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

Corn Field
HerbicidesNew Wilbur‐Ellis Herbicide Targets Volunteer Corn
April 28, 2016
Wilbur‐Ellis Co., a recognized leader in marketing and distribution of crop protection products, as well as precision agriculture technology, has Read More
HerbicidesHELM AGRO Launches New HELM Sulfentrazone 4F Herbicide
April 26, 2016
HELM AGRO US, Inc. has announced the federal Section 3 registration of its new HELM Sulfentrazone 4F herbicide. Classified as a Read More
HerbicidesHerbicide Resistance In Waterhemp Continues To Grow
April 22, 2016
Twenty-five years ago, waterhemp was virtually unknown to Illinois farmers. Today, the broadleaf weed blankets corn and soybean fields across Read More
Soybean Field
HerbicidesHELM AGRO US Unveils New Sheridan 25 WG Herbicide
April 22, 2016
HELM AGRO US, Inc. has announced the federal Section 3 registration of its new Sheridan 25 WG herbicide for selective Read More
Trending Articles
Eric SfiligojMissing The Family Ties In Agriculture
April 25, 2016
By its nature, the agriculture market is cyclical. Up and down cycles come and go with a regular pattern. For Read More
HerbicidesHerbicide Resistance In Waterhemp Continues To Grow
April 22, 2016
Twenty-five years ago, waterhemp was virtually unknown to Illinois farmers. Today, the broadleaf weed blankets corn and soybean fields across Read More
Eric SfiligojBiotech Corn Drop More Economics Vs. Consumer Rejection
April 18, 2016
Since their introduction into the agricultural marketplace, biotech crops has steadily grown in acreage in the countries of the world Read More
Crop InputsBayer: 5 Reasons We Disagree With Maryland Neonic Ban
April 12, 2016
We’re disappointed that the Maryland legislature chose not to stand up for sound science and the rights of Maryland’s homeowners Read More
Corn
OpinionChallenging Global Economic Conditions Putting More Heat On U.S. Ag Retailers
April 7, 2016
There it is. That vague, slightly sickening feeling in the pit of my stomach. Things are getting weird out there. Read More
ScoutPro iPad
Precision AgWhat’s Trending In Soil Testing And Scouting
April 6, 2016
Keeping up on soil testing and scouting, especially following unusual weather conditions, can play a role in maximizing yields. Soil Read More
Latest News
West Central Dome structure
FertilizerFertilizer Logistics 2016: All Clear, So Far
May 1, 2016
After a relatively hairy early start to the spring fertilizer movement season with flooding in Louisiana and St. Louis, things Read More
BAICOR
CropLife 100BRANDT Acquires Utah Specialty Fertilizer Company
April 28, 2016
BRANDT, a leading agriculture retailer and manufacturer of specialty ag products, has acquired a majority interest in BAICOR, L.C. BAICOR, Read More
Soybean field
Industry NewsVerdesian Life Sciences Adds Vice President Of Business…
April 28, 2016
Verdesian Life Sciences has announced the addition of Marc Treurniet to the plant health and nutrition company’s management team. Treurniet Read More
ManagementBiotech Crops Developments and Millennials
April 28, 2016
Editors Eric Sfiligoj and Paul Schrimpf talk about the latest activities in biotech crops and a major shift in the Read More
Industry NewsNachurs-Alpine, Pathway Biologic Extend Marketing Agree…
April 28, 2016
Nachurs-Alpine Solutions (Marion, OH) and Pathway Biologic (Plant City, FL) announce an extension to their exclusive marketing agreement specific to Read More
MicronutrientsCharah Add Three Warehouses For SUL4R-PLUS Fertilizer
April 28, 2016
Charah, Inc., a total solutions company providing unparalleled service and innovation for the coal-fired power generation industry, announced today that Read More
Spenser Forgey, Yahama Fortix
FungicidesIndiana Farmer Wins Arysta LifeScience FORTIX Fungicide…
April 28, 2016
Arysta LifeScience North America recently presented Spenser Forgey, a grower from Young America, IN, with a new 2016 Yamaha Grizzly Read More
Corn Field
HerbicidesNew Wilbur‐Ellis Herbicide Targets Volunteer Corn
April 28, 2016
Wilbur‐Ellis Co., a recognized leader in marketing and distribution of crop protection products, as well as precision agriculture technology, has Read More
StewardshipNRCS To Help Farmers Measure Conservation Impacts On Wa…
April 27, 2016
The USDA has announced the availability of $2 million to help farmers install edge-of-field stations that monitor water quality as Read More
Planter in Iowa
EquipmentTop 10 Twitter Pics For #Plant16
April 27, 2016
Despite some wet weather in parts of the Midwest, growers and ag retailers are working feverishly to plant this year’s Read More
HerbicidesHELM AGRO Launches New HELM Sulfentrazone 4F Herbicide
April 26, 2016
HELM AGRO US, Inc. has announced the federal Section 3 registration of its new HELM Sulfentrazone 4F herbicide. Classified as a Read More
Valley Irrigation Valley 8000 series on corn
EquipmentValley Irrigation Receives Environmental Stewardship Ho…
April 26, 2016
Valley Irrigation has been recognized as a Groundwater Guardian Green Site by the Groundwater Foundation. The Groundwater Guardian Green Site Read More
Eric SfiligojMissing The Family Ties In Agriculture
April 25, 2016
By its nature, the agriculture market is cyclical. Up and down cycles come and go with a regular pattern. For Read More
HerbicidesHerbicide Resistance In Waterhemp Continues To Grow
April 22, 2016
Twenty-five years ago, waterhemp was virtually unknown to Illinois farmers. Today, the broadleaf weed blankets corn and soybean fields across Read More
ManagementIllinois Leads The Nation In 4R Certified Nutrient Mana…
April 22, 2016
As Illinois agriculture works to educate farmers about minimizing environmental impact and optimizing the use of ag nutrients, Illinois now leads Read More
Social media
Matt Hopkins7 Social Media Trends Shaping Ag Retail In 2016
April 22, 2016
After writing my last column on why social media is a must for ag retailers, I heard from several readers Read More
Industry NewsArysta LifeScience Adds Corey Abels To Midwest Sales Te…
April 22, 2016
Arysta LifeScience North America recently announced the addition of Corey Abels as Territory Sales Manager for the Midwest region. In Read More
Soybean Field
HerbicidesHELM AGRO US Unveils New Sheridan 25 WG Herbicide
April 22, 2016
HELM AGRO US, Inc. has announced the federal Section 3 registration of its new Sheridan 25 WG herbicide for selective Read More