Balancing steel and herbicides to reduce resistance – why

resistance?

Micheal D. K. OwenIowa State UniversityAmes, IA 50011 USAmdowen@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

workthat 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*19

9619

9719

9819

9920

0020

0120

0220

0320

0420

0520

0620

0720

0820

0920

1020

11

0

5

10

15

20

25

GlobalUSA

*www.weedscience.com

Glyphosate-resistant weeds found in Iowa

Common waterhemp

Horseweed

Giant ragweed

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 inhibitorsMissouri 1994 PS II inhibitorsIllinois 1996 ALS and PSII inhibitorsKansas 2001 ALS and PPO inhibitorsIllinois 2002 ALS, PS II and PPO inhibitorsMissouri 2005 ALS and PPO inhibitors and

glycinesIowa and Illinois 2009 ALS, PS II and HPPD inhibitorsNebraska 2009 Synthetic auxinsIowa 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”

Year 4 with the same weed managementprogram

Year 2+ - Adapted biotype quickly becomes the dominant weed

Poor control – the weed population shiftis obvious and a serious problem

Herbicide resistance• The evolution of herbicide

resistance is not a herbicide problem

• The evolution of herbicide resistance is not a trait problem

• The evolution of herbicide resistance is not a glyphosate problem

• The evolution of herbicide resistance is a behavioral problem

Integrated Weed Management

Mecha

nical

Cultural

HerbicidesHerbicides

Mecha

nical

Cultural

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?