Herbicide Resistance: Are Soil-Applied Herbicides Immune?

The continual evolution of herbicide-resistant weeds is one of the most daunting problems faced by weed management practitioners. According to Aaron Hager, associate professor of weed science at the University of Illinois, biotypes of 12 weed species in Illinois are known to be resistant to one or more herbicide action mechanisms.

“The occurrence of multiple herbicide resistances within individual plants and/or fields is a particular challenge,” Hager said. “Waterhemp has evolved resistance to more herbicide mechanisms of action than any other Illinois weed species.”

Frequently, soil-residual herbicides are proposed as part of an integrated weed management program. They provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.

Many growers believe that weeds demonstrate resistance only to herbicides applied to the foliage. In fact, weeds can also develop resistance to soil-applied herbicides.

For example, imazethapyr (the active ingredient in Pursuit and some herbicide premixes) is an acetolactate synthase (ALS)-inhibiting herbicide that can be applied to the soil or plant foliage. Worldwide, biotypes of 128 weed species have evolved resistance to ALS-inhibiting herbicides. Many Illinois waterhemp populations contain plants that are resistant to these herbicides whether they are applied to the soil or plant foliage.

The story is similar for herbicides that share a common mechanism of action but that are usually applied to either the soil or the foliage, such as herbicides that inhibit the protoporphyrinogen oxidase (PPO) enzyme.

“Waterhemp resistance to PPO-inhibiting herbicides is becoming increasingly common across Illinois,” Hager said. “Growers began to suspect it was present after a foliar-applied PPO inhibitor failed to control plants that were treated within label guidelines. The treated plants demonstrated injury symptoms, such as leaf necrosis, that are characteristic of this herbicide family. However, the injury was less than normal, and the resistant plants begin to recover within seven to 10 days after the application.”

A common misconception is that resistance to PPO inhibitors exists only to those that are foliar-applied. In reality, biotypes of waterhemp that are resistant to PPO-inhibiting herbicides are resistant regardless of whether they are applied to the soil or foliage. However, when a PPO-inhibiting herbicide is applied in the soil, the waterhemp control level is often comparable to that of a susceptible population.

Why do soil-applied PPO-inhibiting herbicides seem to control PPO-resistant waterhemp populations that foliar-applied PPO-inhibiting herbicides do not control?

“Because the dose of the soil-applied herbicide is high enough to overcome the resistance mechanism, at least for a while,” Hager explained. “Foliar-applied PPO-inhibiting herbicides are applied at rates to control the weeds that are present when the application is made. Rates for soil-applied PPO-inhibiting herbicides are calculated to provide several weeks of residual weed control and thus are much higher than what is needed to control weeds that are present when they are applied. The higher rates overcome the mechanism of resistance to PPO-inhibiting herbicides because the magnitude of resistance is relatively low.”

If a waterhemp population contains a mix of PPO-resistant and sensitive individuals, a soil-applied PPO-inhibiting herbicide will control both resistant and sensitive plants for a period of time after application. As the herbicide degrades, a threshold concentration is reached at which the sensitive individuals are still controlled, but the resistant individuals survive. This provides a selective advantage for the resistant individuals and may partially explain the increasing frequency of PPO-resistant waterhemp populations in Illinois.

Selection for herbicide resistance for any herbicide takes place each time it is applied. However, the overall intensity of selection for resistance to a specific herbicide or mechanism-of-action family is reduced when multiple, different tactics are used to control the weed population.

“Not every individual waterhemp plant is herbicide-resistant,” Hager concluded. “An integrated weed management approach that applies soil- and foliar-applied herbicides at labeled rates in combination with other management tactics can help slow the selection for additional resistances in individual plants and populations.”


Leave a Reply

Herbicides Stories

HerbicidesBREAKING: EPA Pulls Registration On Dow’s Enlist Duo
November 25, 2015
EPA on Wednesday withdrew approval of a controversial new weed killer to be used on genetically modified corn and soybeans, Read More
Cornfield near Sioux City Iowa
HerbicidesDow AgroSciences Sells Oxyfluorfen Herbicides To China’s Nutrichem
November 16, 2015
Dow AgroSciences LLC, a wholly owned subsidiary of The Dow Chemical Co., has entered into definitive agreements under which Nutrichem Read More
HerbicidesEuropean Agency Declares Glyphosate Unlikely To Cause Cancer
November 13, 2015
Glyphosate is unlikely to cause cancer in humans, the European Food Safety Authority (EFSA) said on Thursday, and the agency Read More
Enlist spray Demo
Crop InputsGrowers See Benefits Of Enlist Weed Control In First Year Of Commercial Use
November 12, 2015
Growers report seeing strong stands, good ear length and fill, full pods and clean fields where Enlist corn and Enlist soybeans Read More
Top 100 Articles
CropLife 100United Cooperative Acquires Four ADM Wisconsin Facilities
November 12, 2015
United Cooperative, based out of Beaver Dam, WI, recently acquired four grain and agronomy locations from Archer Daniels Midland (ADM) Read More
CropLife 100BRANDT Car Makes NASCAR Xfinity Series
October 29, 2015
BRANDT will join JR Motorsports with driver, Justin Allgaier, for the 2016 NASCAR Xfinity (NXS) Series season. Allgaier will pilot Read More
CHS Building exterior
CropLife 100CHS Leaders Help Shape Succession Report
October 22, 2015
Succession planning was listed as a main concern of cooperative leaders at this summer’s 15 regional CHS owner town hall Read More
Fall Harvest
CropLife 100Pinnacle Purchases Eight Regional Seed Companies
October 12, 2015
Pinnacle Agriculture Holdings has announced the purchase of eight regional seed companies to advance its seed business. Pinnacle has acquired: Read More
MRM Agricultural Service
CropLife 100Pinnacle Acquires CropLife 100 Ag Retailer
October 8, 2015
Pinnacle Agriculture Holdings, LLC has successfully acquired the Missouri-based agronomy business of MRM Agricultural Service, LLC and its affiliates (MRM). Operating Read More
Eric Jenks, Wilbur-Ellis
CropLife 100Wilbur-Ellis Opens Dry Fertilizer Storage And Blending Facility In Washington
October 8, 2015
Wilbur-Ellis Co., ranked No. 4 on the CropLife 100, has constructed and opened a 22,000-ton dry fertilizer storage and blending Read More
Latest News
HerbicidesBREAKING: EPA Pulls Registration On Dow’s Enlist …
November 25, 2015
EPA on Wednesday withdrew approval of a controversial new weed killer to be used on genetically modified corn and soybeans, Read More
FertilizerYara Tampa Ammonia Terminal Garners ResponsibleAg Honor…
November 25, 2015
Yara has received its first ResponsibleAg Certification for its Tampa, FL, Ammonia terminal location. This achievement demonstrates Yara’s commitment to Read More
ManagementReviewing The Top Ag Stories For 2015
November 24, 2015
Editors Eric Sfiligoj and Paul Schrimpf discuss the most noteworthy events from this past year in agriculture. Read More
EmployeesAgriculture Jobs May Be Plentiful, But New Grads Are Sc…
November 24, 2015
Close to 60,000 jobs are set to open up in agriculture each year for the next five years, but there's not enough grads to fill them. Read More
Radish cover crop
StewardshipCover Crops Guide Helps Growers Improve Water Quality
November 23, 2015
A publication that teaches growers the advantages of using cover crops to improve soil health and crop yields has won Read More
Crop InputsThe Andersons’ Products Receive California Organi…
November 23, 2015
The Plant Nutrient Group of The Andersons, Inc. announced today that its Humic DG and Black Gypsum DG products are Read More
Eric SfiligojNew European Study Seems To Support Glyphosate Safety
November 23, 2015
Looking at any international news over the past few years, it might seem as if the European Union (EU) is Read More
Toggle Biostimulant corn roots
Crop InputsTwo New Biostimulants Added To United Suppliers’ …
November 20, 2015
Winfield US announces the launch of two new products in the United Suppliers portfolio for the 2016 crop season, Optify Read More
StewardshipNRDC Report: Cover Crops Contain Vast Benefits
November 19, 2015
As harvest season ends and farmers in the United States ready themselves for winter, one small change could make a Read More
Fall Harvest
Seed/BiotechArysta Launches New Seed Protectant
November 19, 2015
As spring wheat growers begin planning for a strong start to their season, Arysta LifeScience North America announces RANCONA V Read More
Crop InputsInocucor Receives U.S. Patent For Microbials
November 19, 2015
Inocucor Technologies Inc., the Montreal-based agriculture biotech company, was issued patent No. 9,175,258 by the U.S. Patent and Trademark Office to protect Read More
ManagementSyngenta Rumors and ARA 2015 Preview
November 19, 2015
CropLife Editor Eric Sfiligoj looks at the latest rumors involving Syngenta selling its crop protection business and talks about events Read More
EquipmentAGCO Announces Finalists For Operator Of The Year Honor
November 19, 2015
AGCO Corp. has announced the four top operators chosen as finalists for AGCO’s 10th Annual Operator of the Year program, Read More
High Plains Aquifer
StewardshipStudy: High Plains Aquifer Overall Usage In Decline
November 18, 2015
A new Kansas State University study finds that the over-tapping of the High Plains Aquifer’s groundwater beyond the aquifer’s recharge Read More
Golden Harvest Corn healthy leaves
FungicidesArysta Launches New Corn And Soybean Fungicide
November 17, 2015
Arysta LifeScience North America has recently launched ZOLERA FX Fungicide. The highly systemic, powerful fungicide delivers broad-spectrum disease control with Read More
Young corn plants in soil
Crop InputsBioAg Alliance: Microbials Targeted For 250-500 million…
November 17, 2015
Monsanto and Novozymes have announced a new 2025 acreage target that will guide the companies’ microbials business for the next Read More
Syngenta Sign
Crop InputsSyngenta Still Being Eyed For China Takeover
November 16, 2015
China National Chemical Corp. approached sovereign wealth funds including China Investment Corp. to help pay for a potential acquisition of Read More
Cedar Country Cooperative
Eric SfiligojCooperative Consolidation Continues
November 16, 2015
As I put this enewsletter column together, we’ve just put the finishing touches on our magazine’s annual CropLife 100 report. Read More