Six Rs Of Herbicide-Resistant Weed Management

Kevin Bradley, University of Missouri Associate Professor and State Extension Weed Scientist, offers the six “Rs” of herbicide-resistant weed management.

1. Remember how we got here.
Herbicide resistance in weeds is nothing new. The first reported instance of herbicide resistance in a weed species occurred in wild carrot in 1957 after several seasons of consecutive use of 2,4-D. In 1970, the first reported case of atrazine resistance was reported in common groundsel. Since those initial reports, the occurrence of herbicide-resistant weeds has risen each year. To date, there are 362 confirmed instances of weed resistance across 198 species, one-third of which occur in the U.S.

The point is, weed resistance is not a new phenomenon, we have dealt with triazine-resistant weeds, ALS-resistant weeds, and a host of other types of weed resistance for years. The only thing that is different now with respect to the glyphosate-resistant weed problem we are currently experiencing, is the extent of the acreage that is impacted and the degree to which we currently rely on one active ingredient–glyphosate–for weed control in our cropping systems.

We have glyphosate-resistant horseweed (a.k.a. marestail) in a number of areas throughout the state, glyphosate-resistant giant ragweed that occurs sporadically throughout the state, glyphosate-resistant palmer amaranth that occurs as a significant problem in the boot heel of Missouri, and glyphosate-resistant waterhemp that now occurs as our biggest weed problem in almost all of the counties north of I-70 and some counties south of St. Louis and Kansas City as well. All of these resistant weeds have arisen as a result of our continual use of glyphosate for weed control in the same fields over time. We have selected for these resistant weed biotypes by applying glyphosate over and over, and now the only way to manage this problem is to apply herbicides with an alternate mode-of-action (more on this later).

2. Respect the biology of the weed(s) present.
If we were to truly respect and understand the biology of the weed or weeds present, chances are we wouldn’t have problems with herbicide-resistant weeds in the first place. This means recognizing the strengths of the weeds you’re trying to control but more importantly recognizing their weaknesses. Once you understand a given weed’s weakness, you must exploit it. For example, we know that waterhemp seed is relatively short-lived in the soil and that it does not germinate from lower soil depths. This means that if you were to really attack this weed for a three or four year time period and not let any plants produce seed that will go back into your soil, then you can virtually eliminate this weed as a problem from your fields.

As for giant ragweed, it is one of the first summer annual weeds to germinate each spring, and a large percentage of the seedlings that are going to emerge will do so in that initial flush. We should take advantage of this biological characteristic by dealing with giant ragweed prior to planting with appropriate burndown herbicides or tillage. Horseweed is similar in this respect; it is a weed that will be present well before planting and we should take advantage of that fact. Whatever your problem weed is, think about the weaknesses of that plant and take advantage of those characteristics.

3. Rotating herbicide modes of action is the key. If you don’t know what mode of action means, you will need to become familiar with that term in the future. The mode of action of a herbicide is the way in which a herbicide kills a plant. Some common modes of action you may have heard before include the PPO-inhibiting herbicides, ALS-inhibiting herbicides, ACCase-inhibiting herbicides, and others.

In an effort to make this simpler, herbicide mode of actions are now being identified on herbicide labels with a number. For example, 2, 4-D is a group 4 herbicide, glyphosate is a group 9 herbicide, and so on. As already discussed, the way we select for any herbicide-resistant weed biotype is by applying a herbicide or herbicides with the same mode of action in the same place over time. As a result, the primary way to manage a herbicide-resistant weed biotype is to apply a different herbicide that has an alternate mode of action; hopefully one that has good activity on your resistant weed biotype. So if you have been applying a group 9 herbicide (glyphosate) and believe you now have resistant weeds, then it is imperative that you switch to a different family of herbicides that have a different group number other than 9, making sure that the herbicide you have chosen has good activity on the weed(s) in question.

4. Relying on post-emergence applications of PPO-inhibiting herbicides will likely lead to more failures in the future. With the herbicide options we currently have available in soybeans, post-emergence applications of glyphosate tank-mix partners are not the way to solve this problem. Period. For one thing, almost every one of our tank-mix partners is a PPO-inhibiting herbicide, and we have plenty of waterhemp populations with resistance to these herbicides as well. We now have many cases of multiple-resistant waterhemp around the state; waterhemp with resistance to glyphosate, to the ALS-inhibitors, and to the PPO-inhibitors. So in these cases, applying a PPO-inhibiting herbicide like Cobra, Flexstar, Cadet, Ultra Blazer or others for the control of your glyphosate-resistant waterhemp is futile, because those plants are also resistant to the PPO-inhibitors.

Perhaps even a bigger problem than this is the amount of pressure that we are currently placing on our PPO-inhibiting herbicides. What do I mean by that? Just look at our history with this weed species; decades ago we used ALS-inhibiting herbicides until we got waterhemp with resistance to that mode of action throughout the state. We then started switching to the PPO-inhibitors to control the ALS-resistant waterhemp. We were starting to see PPO-resistant waterhemp when glyphosate and the Roundup Ready system were introduced onto the market. We then used that system for years for the control of our ALS- and PPO-resistant waterhemp. Now we have widespread problems with glyphosate-resistant waterhemp around the state. Many have attempted to “solve” this most recent problem by putting a PPO-inhibitor like Cobra, Flexstar, Cadet, Ultra Blazer, or others with their post-emergence glyphosate application for the control of their glyphosate-resistant waterhemp.

See a pattern here? So we essentially keep moving from relying solely on one mode of action for the control of waterhemp to relying solely on another mode of action, usually to the point that we break each one. I believe that if we continue to use the PPO-inhibitors as post-emergence products in this way, then we will be dealing with a bigger problem in the future — multiple-resistant waterhemp that we can’t kill with any current post-emergence herbicide in a Roundup Ready soybean system.

5. Re-calibrate your mind as to what small weeds really means. This fifth “R” is a continuation of the previous one; if you are going to use PPO-inhibiting herbicides post-emergence for the control of glyphosate-resistant weeds like waterhemp (which I’ve already said is a bad idea), then we must spray plants that are NO MORE than 4-inches in size–preferably smaller. The same holds true with giant ragweed, horseweed, or just about any other weed that we are having trouble with lately. You must remember how sensitive these herbicides are to weed size. If you are going to take this route, the use of these tank-mixes must be planned; they cannot be applied as a rescue.

6. Residual herbicides will be the most important component of a resistant weed management program. At this point, I think the only viable option we have moving forward, especially for the control of resistant waterhemp, is the use of pre-emergence, residual herbicides. Fortunately for us, the majority of our pre-emergence herbicides available for use in soybeans have good activity on waterhemp. However, I want to point out that in most cases you don’t have to use the pre-emergence herbicide rates that have been promoted in the past. In many cases, these are what have been referred to as “foundation” (or in many cases 1/2) rates. In other words, these were rates chosen years ago when we knew we could come back with a post-emergence application of glyphosate to clean everything up. The fact is, that’s no longer a reality. So, it is my recommendation that you look closely at these labels and consider full use rates or even combinations of these products. You want to get as much residual activity as you can out of your pre-emergence herbicide application because as already discussed, we’re running out of effective post-emergence herbicide options.

Another program to consider is an approach I refer to as “overlapping residuals”. This may not have a fit for every weed species, but it does have a particular fit for waterhemp. In this type of program, you apply a pre-emergence herbicide at planting, in this case it may not need to be the fully labeled rate, and then early in the season when you come back with a timely post-emergence herbicide application, you apply a residual herbicide as well. Prefix and Warrant are two of the current products that allow this type of use pattern and at some point in 2012, two new products, Anthem and Zidua, are also expected to be labeled in this manner. Keep in mind that these herbicides are for the waterhemp that is going to emerge later in the season, not for what is there at the time of the application.

Soybean fields like this one infested with multiple-resistant waterhemp have become a much too common sight in Missouri. Following the 6 “R’s” of herbicide-resistant weed management will prevent these problems from happening in the future.

Topics:

Leave a Reply

One comment on “Six Rs Of Herbicide-Resistant Weed Management

Herbicides Stories
HerbicidesScouting Key To Next Season’s Soybean Herbicide Program
August 12, 2014
When growing soybeans, growers need to think ahead to stay one step ahead of weeds. That means examining weed threats and evaluating which herbicides work best. Read More
Winter Wheat
HerbicidesArysta Touts Burndown Herbicide For Brome Management In Wheat
June 4, 2014
Arysta LifeScience North America recommends PRE-PARE Burndown Herbicide for effective brome management in winter wheat. Read More
HerbicidesFall Herbicide Applications More Effectively Manage Resistant Italian Ryegrass
May 14, 2014
Field studies show that a fall application of certain herbicides before weed emergence, leads to more successful crop planting and weed control in spring. Read More
Waterhemp in soybean stubble
HerbicidesThe Weed Resistance Problem: A Matter Of Billions
April 1, 2014
Not only is the amount of money being spent to control these yield robbers high, but the toll it has taken on the human population is growing lager by the day. Read More
Top 100 Articles
CropLife 100BRANDT Commemorates National Ag Day
March 20, 2015
The Illinois agriculture community gathered at BRANDT global headquarters on March 18 to celebrate National Ag Day behind this year's theme Sustaining Future Generations. Read More
Wheat Growers, North Central Farmers Elevator Pursue Merger
CropLife 100Wheat Growers, North Central Farmers Elevator Pursue Merger
March 3, 2015
Two CropLife 100 retailers — South Dakota Wheat Growers (ranked No. 11) and North Central Farmers Elevator (No. 19) — have entered into a Letter of Intent to unify the two companies into a newly named cooperative. Read More
Growmark Group
CropLife 100GROWMARK In 2015: Back, To The Future
March 2, 2015
The nation’s third largest ag retail organization is simultaneously moving forward while remembering its past. Read More
CropLife 100Pinnacle Expands Sanders Brand In The South
February 27, 2015
Pinnacle has acquired Hopkins Seed and Chemical in Qulin, MO, which expands the company's Sanders brand to nine Southern states. Read More
CropLife 100Pinnacle Launches New Providence Agriculture Location In Indiana
February 27, 2015
Pinnacle Agriculture Holdings — ranked No. 6 on the CropLife 100 — has established a new retail location in New Castle, IN, which will operate as part of Pinnacle's Providence Agriculture brand. Read More
Carl Casale of CHS
CropLife 100Cooperative CHS Returns $518 Million To Owners
February 23, 2015
The 2015 cash return to owners is based on CHS net income of $1.1 billion, the company's second highest on record. Read More
Latest News
Nachurs Alpine Solutions, Liquid fertilizer, micronutrients, facility
MicronutrientsYara Launching YaraVita Procote Liquid Micronutrients
March 31, 2015
Yara North America, Inc. broadens its fertilizer coating offerings with the introduction of YaraVita Procote, oil-based micronutrient liquid suspensions that Read More
Industry NewsINCOTEC Board Member To Retire
March 31, 2015
JanWillem Breukink has decided to retire from his position on INCOTEC’s Board of Directors, effective April 1, 2015. This was Read More
MicronutrientsAgrium Deals Michigan Micronutrient Facility To Cameron…
March 31, 2015
Cameron Chemicals, Inc. has announced today that it has entered into a definitive agreement with Agrium Inc. to purchase the Read More
Industry NewsH.J. Baker Creates, Fills New Pacific Northwest Sales P…
March 30, 2015
H.J. Baker announced today that Gary Bingham has joined the company’s Crop Performance Division to further deepen its sales portfolio Read More
Algal blooms
StewardshipStudy: Algae From Clogged Waterways Could Be Used As Bi…
March 30, 2015
Water-borne algal blooms from farm fertilizer runoff can destroy aquatic life and clog rivers and lakes, but scientists are reporting Read More
Young corn plants
Stewardship4R Nutrient Stewardship Summit Coming To DC
March 30, 2015
The annual 4R Nutrient Stewardship Summit will be held on May 12 in Washington DC. The summit will include a full Read More
FungicidesSyngenta Suing Willowood Over Azoxystrobin Fungicide
March 27, 2015
Syngenta announced today that it has sued agrochemical maker Willowood, LLC., for patent and copyright infringement, as well as unfair Read More
ManagementRetail Week: The Future Of Mycogen Seeds; The 4Rs At Na…
March 27, 2015
Editors Eric Sfiligoj and Matt Hopkins discuss recent trips, including a look at the future of Mycogen Seeds at Dow Read More
Eric SfiligojMonsanto Hears The WHO
March 27, 2015
Another challenge to the safety of glyphosate, and the responses from supporters and opponents, calls to mind a classic Dr. Seuss story. Read More
Industry NewsMonty’s Plant Food Expands Sales Team
March 27, 2015
Monty’s Plant Food Company, a leader in natural soil enhancement and  plant fertility products, has hired Andrew Bullock as a Read More
Crop InputsSyngenta Louisiana Plant Poised For 2015 Production
March 26, 2015
As the 2015 planting season gets underway, growers across the country will need crop protection products to combat pests and Read More
Lake Erie Nutrient Stewardship
LegislationOhio Lawmakers Finalize Phosphorus Restrictions
March 25, 2015
State lawmakers on Wednesday finalized new rules designed to curb toxic algal blooms on Lake Erie, calling the regulations a major step forward in addressing the problem. Read More
Spreaders17 Fertilizer Spreaders For 2015
March 25, 2015
Manufacturers shoot for versatility and accuracy in this year's crop of fertilizer spreaders. Read More
Industry NewsMonty’s Hires Mid-South Product Consultant
March 25, 2015
Monty's Plant Food Co. has hired Matt Woodring as a Product Consultant for portions of Central Kentucky and Tennessee.    Read More
StewardshipMapShots Integrates With DriftWatch
March 23, 2015
Growers and agricultural providers using AgStudio FARM and AgStudio PRO can now view vital information about specialty crops and apiaries through a recent integration with the DriftWatch Specialty Crop Site Registry from FieldWatch, Inc. Read More
Crop InputsWorld Health Organization Report Contradicts Scientific…
March 23, 2015
A new report from the World Health Organization has classified glyphosate with a “2A” rating as a probable carcinogen, a Read More
Eric SfiligojSeed Treatment Stays Necessary
March 20, 2015
Grower-customers looking to scale back spending in 2015 won’t consider seed treatment, say experts. Read More
HerbicidesMARCH MADNESS: Industry Rallies Around Glyphosate Safet…
March 20, 2015
A newly published report from the International Agency for Research on Cancer (IARC) classifies the herbicide glyphosate as a "2A-rated possible carcinogen" and the ag industry responds by circling the wagons. Read More