ALS Resistant Common Ragweed
Glenn Nice, Weed Diagnostician, Department of Botany and
Plant Pathology, Purdue University
Bill Johnson, Assistant Professor of Weed Science, Department of Botany and
Plant Pathology, Purdue University
To people who use herbicides, the term ‘resistance’ is
not new. Like bacteria to antibiotics and insects to insecticides,
plants can develop the ability to tolerate herbicide applications
that at one time could control them. The process of developing
resistance is based on natural selection. Variation in a plant’s
physiology or morphology can make it possible for the plant to
survive a herbicide application. The plant that can survive the
herbicide application is allowed to produce the next generation
with the same ability to survive the application of that specific
family of herbicides. Continued use of this herbicide or herbicides
that are similar in their activity, will suppress the non-resistant
(or sensitive) plants and allow the resistant plants to survive.
This will build up a seed bank in the soil of the resistant plants
waiting to emerge when the conditions are suitable. Common ragweed
is no exception to this.
Common ragweed (Ambrosia artemisiifolia L.)
is no stranger to Indiana’s
production fields. In surveys conducted by the Purdue University’s
extension weed science team in 1996 and 2000, common ragweed earned a spot
in the top ten most common and problematic weeds. This member of the composite
family can grow up to 6 ft tall. Its leaves are highly lobed sort of reminding
me of a carrot leaf (figure 1). On older plants the lower leaves can be arranged
opposite and the upper leaves can be alternately arranged on the stem. There
are two types of flowers, one pollen producing found in clusters at the tip
of the branches, and another seed producing flower found in the axils of
There are reports of common ragweed being resistant
a few families of herbicides. In Michigan and New York populations
of common ragweed have been found to be resistant to photosystem
II inhibitors (atrazine, Sencor, Velpar). Resistance to another
family of herbicides, the ALS inhibiters, has been found in Indiana
and Ohio. It is suspected that this could be found through out the state
of Indiana, but is more common in the Southeastern and central counties.
In this situation the use of herbicides that would normally control common
ragweed such as Canopy XL, Firstrate, and Scepter in soybean and Beacon,
Permit, and Spirit in corn are no longer effective.
This is a situation that can easily sneak up on
a person leaving a person with a field of uncontrolled common
ragweed (figure 2). Possibly difficult to detect in a large field,
it may look like few late season escapes. Then depending on the
environmental conditions a seed bank with a large percentage
of seed from resistant plants can spring a surprise, if ALS herbicides
are products regularly used alone.
The best solution for this is prevention.
Utilize herbicides that have different modes of action. The
ALS herbicides are effective tools for weed control; however,
it is not a good idea to become to dependent on one herbicide
group. A breakdown of different herbicide families can be found
at this web address, http://www.btny.purdue.edu/weedscience/inj/MOAinjury.html.
Rotate crops to allow for the use of different herbicide groups. Utilize
the tools available to you, including tillage in bad cases if the site
is not conducive to soil erosion. Consider using PRE and POST applications
of herbicides with activity on this weed. Consult weed response tables
in the Weed Control Guide for Ohio and Indiana (http://www.btny.purdue.edu/Pubs/WS/WS-16/)
to determine the spectrum of weed control by commonly used herbicides
in corn and soybean.
In soybean with ALS resistant common ragweed use
Boundary, Command, Gangster, Sencor, or Valor for soil applied
suppression. None of these offer great control, but they will
suppress emerging ALS resistant common ragweed. If there is common
ragweed up before planting in no-till fields use a glyphosate
product as a burndown and add 1 pt of 2,4-D at least 7 days before
planting (if more than 1 pt is used you must wait to at least
30 days before planting) to help provide a fresh start. In a
conventional soybean planting system, POST options are limited
to the use of diphenylethers such as Cobra, Flexstar, Reflex, or
Ultra Blazer. These may require more than one application to
control consecutive flushes that may occur during the growing
season. The diphenylethers can cause slight injury, such as leaf
spackling or bronzing, but if applied at labeled times before
blooming, research suggests that yields are not effected. Another
option in soybean is to use Roundup Ready soybeans. Glyphosate
products will control common ragweed from good to excellent.
Keep in mind that more than one application may be required due
to multiple flushes of weeds.
In corn, there are a few more options available.
Although triazine resistance common ragweed has been found in
Michigan, we have not found it yet in Indiana. PRE applications
of atrazine, Balance Pro, Bicep II Magnum, Epic, Guardsman Max,
Lariate, Bullet, or Lumax effectively control emerging ALS resistant
common ragweed. POST options include growth regulators (2,4-D
or dicamba, Stinger), atrazine + oil, or Buctril alone or with
atrazine, Liberty in Liberty Link corn or glyphosate in Roundup
ALS resistant common ragweed can be effectively
controlled, however, it is just one more thing we have to keep
on our minds while trying to make a living in production agriculture.
Please read specific herbicide labels for rates and restrictions.
For more information on herbicide resistance and a list of weeds
that have developed resistant visit the following web sites.