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
– Occurrence
– Effect on farmers
• My Perspective….
My 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
• 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
• 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?
• 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
• LibertyLink Crops (Bayer)
– Tolerant to glufosinate
– Canola
– Corn, soybeans
• Some very promising lines
Some resistance to GMOs
• 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
• 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)
(Amaranthus palmerii)
Distribution of HR weeds
• More prevalent where glyphosate
continually used
• Cotton/soybean farms
• Southeastern United States
Two main weedy species
• Conyza canadensis
– Winter annual
• Amaranthus palmerii
– Summer annual
Conyza canadensis
• 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)
Photo credit: Chism Craig
Conyza canadensis
Resistant to glyphosate in TN
• Wind-blown seed
• Need more herbicides
• Need more tillage
Photo courtesy of
Chism Craig
Amaranthus palmerii
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Dioecious summer annual
Prolific seed producer
Can grow 2.5 cm/day in summer
Greatly reduces crop yield
Cotton field 2004
0.8 kg ae/ha
glyphosate 14 DAA
West Tennessee 2008
4 kg/ha glyphosate in 1st spray
3 kg/ha glyphosate applied in 2nd application
HR Weeds effect on Farmers
• Use more herbicides
• In extreme cases, hire people to hoe the
fields
• Still using RR crop varieties
Other areas
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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
• “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
• RR corn “volunteers” in soybeans
• A RR crop (corn) becomes a weed in a
different RR crop (soybeans)
Do you see RR corn plants?
Final thoughts
• HT crops widely used
• Few HR weeds, but
more each year
• Once a farmer gets
them, is a major
problem
One sees what one wants to see…..
Questions??
• Canada