Managing Herbicide

ResistanceBob Hartzler

Extension Weed ScienceDepartment of Agronomy

Evolution of Row CropWeed Management

1900 1920 1940 1960 1980 2000 2010

Cultural + Chemical + mechanical mechanical

Chemical

Important Resistant Weeds in Iowa

Decade Herbicides

Weeds

1980’s Triazines Pigweed, lambsquarter1990’s ALS inhibitors Waterhemp, cocklebur,

foxtails, several others2000’s Glyphosate Waterhemp, horseweed,

giant ragweedPPO inhibitors

Waterhemp

2010’s HPPD inhibitors

Waterhempwww.weedscience.org

Resistance Mechanisms in Waterhemp

Family Herbicides MechanismTriazines Atrazine,

metribuzinAltered target site,Enhanced metabolism

ALS inhibitors Pursuit, Classic Altered target sitePPO inhibitors Blazer, Cobra,

ReflexAltered target site

Glyphosate Roundup Target site amplification, perhaps others

HPPD inhibitors Callisto, Laudis UnknownGrowth regulators

2,4-D Unknown

Why Waterhemp?

Seed ProductionResistance is a numbers

game

Time of emergence

Soybean Corn

With crop 320,000 48,000V4 20,000 2,000

‘Standard’ Resistance Management Strategies

• Fundamentals– Start with clean fields– Use weed-free seed– Scout fields– Control weed escapes– Clean equipment between fields

www.monsanto.comwww.resistancefighter.com

‘Standard’ Resistance Management Strategies

• Herbicide management– Rotate crops to allow alternate

herbicides– Use multiple herbicides– Use full rates

www.monsanto.comwww.resistancefighter.com

‘Standard’ Resistance Management Strategies

• Miscellaneous tactics– . . . and add cultural practices– Consider mechanical control– Use cover crops

www.monsanto.comwww.resistancefighter.com

YEAR PRE POST MOA1999 Dual Basagran + atrazine 32000 Prowl Glyphosate 22001 Dual Callisto 22002 Prowl Glyphosate 22003 Dual Accent + atrazine (E-POST)

Callisto + atrazine (L-POST)4

2004 Prowl Glyphosate 22005 Dual Accent + Callisto + atrazine 42006 Prowl Glyphosate 22007 Dual Callisto + atrazine (E-POST)

Impact + atrazine (L-POST)3

2008 Prowl Glyphosate 22009 Dual Callisto + atrazine 3

Keys to Resistant Weed Management

• Know your enemy• Optimize herbicide

effectiveness

Evolution of Herbicide RatesFull label rate Acetochl

orFlumetsulam

Clopyralid

Pre RR 3 pts Harness +5 oz Hornet

2.6 lbs 0.9 oz 2.5 oz

Post RR 2 pts SureStart 36% 50% 50%Post RRv2.0

3 pts SureStart 54% 75% 75%

April May June July

Waterhemp emergence

# em

erge

d/we

ek

% h

erbi

cide

rem

aini

ng

Keys to ResistantWeed Management

• Cultural practices– Manipulation of cropping conditions to

reduce the density and competitiveness of weeds

or

Effects of Cultural Practices in Nebraska

Wheat FieldsRow Direction

LQ Pig Grass0

10

20

30

40E-W N-S

Drill Type

Fox Kochia Stinkgrass 8 Others0

2

4

6Disk Hoe

Not s

igni

fican

t

Wicks et al. 2003. Weed Technol.

Value of Cultural Practices

Value of Cultural Practices

Preemergence No Pre Glyphosate at V3 Glyphosate at V3

Value of Cultural Practices

• Row spacing• Planting rate• Planting date• Variety characteristics

Cultural Practices For An Herbicide Era

Herbicide Resistance• Weeds adapt to all tactics, all

herbicides

• The greater the reliance on a tactic, the greater the speed of adaption

• Integrated management programs needed to stay ahead of evolution

Questions