall rows of corn and soybeans — weeds. In many cases, weeds such as ragweed, marestail, and waterhemp were easily two to three times the height of the surrounding crops, especially in soybean fields across states such as Indiana and Ohio.


According to Dr. David Hillger, Enlist Field Specialist for DowDuPont, this “explosion” of tall weeds across the nation’s Corn Belt in 2017 isn’t that surprising, considering how wet parts of the Midwest remained throughout the spring planting season.

“When it’s wet, some growers may consider skipping burndown,” says Hillger. “A rainy spring can make it difficult to clean up fields before planting. Still, it’s usually best to delay planting and take time for a burndown. Soybeans in particular are somewhat forgiving of planting date.”

Of course, besides a wet spring, all these weeds in crop fields is an illustration of an even bigger problem for today’s agriculture — the spread of herbicide-resistance. By last count, there were just under 250 confirmed herbicide-resistant weeds present across the U.S., impacting crop fields in virtually every state. A handful of states — California, Nebraska, Arkansas, Illinois, and Michigan — have more than 20 different types of herbicide-resistant weeds growing within their borders. In fact, when CropLife® magazine recently surveyed weed Extension representatives across the country, 100% of them said that herbicide-resistant weeds “remain a major problem in our area.”

“Glyphosate-resistance is widespread,” said Dane Bowers, Herbicide Technical Product Lead for Syngenta, during a spring 2017 interview. “And this is not limited to just glyphosate. All herbicides are now at risk.”

The Worst Weeds

Although there are a host of herbicide-resistant weeds for agriculture to contend with, by far the most troublesome one seems to be Palmer amaranth (or pigweed). In the CropLife Weeds survey, almost half of respondents (45%) indicated this single weed type was the most difficult to control in their parts of the country.

A recent survey of 200 weeds scientists conducted by the Weed Science Society of America (WSSA) came to a similar conclusion regarding Palmer amaranth. Here, Palmer amaranth topped the list of “most troublesome weeds,” outpacing equally hard-to-control types such as common lambsquarters, marestail, morning glory, and waterhemp. On the most common weeds list, Palmer amaranth finished in fourth place, behind common lambsquarters, foxtail, and morning glory.

“Weed scientists have confirmed multiple cases of herbicide resistance in Palmer amaranth, common lambsquarters, marestail, morning glory, waterhemp, and common ragweed, except for the morning glories, where there is suspected resistance to glyphosate,” said Dr. Lee Van Wychen, Science Policy Director for WSSA, of the survey results. “While each of these species has evolved traits that make them widespread and tough competitors in broadleaf crops such as soybeans and cotton, there is no question that their difficulty to control with herbicides has pushed them to the top of the list in the survey.” He went on to add that although Palmer amaranth was only listed as the most troublesome weed in cotton, it ranked first in the overall survey based upon the number of respondents who cited it as a problem.

“Palmer amaranth differs from other pigweed and waterhemp species due to its rapid growth,” says Dave Ruen, Field Scientist for DowDuPont. “Farmers cannot let this weed get ahead of the crop.”

Based upon the data, Palmer amaranth is an extremely hardy weed, dropping an average of 100,000 seeds each season that can quickly spread its presence to neighboring fields. At last count, Palmer amaranth had developed resistance to multiple herbicides including glyphosate, with more appearing all the time.

“PPO-resistant Palmer amaranth was identified in our geography in fall 2016,” says Dr. Karla Leigh Gage, Assistant Professor of Weed Science and Plant Biology at Southern Illinois University Carbondale. “This was the first full growing season over which we tried to make growers aware of this potential issue.”

Among weed scientists surveyed by CropLife, waterhemp ranked just behind Palmer amaranth in herbicide-resistance issues at 33%. According to DowDuPont’s Ruen, waterhemp can cause devastating yield loss if left uncontrolled. “Research from the University of Illinois has shown waterhemp can cause up to 40% yield loss in soybeans,” he says. “Commonly known as a summer annual, waterhemp germination and emergence also can extend late into the season.”

Finishing tied in third place on the CropLife Weeds survey were marestail (horseweed) and ragweed, both cited by 11% of respondents as the most troublesome weeds to control. In particular, says Dr. Jeff Ellis, Field Scientist for DowDuPont, marestail has particular notoriety among scientists since it was the first weed to show glyphosate resistance back in 2000. “If left uncontrolled, herbicide-resistant marestail can present huge challenges for farmers at planting and throughout the season,” says Ellis. “A single female marestail plant can produce approximately 200,000 seeds that are transported by wind, perpetuating the spread of herbicide-resistant populations.”

The New Control Options

Naturally, the agricultural marketplace has spent years and millions of dollars to find new options for fighting back against herbicide-resistant weeds. The first of these began appearing a few years ago in the form of blended herbicides such as Acuron from Syngenta. These new offerings mixed three or four different active ingredients into their formulations featuring different modes of action in an effort to keep weeds under control.

Just this year, dicamba-resistant crops began appearing on a widespread basis. Supported by Monsanto, DowDuPont, and BASF, these crops offered growers the option to use dicamba herbicide in-season as a way to control herbicide-resistant weeds.

However, many weed scientists were quick to caution applicators and growers regarding the use of these products/crops because of the potential for off-target drift damaging nearby non-resistant crops. “Concerns about drift led the U.S. EPA to issue time-limiting registrations for the auxin herbicides dicamba and 2,4-D of two years and five years, respectively,” said Dr. Kevin Bradley, Associate Professor at the University of Missouri. “Unless growers show they can use these herbicides as labeled, the registrations could easily be revoked.”

Since that time, unfortunately, there have been some widely reported issues with dicamba application and off-target movement. In fact, more than 1,000 complaints have been filed with state officials in such places as Arkansas, Missouri, and Tennessee regarding suspected dicamba drift problems.

Some of the weed scientists surveyed by CropLife noted this as well. “Dicamba cropping systems in our area performed just as expected in 2017,” says Dr. Bill Johnson, Professor of Weed Science at Purdue University. “It offered good control of ragweed and provided some help on marestail, waterhemp, and Palmer amaranth, but there were too many instances of off-target movement.”

Still, the marketplace can expect more such efforts at herbicide-resistant weed control from suppliers in 2018. Combining both blended product and new cropping system strategies, Syngenta is hoping to introduce Tavium plus VaporGrip Technology. This product blends dicamba and S-metolachlor and can be used on Roundup Ready 2 Xtend soybeans and Bollgard II XtendFlex cotton.

And now that they’ve received formal approval for export to China, the 2,4-D-based Enlist cropping systems should begin appearing in the marketplace for the 2018 season. “According to experts, resistant weeds can be found today on more than 100 million acres of farmland in the U.S.,” says John Chase, U.S. Commercial Leader, Enlist Weed Control System for DowDuPont. “We’ve already seen some success in this fight with the launch of Enlist cotton earlier this year, and it is our hope that corn growers will be able to gain this same level of success against stubborn weeds in their fields.”