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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
  • 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
  • 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
  • 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
  • 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?
  • 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
  • LibertyLink Crops (Bayer)
    • Tolerant to glufosinate
    • Canola
    • Corn, soybeans
  • Some very promising lines
some resistance to gmos
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
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
slide22

(Conyza canadensis)

(Amaranthus palmerii)

distribution of hr weeds
Distribution of HR weeds
  • More prevalent where glyphosate continually used
  • Cotton/soybean farms
  • Southeastern United States
two main weedy species
Two main weedy species
  • Conyza canadensis
    • Winter annual
  • Amaranthus palmerii
    • Summer annual
conyza canadensis
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 resistant to glyphosate in tn
Conyza canadensisResistant to glyphosate in TN
  • Wind-blown seed
  • Need more herbicides
  • Need more tillage

Photo courtesy of

Chism Craig

amaranthus palmerii
Amaranthus palmerii
  • Dioecious summer annual
  • Prolific seed producer
  • Can grow 2.5 cm/day in summer
  • Greatly reduces crop yield
cotton field 2004
Cotton field 2004

0.8 kg ae/ha glyphosate 14 DAA

slide34

West Tennessee 2008

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
  • Use more herbicides
  • In extreme cases, hire people to hoe the fields
  • Still using RR crop varieties
other areas
Other areas
  • 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
  • “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
  • RR corn “volunteers” in soybeans
  • A RR crop (corn) becomes a weed in a different RR crop (soybeans)
final thoughts
Final thoughts
  • HT crops widely used
  • Few HR weeds, but more each year
  • Once a farmer gets them, is a major problem
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