Balancing steel and herbicides to reduce resistance – why resistance? Micheal D. K. Owen Iowa State University Ames, IA USA

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* *

Evolution of glyphosate-resistant weeds* *

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* YearSite of action Iowa and Illinois1993ALS inhibitors Missouri1994PS II inhibitors Illinois1996ALS and PSII inhibitors Kansas2001ALS and PPO inhibitors Illinois2002ALS, PS II and PPO inhibitors Missouri2005ALS and PPO inhibitors and glycines Iowa and Illinois2009ALS, PS II and HPPD inhibitors Nebraska2009Synthetic auxins Iowa2011ALS and HPPD inhibitors and glycines *

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 management program Year 2+ - Adapted biotype quickly becomes the dominant weed

Poor control – the weed population shift is 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 Mechanical Cultural Herbicides Mechanical 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?