Herbicide resistant weeds crops a north american perspective
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Herbicide Resistant Weeds & Crops: A North American Perspective. Tom Mueller University of Tennessee Knoxville, TN, USA. Overview of presentation. Herbicide resistant crops How widespread? Effect on weed control Effect on development of herbicide resistance Herbicide resistant weeds

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Herbicide resistant weeds crops a north american perspective

Herbicide Resistant Weeds & Crops: A North American Perspective.

Tom Mueller

University of Tennessee

Knoxville, TN, USA


Overview of presentation
Overview of presentation Perspective.

  • Herbicide resistant crops

    • How widespread?

    • Effect on weed control

    • Effect on development of herbicide resistance

  • Herbicide resistant weeds

    • Occurrence

    • Effect on farmers

  • My Perspective….


My perspective
My perspective Perspective.

  • Herbicide tolerant crops can be an emotional issue

  • If one wants to see a problem, one will see one

  • If one does not want to see a problem, one will not see one

  • As is often the case, we only see what we want to see…..


Defining a few terms
Defining a few terms Perspective.

  • Herbicide Resistant Crop

    • Genetically Modified Organisms (GMOs)

    • Transgenic

  • Herbicide Resistance =

    • Inherited ability of a weed population to survive and reproduce after exposure to a herbicide dose (rate) that would control an unselected (sensitive) population


Glyphosate tolerant crops
Glyphosate Tolerant Crops Perspective.

  • RoundupReady (RR) varieties now common in USA

    • Soybean (>90%)

    • Cotton (70%)

    • Corn (~50%, still increasing)

    • Canola (not a US crop, but in Canada)

  • Mainly Monsanto Ag Products


Why do us farmers like rr
Why do US farmers like RR? Perspective.

  • The System works!!

    • Kills many weeds, both small and large

    • Crop safety, can have overlaps

    • Safe to people spraying (glyphosate is not toxic)

  • Monsanto owns seed companies….

  • System is simple

    • One chemical… very easy


Other types of ht crops
Other types of HT crops Perspective.

  • LibertyLink Crops (Bayer)

    • Tolerant to glufosinate

    • Canola

    • Corn, soybeans

  • Some very promising lines


Some resistance to gmos
Some resistance to GMOs Perspective.

  • In practice, the United States has conducted a large-scale feeding trial

  • ~300 million US citizens have consumed GMO crops for ~ 12 years.

  • No negative dietary effects

  • None…

  • There is no toxicological reason not to allow GMO crops to be used.


Herbicide resistant weeds
Herbicide Resistant Weeds Perspective.

  • Weeds have been adapting to herbicide use for many years

  • Selection pressure allows for survivors to make seed and fill that open niche


(Conyza canadensis) Perspective.

(Amaranthus palmerii)


Distribution of hr weeds
Distribution of HR weeds Perspective.

  • More prevalent where glyphosate continually used

  • Cotton/soybean farms

  • Southeastern United States


Two main weedy species
Two main weedy species Perspective.

  • Conyza canadensis

    • Winter annual

  • Amaranthus palmerii

    • Summer annual


Conyza canadensis
Conyza canadensis Perspective.

  • Very widespread

    • Several million hectares infested

  • Only a problem in no-tillage systems

  • Farmers using additional chemicals

    • 2,4-D/dicamba + flumioxizan prior to planting


(Conyza canadensis) Perspective.



Conyza canadensis resistant to glyphosate in tn
Conyza canadensis Perspective.Resistant to glyphosate in TN

  • Wind-blown seed

  • Need more herbicides

  • Need more tillage

Photo courtesy of

Chism Craig


Amaranthus palmerii
Amaranthus palmerii Perspective.

  • Dioecious summer annual

  • Prolific seed producer

  • Can grow 2.5 cm/day in summer

  • Greatly reduces crop yield


Cotton field 2004
Cotton field 2004 Perspective.

0.8 kg ae/ha glyphosate 14 DAA


West Tennessee 2008 Perspective.

4 kg/ha glyphosate in 1st spray

3 kg/ha glyphosate applied in 2nd application


Hr weeds effect on farmers
HR Weeds effect on Farmers Perspective.

  • Use more herbicides

  • In extreme cases, hire people to hoe the fields

  • Still using RR crop varieties


Other areas
Other areas Perspective.

  • Canada, upper midwest US

  • Minimal glyphosate Resistance problems

  • No GMO wheat

  • GMO canola has several modes of action

    • 50% RR, 30% LL, 15% Imi, 5% non HT

    • Can rotate between alternate modes of action

  • Other weeds have developed resistance

    • ACCase or ALS resistance


Over the larger area
Over the larger area Perspective.

  • “Most” farmers have no “major” weed resistance problems

  • More farmers will have HR weeds in the next few years

  • As RR corn use increases, will see more HR problems


The most common problem
The most common problem Perspective.

  • RR corn “volunteers” in soybeans

  • A RR crop (corn) becomes a weed in a different RR crop (soybeans)



Final thoughts
Final thoughts Perspective.

  • HT crops widely used

  • Few HR weeds, but more each year

  • Once a farmer gets them, is a major problem



Questions?? Perspective.

  • Canada


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