Balancing steel and herbicides to
reduce resistance – why
resistance?
Micheal D. K. Owen
Iowa State University
Ames, IA 50011 USA
mdowen@iastate.edu
www.weeds.iastate.edu
Herbicide resistance =
superweeds?
• While this “description” draws considerable
attention (i.e. NY Times), it is clearly incorrect
and inappropriate!
• Organisms adapt to the selective forces that
exist
• Plants have impacted man since the transition
from hunter/gatherer to an agrarian society –
approximately 8 millennia
Superweeds?
Evolution does not
work
that way!
Major weed shifts in Iowa
1950’s
2,4-D
1960’s
Treflan, Prowl
(trifluralin,
pendimethalin)
70-80’s
Lasso, Dual
(alachlor, metolachlor)
80’-90’s
Pursuit, Classic, etc.
(imazethapyr,
chlorimuron)
2000’s
Roundup
(glyphosate)
Herbicide resistance – a
historical perspective
• Concern for herbicide-resistant weeds predates
glyphosate by more than 50 years
• The USA has a long history of herbicide-resistant
weeds
– Triazine resistance and ALS resistance are
currently most common
– Globally, resistance evolved for ~21 herbicide
MOAs in 201 weed species represented by 372
weed biotypes*
*www.weedscience.com
Evolution of glyphosate-resistant weeds*
25
20
15
10
Global
USA
5
0
*www.weedscience.com
Glyphosate-resistant weeds found in Iowa
Giant ragweed
Common waterhemp
Horseweed
Marestail
Ohio Weed Gothic
(a study in giant ragweed)*
*with apology to Grant Wood
Evolution of herbicide resistance in
common waterhemp
State where first
reported*
Year
Site of action
Iowa and Illinois
1993
ALS inhibitors
Missouri
1994
PS II inhibitors
Illinois
1996
ALS and PSII inhibitors
Kansas
2001
ALS and PPO inhibitors
Illinois
2002
ALS, PS II and PPO inhibitors
Missouri
2005
ALS and PPO inhibitors and
glycines
Iowa and Illinois
2009
ALS, PS II and HPPD inhibitors
Nebraska
2009
Synthetic auxins
Iowa
2011
ALS and HPPD inhibitors and
glycines
*www.weedscience.com
Soybean in a common waterhemp field
“Causes” of herbicide
resistance
• Herbicides do not “cause” weeds to evolve
resistance per se
• How herbicides are used “causes” weeds to evolve
resistance (e.g. management)
• Factors to consider
– Frequency of the resistance trait in weeds
– “Effectiveness” of the herbicide
– Management strategies
– Others (e.g. herbicide marketing)
Original population – year 1
99% Control - Population still
appears like the “original”
2+the
- Adapted
biotype
quickly
YearYear
4 with
same weed
management
becomes the dominant weed
program
Poor control – the weed population shift
is obvious and a serious problem
Herbicide resistance
• The evolution of herbicide
not a herbicide problem
• The evolution of herbicide
not a trait problem
• The evolution of herbicide
not a glyphosate problem
• The evolution of herbicide
a behavioral problem
resistance is
resistance is
resistance is
resistance is
Integrated Weed
Management
Cultural
Cultural
Herbicides
Herbicides
Current
Future
Conclusions
• Weeds have and will inevitably adapt to what
mankind provides, irrespective of technology
• Current issues with Darwinian evolution (herbicide
resistance – aka. “Superweeds”) are widespread and
of great economic importance
• The “causes” of herbicide resistance are more of a
socioeconomic rather than agronomic issue
• Weeds threaten global food security and society just
as they have always done
Questions?