Attendees will gain a basic understanding of the processes influencing herbicide availability which is essential to diagnose performance issues in the field.

1. Managing
Herbicide Resistance
Bob Hartzler
Extension Weed Science
Department of Agronomy

2. Evolution of Row Crop
Weed Management
Cultural + Chemical +
Chemical
mechanical mechanical
1900 1920 1940 1960 1980 2000 2010

3. Important Resistant
Weeds in Iowa
Decade Herbicides Weeds
1980’s Triazines Pigweed, lambsquarter
1990’s ALS inhibitors Waterhemp, cocklebur,
foxtails, several others
2000’s Glyphosate Waterhemp, horseweed,
giant ragweed
PPO inhibitors Waterhemp
2010’s HPPD inhibitors Waterhemp
www.weedscience.org

4. Resistance Mechanisms
in Waterhemp
Family Herbicides Mechanism
Triazines Atrazine, metribuzin Altered target site,
Enhanced metabolism
ALS inhibitors Pursuit, Classic Altered target site
PPO inhibitors Blazer, Cobra, Reflex Altered target site
Glyphosate Roundup Target site amplification,
perhaps others
HPPD inhibitors Callisto, Laudis Unknown
Growth regulators 2,4-D Unknown

5. Why Waterhemp?

6. Seed Production
Resistance is a numbers game
Time of Soybean Corn
emergence
With crop 320,000 48,000
V4 20,000 2,000

9. ‘Standard’ Resistance
Management Strategies
• Fundamentals
– Start with clean fields
– Use weed-free seed
– Scout fields
– Control weed escapes
– Clean equipment between fields
www.monsanto.com
www.resistancefighter.com

10. ‘Standard’ Resistance
Management Strategies
• Herbicide management
– Rotate crops to allow alternate herbicides
– Use multiple herbicides
– Use full rates
www.monsanto.com
www.resistancefighter.com

11. ‘Standard’ Resistance
Management Strategies
• Miscellaneous tactics
– . . . and add cultural practices
– Consider mechanical control
– Use cover crops
www.monsanto.com
www.resistancefighter.com

12. YEAR PRE POST MOA
1999 Dual Basagran + atrazine 3
2000 Prowl Glyphosate 2
2001 Dual Callisto 2
2002 Prowl Glyphosate 2
2003 Dual Accent + atrazine (E-POST) 4
Callisto + atrazine (L-POST)
2004 Prowl Glyphosate 2
2005 Dual Accent + Callisto + atrazine 4
2006 Prowl Glyphosate 2
2007 Dual Callisto + atrazine (E-POST) 3
Impact + atrazine (L-POST)
2008 Prowl Glyphosate 2
2009 Dual Callisto + atrazine 3

13. Keys to Resistant
Weed Management
• Know your enemy
• Optimize herbicide effectiveness

14. Evolution of Herbicide Rates
Full label rate Acetochlor Flumetsulam Clopyralid
Pre RR 3 pts Harness + 2.6 lbs 0.9 oz 2.5 oz
5 oz Hornet
Post RR 2 pts SureStart 36% 50% 50%
Post RRv2.0 3 pts SureStart 54% 75% 75%
% herbicide remaining
Waterhemp emergence
# emerged/week
April May June July

15. Keys to Resistant
Weed Management
• Cultural practices
– Manipulation of cropping conditions to
reduce the density and competitiveness of
weeds
or

16. Effects of Cultural Practices
in Nebraska Wheat Fields
Row Direction Drill Type
E-W N-S Disk Hoe
40 6
30
4
20
2
10
0 0
LQ Pig Grass Fox Kochia Stinkgrass 8 Others
Wicks et al. 2003. Weed Technol.

17. Value of Cultural Practices

18. Value of Cultural Practices
Preemergence No Pre
Glyphosate at V3 Glyphosate at V3

19. Value of Cultural Practices

20. Cultural Practices For
An Herbicide Era
• Row spacing
• Planting rate
• Planting date
• Variety characteristics

21. 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

22. Questions