1. Herbicide-Resistant Weeds and Herbicide-Tolerant Turfgrasses
Tim R. Murphy
The University of Georgia

2. Herbicide Resistance
Tolerance
Susceptible
Mode-of-action
Resistant

3. Herbicide Resistance Tolerance
The ability of a turfgrass or weed species to withstand the effects of an applied herbicide naturally.
For example, most turfgrasses are tolerant of broadleaf herbicides. Dandelions are tolerant of crabgrass herbicides (Barricade, Pre-M, etc…).

4. Herbicide Resistance Susceptibility
The inability of either a desirable species or weed to withstand the effects of an applied herbicide.
For example, common chickweed is controlled by (susceptible to) Confront. A tall fescue lawn is tolerant of Confront and is susceptible to Roundup.

5. Herbicide Resistance
Mode-of-Action is a herbicide’s sequence of events, including absorption, movement in the plant (translocation) to the target site, and activity at the target site (e.g. simazine, atrazine blocks photosynthesis) which, together, result in the herbicide killing susceptible plants.

6. Herbicide Resistance Definition
inherited ability of a weed or crop biotype to survive a herbicide application to which the original population was susceptible.
example, goosegrass or annual bluegrass plants that survive herbicides that normally control goosegrass or annual bluegrass are considered resistant plants.
Biotype = a group of plants within a species that has
biological traits that are not common to the population
as a whole.

7. Herbicide Resistance
cross resistance - weed biotype that has gained resistance to more than 1 herbicide with the same mode of action. Same or different families.
multiple resistance - weed biotype that has developed tolerance to more than one herbicide brought about by different selection pressures (different modes of action).

8. Why Are Plants Resistant to Herbicides?
altered site of action
enhanced metabolism
sequestration

9. Altered Site of Action
Source: J.L. Gunsolus. Herbicide Resistant Weeds
North Central Region Extension Publication 468.

10. Herbicide Resistance
How does it occur?

11. Herbicide Selection Pressure
Source: J.L. Gunsolus. Herbicide Resistant Weeds
North Central Region Extension Publication 468.

12. Herbicide Resistance Around the World
304 Resistant Biotypes
182 Species (109 dicots and 73 monocots)
over 270,000 fields
Source: Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet.
October 20,

13. The Beginning of Weed Resistance
1968 (Washington)
nursery crops
common groundsel
atrazine
simazine
Photo: The Missouri Flora Web-Site

15. Weed Characteristics That Favor Resistance
high reproductive capability
seed dispersal mechanisms

16. Worldwide
Source: Dr. Ian Heap (

17. Herbicide Characteristics That Influence Weed Resistance
herbicides with a single site of action
herbicides used multiple times during the growing season
herbicides used for consecutive growing seasons
herbicides used without other control strategies

18. Herbicide Resistant Weeds By Mode of Action

19. Glyphosate Resistance Around the World (8 species)
rigid ryegrass (1996)
goosegrass (1997)
horseweed (2000)
Italian ryegrass (2001)
hairy fleabane (2003)
buckhorn plantain (2003)
common ragweed (2004)
Palmer amaranth (2005)

20. Glyphosate Resistant Horseweed in US (confirmed in 12 states)
DE (2000)
TN (2001)
IN (2002)
MD (2002)
NJ (2002)
OH (2002)
AK (2003)
MS (2003)
NC (2003)
OH (2003)
PA (2003)
CA (2005)
Horseweed pictures from SWSS Weed ID Guide

21. Other Weeds in the U.S. that have Developed Resistance to Glyphosate
Palmer amaranth
(GA-2005)
Italian ryegrass (OR-2004)
SWSS
SWSS
Common ragweed
(MO-2004)
USDA
L. Hall - UGA
Rigid ryegrass (CA-1998)

22. U.S. Acres Treated with Glyphosate 1996-2003
Source: USDA - National Agricultural Statistics Service – Agricultural Chemical Usage Reports

23. Herbicide Resistance Why?
Herbicide resistant weeds should be a concern to anyone that applies herbicides on a regular basis. This is true even where successful herbicide programs exist.
Why?

24. Herbicide Resistance
Example: Dinitroaniline herbicides provide excellent crabgrass and very good goosegrass control in turfgrasses (home lawns, golf courses, nonresidential turf, etc…).
Doesn’t it make sense to use a DNA herbicide every single year since they are so effective?

25. Herbicide Resistance
The problem with using any herbicide family without change is that weed populations are not identical (biotypes within a population).
Resistance does not appear and become a problem because of genetic mutations or changes in plants.
Resistant plants already exist within the population.

26. Herbicide Resistance
Resistant plants become a problem when the susceptible members of the population are killed by herbicide applications.
The resistant plants escape, reproduce, and over time, can become the majority of the population. The susceptible plants eventually are replaced by the resistant plants.

27. Herbicide Resistance - Turfgrasses
Resistance is encouraged when the same herbicide or herbicide family is used year after year for the same weed control.
Goosegrass – resistant to dinitroanilines and Illoxan.
Annual bluegrass - resistant to triazines, ethofumesate, dinitroanilines.

28. Herbicide Resistance
Goosegrass that is resistant to Barricade (DNA herbicide), is resistant to Pre-M, Balan, Surflan, etc…(other DNA herbicides), and Dimension because they all control weeds by the same mode of action. Remember, DNA herbicides are root inhibitors that affect cell division.

29. DNA Root inhibition Swollen, club shaped roots
Surflan on St. Augustinegrass
pendimethalin on zoysiagrass
Dimension on bermudagrass
Swollen, club shaped roots

30. Resistant Goosegrass
Susceptible Goosegrass

31. Herbicide Resistance
Annual bluegrass that is resistant to atrazine, is resistant to simazine and metribuzin. These herbicides are all photosynthetic inhibitors.

32. How long does resistance last in absence of further selection pressure?
DNA resistant green foxtail
% of resistant seeds after 7 years
initial resistance was at least 90%
Source: Weed Technology (1997): 11:

33. Herbicide Resistance Should Only Be Suspected When .……...
other causes of herbicide failure have been ruled out.
the same herbicide or herbicides with the same mode of action have been used year after year.
one weed that is normally controlled is not controlled
healthy weeds are mixed with controlled weeds (same species)
a patch of uncontrolled weed is spreading.

34. Causes of Herbicide Failures
weed size**
moisture
temperature
humidity
rate
application method
calibration
others
All possible reasons for poor performance should be investigated
before considering the possibility of resistance!!!

35. Herbicide Resistance
The way to avoid or delay herbicide resistance is to rotate not just herbicides from year to year, but to use herbicides that have different mode-of-action.
To best avoid the proliferation of resistant weeds, a herbicide with a different mode-of-action should be used at least one year out of every four.

36. Herbicide Resistance
Why would rotating herbicide families be a potential problem for turfgrass managers???
Several Reasons
Turfgrasses are usually perennial plantings. They remain the same year after year. Depending on the turfgrass species, you may not have many effective available options. Some of your alternative options may not provide acceptable control, or cost may be an issue.

37. Herbicide Resistant Weeds Strategies for Control/Prevention
proactive vs. reactive
rotate herbicides with different MOA
scout sites
prevent seed production
clean mowers and equipment

38. Herbicide Tolerant Turfgrasses
Technology developed for glufosinate and glyphosate
Creeping bentgrass is scheduled for first release for glyphosate
Transgenic (GMO) cultivars
Glufosinate-tolerant turfgrasses may be released

39. Herbicide Tolerant Turfgrasses
May see stacked gene releases
Ex. St. Augustinegrass with tolerance to glyphosate and with a reduced growth rate gene
Unlike glyphosate, glufosinate tolerance can be inserted into an existing cultivar

40. Herbicide Tolerant Turfgrasses
Advantages
Nonselective herbicides
Broad weed control spectrum
Very favorable environmental characteristics with glyphosate and glufosinate
Simplifies weed control
May reduce total amount of herbicides applied to turfgrasses

41. Herbicide Tolerant Turfgrasses
Potential Problems
Requires federal review for registration
Some are concerned about transgenic plants
Some potential for outcrossing
May act as weeds in some crops
May be high in cost –annual fee vs. high price on seed

42. Herbicide Tolerant Turfgrasses
Transgenic technology is actually leased
One never owns technology
User will pay a technology fee.
User will not be able to sell transgenic turfgrasses or turfgrass seed without permission of registrant

43. Roundup Ready Crops
The most significant item with glyphosate has been the introduction of glyphosate-resistant crops in Today there are Roundup Ready soybean, cotton, corn, and canola on the market with wheat, potatoes, and other small grains awaiting approval.

44. How does glyphosate resistance work
How does glyphosate resistance work? Essentially, a resistant form of the EPSP synthase enzyme has been taken from bacteria and placed in plant so that the plant essentially bypasses the blockage caused by glyphosate.