Herbicide-Resistant Weeds and
Herbicide-Tolerant Turfgrasses
Tim R. Murphy
The University of Georgia
Herbicide Resistance
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Tolerance
Susceptible
Mode-of-action
Resistant
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…).
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.
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.
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.
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).
Why Are Plants Resistant to
Herbicides?
 altered site of action
 enhanced metabolism
 sequestration
Altered Site of Action
Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998.
North Central Region Extension Publication 468.
Herbicide Resistance
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How does it occur?
Herbicide Selection Pressure
Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998.
North Central Region Extension Publication 468.
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, 2005 .
The Beginning of Weed Resistance
 1968 (Washington)
 nursery crops
 common groundsel
 atrazine
 simazine
Photo: The Missouri Flora Web-Site
Weed Characteristics That
Favor Resistance
 high reproductive capability
 seed dispersal mechanisms
Worldwide
Source: Dr. Ian Heap (www.weedscience.com)
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
Herbicide Resistant Weeds
By Mode of Action
Glyphosate Resistance Around the
World (8 species)
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rigid ryegrass (1996)
goosegrass (1997)
horseweed (2000)
Italian ryegrass (2001)
hairy fleabane (2003)
buckhorn plantain (2003)
common ragweed (2004)
Palmer amaranth (2005)
Glyphosate Resistant Horseweed
in US (confirmed in 12 states)
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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
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)
U.S. Acres Treated with Glyphosate
1996-2003
1996
1997
1998
1999
2000
2001
2002
2003
100
% of Acres
80
60
69
30
20
4 4 5 9 9 13 9 19
Corn
78
62 62
56 5758
40
0
73
46
36
2528
1314
Cotton
Soybean
Crop
Source: USDA - National Agricultural Statistics Service – Agricultural Chemical Usage Reports
Herbicide Resistance
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?
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?
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.
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.
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.
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.
DNA Root inhibition
pendimethalin on
zoysiagrass
Surflan on St.
Augustinegrass
Swollen, club shaped roots
Dimension on
bermudagrass
Resistant Goosegrass
Susceptible
Goosegrass
Herbicide Resistance
Annual bluegrass that is resistant to atrazine, is
resistant to simazine and metribuzin. These
herbicides are all photosynthetic inhibitors.
How long does resistance last in absence
of further selection pressure?
100
98
75
%
 DNA resistant green
foxtail
 % of resistant seeds
after 7 years
 initial resistance was
at least 90%
87
81
50
25
0
0 DNA
1 DNA
DNA'a Applied in 7 years
Source: Weed Technology (1997): 11:369-372.
2+ DNA
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.
Causes of Herbicide Failures
 weed size**
 rate
 moisture
 temperature
 humidity
 application method
 calibration
 others
All possible reasons for poor performance should be investigated
before considering the possibility of resistance!!!
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-ofaction.
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.
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.
Herbicide Resistant Weeds
Strategies for Control/Prevention
 proactive vs. reactive
 rotate herbicides with different MOA
 scout sites
 prevent seed production
 clean mowers and equipment
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
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
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
Herbicide Tolerant Turfgrasses
Potential Problems
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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
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
Roundup Ready Crops