Field Insights

Plan Programs to Help Fight Herbicide-Resistant Weeds

Managing herbicide-resistant weeds in corn requires a multifaceted approach.

Corn popping through freshly planted soil gives farmers a little spring in their steps. There’s nothing quite like it for folks who love farming.

All too often, however, that step goes flat when farmers see tough weeds emerge alongside those corn plants. Those weeds reduce yield starting with that first leaf. With widespread herbicide resistance increasingly commonplace in weeds, farmers must develop management plans to wipe out weeds and protect their yield.

Crop rotation, which in corn country generally means planting soybean in alternate seasons, is one tactic to consider. Crop rotation gives farmers the option to use different herbicides, increasing the modes of action available to combat weeds.

“A comprehensive weed control program should include tools to reduce incidence of herbicide resistance,” says Sudeep Mathew, mid-Atlantic area agronomic service representative at Syngenta. “In the mid-Atlantic, we not only have resistant Palmer amaranth, common ragweed and grasses like barnyardgrass, foxtail and Italian ryegrass, we also have resistant horseweed, which was first identified in this area.”

Resistant weeds vary between regions. In North Dakota, waterhemp and kochia show resistance to both glyphosate and ALS inhibitor herbicides, says Joe Ikley, Ph.D., assistant professor and extension weed specialist at North Dakota State University.

Waterhemp also causes problems in the western Corn Belt, says Bill Johnson, Ph.D, Purdue University weed science professor. However, Johnson points out, farmers farther east are more concerned with weeds like giant ragweed, foxtail, lambsquarter and morningglory.

In the mid-South, Palmer amaranth resistance to glyphosate appeared more than a decade ago and presents a particular challenge.

“Mississippi may rank at the top of the list for resistant weeds,” says Tripp Walker, Syngenta area agronomy service representative in Mississippi and north Alabama. “I think there are 13 weed species here with herbicide resistance. To get control now, you have to diversify your strategy.”

Plan Intensively

Starting with a solid weed management plan is essential.

“Crop rotation is really your biggest strength,” Walker says. “If you’re having a resistant-weed problem, rotate to a monocot crop like corn on some acreage. Then you have more tools for effective weed control. You can reduce the weed seed bank that way.”


We spray Acuron as we plant, right behind the planter. That way, the ground is fresh. There’s a little moisture in the soil, which helps activate the herbicide.

Al Hill Owner of High Yield Farms in Deep Run, North Carolina

Walker and Mathew recommend a two-pass herbicide approach on fields where potentially resistant weeds are present. They say the best approach is to consider these weeds herbicide-resistant from the beginning.

“You have to think differently these days. Using multiple modes of action is the key,” Mathew says. “Be sure to use herbicides effective against the weeds. If you use a preemergence herbicide at corn planting, plan to come back within 21 to 28 days with a post-emergence application of a different herbicide. Overlay a residual product before canopy closure.”

According to Walker, growers should design weed-control programs that include pre- and post-emergence applications.

“You should design a total weed control program with a preemergence herbicide followed by a post-emergence material as a priority, and use multiple modes of action,” Walker says.

Additionally, Johnson says, “Scout fields. See what’s leaking through.”

For effective control and to anticipate herbicide resistance, Johnson says, “Most Indiana farmers use a glyphosate product along with something else as a post-emergence program.”

Acuron® herbicide from Syngenta is effective preemergence because it combines four different active ingredients with three modes of action. Acuron contains atrazine, mesotrione, S-metolachlor — also sold under the brand names of AAtrex®Callisto®, and Dual II Magnum® herbicides, respectively, — and bicyclopyrone, a novel active ingredient designed to improve weed control and consistency. The four active ingredients together provide broadleaf and grass control for over 70 weed species.

Ikley likes to see farmers use a multiple mode-of-action approach like that.

“When you get three or four active ingredients spanning three modes of action, you’re more likely to help your situation with resistance,” he says. “Most of our soybean farmers have gone to Roundup Ready 2 Xtend® or Enlist E3® soybean technology, which helps with resistance.”

Choose Wisely

It is important for farmers to stay alert and inspect your fields for escaped weeds. Johnson says weeds like velvetleaf, cocklebur and jimsonweed, which seemed to be easily controlled a few years ago, have returned in some areas. In North Dakota, Ikely says he’s seeing more wild oats and green foxtail. Waterhemp, too, seems to be a never-ending challenge — with seeds arriving in floodwaters as well as being moved by harvest equipment.

Fortunately, growers have choices. Halex® GT and Acuron® GT herbicides are effective post-emergence herbicide options because they provide both the control of emerged and unemerged weeds. Acuron GT is a new herbicide developed specifically for postemergence. Because it contains the full-season label rate of bicyclopyrone, it cannot be used if Acuron was used preemergence. However, if growers want to use Acuron GT postemergence, they can use Lexar® EZLumax® EZ or Bicep II Magnum® preemergence.

Al Hill, who farms 3,100 acres of corn and soybeans in Deep Run, North Carolina, says a one-pass approach in corn using Acuron herbicide, a premix formulation of bicyclopyrone, mesotrione, S-metolachlor and atrazine, preemergence with a burndown application, such as Gramoxone® SL 3.0 herbicide, works well for him.

“Doing it in one pass simplifies things and works for me,” Hill says. “I start with a fall application of [herbicide] to keep weeds from overwintering. We tend to have mild winters in eastern North Carolina, and this keeps weeds from popping up. Then I come back with Gramoxone® SL 3.0 and with Acuron when we plant corn in late March and early April. We spray Acuron as we plant, right behind the planter. That way, the ground is fresh. There’s a little moisture in the soil, which helps activate the herbicide.”

Hill also strip-tills corn in 20-inch rows so it can canopy quickly, shading out late weed emergence.

Walker notices farmers taking more comprehensive approaches to weed management, which he sees as necessary to deal with today’s weeds. Farmers, he says, are more innovative because they’ve seen the writing on the wall and know long-term farming requires maintaining soil health and reducing the seed bank.

Don’t Delay

A quick start to corn weed control programs protects against yield loss. Mathew advises initiating weed removal at least four to five days before the critical period of weed control, which is the V1 to V11 corn growth stages.

Some weeds can surprise you with lightning-fast growth. Palmer amaranth, for example, can grow two-to-three inches a day.

“People underestimate it,” Mathew says. “They see it, then look at it again in four or five days, and it’s gone from a two-leaf weed to a six-leaf weed. Don’t underestimate these weeds. Be sure you apply proper herbicide rates to control them.”

“Research tells us that for every leaf stage of delay from V1 to V11, there’s a 2% yield loss,” Mathew continues. “You can easily get a 10% yield loss just by delaying post-emergence herbicides. For $6 per bushel corn, that adds up.”

  • Palmer amaranth can grow two to three inches a day.
  • Research shows every leaf stage of delay from V1 to V11 causes a 2% yield loss.
  • Growers may end up with a 10% loss in yield if they delay use of postemergence herbicides