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Herbicide Resistance
Discussion Topics
What is resistance?
What causes resistance?
Terminology
Sites of Action
Inhibition of ACCase
Inhibition of ALS
Inhibition of PS II
PS I electron diversion
Inhibition of PPO
Bleaching: inhibition of carotenoid biosynthesis at PDS
Bleaching: inhibition of HPPD
Bleaching: inhibition of carotenoid biosynthesis, unknown target
Inhibition of EPSPS
Inhibition of glutamine synthetase
Microtubule assembly inhibition
Inhibition of long-chain fatty acid synthesis
Inhibition of cell wall synthesis
Inhibition of lipid synthesis; not ACCase inhibition
Synthetic auxins
Inhibition of auxin transport
Unknown
WSSA Code Site of Action
1 Inhibition of ACCase
2 Inhibition of ALS
5 Inhibition of PS II
22 PS I electron diversion
14 Inhibition of PPO
12 Bleaching: inhibition of carotenoid biosynthesis at PDS
27 Bleaching: inhibition of HPPD
13 Bleaching: inhibition of carotenoid biosynthesis, unknown target
9 Inhibition of EPSPS
10 Inhibition of glutamine synthetase
3 Microtubule assembly inhibition
15 Inhibition of long-chain fatty acid synthesis
20 and 21 Inhibition of cell wall synthesis
8 Inhibition of lipid synthesis; not ACCase inhibition
4 Synthetic auxins
19 Inhibition of auxin transport
17 Unknown
From 2011* NC AgChem Manual (>150 entries)
Table 8-11. Herbicide Modes of Action.
Brand names Active ingredient(s) Chemical family
Mode of Action
AAtrex atrazine Triazine 5
Accent nicosulfuron Sulfonylurea 2
Aim carfentrazone Triazolinone 14
Alachlor alachlor Chloroacetamide 15
Alanap naptalam Phthalamate semicarbazone
19
Arrow Clethodim Cyclohexadione 1
Atrazine atrazine Triazine 5
Assure II quizalofop Aryloxyphenoxy-propionate
1
Axiom flufenacet + metribuzin
Oxyacetamide + triazinone
15 + 5
Backdraft Glyphosate + imazaquin
Glycine + imidazolinone
9 + 2
* Revised annually.
Mode of Action Codes
1. ACCase inhibition
2. ALS inhibition
3. Microtubule assembly inhibition
4. Synthetic auxin
5. PS II inhibition, not groups 6 or 7
6. PS II inhibition, not groups 5 or 7
7. PS II inhibition, not groups 5 or 6
8. Lipid synthesis inhibition, not ACCase
inhibition
9. EPSP synthase inhibition
10.Glutamine synthase inhibition
12.Carotenoid biosynthesis inhibition at
PDS
13.Carotenoid biosynthesis inhibition,
unknown target
14.PPO inhibition
15.Inhibition very long-chain fatty acids
17.Unknown MOA
19.Auxin transport inhibition
22.PS I electron diversion
27.Inhibition of HPPD
Herbicide Resistance Defined (WSSA definition)
Inherited ability of a biotype* of a weed to
survive and reproduce following exposure to a
dose of herbicide normally lethal to the wild
type.
*Biotype is a subset of the population having some genetically controlled characteristic not
common to the population as a whole
Herbicide Resistance Defined (WSSA definition)
Inherited ability of a biotype of a weed to
survive and reproduce following exposure
to a dose of herbicide normally lethal to the
wild type.
Herbicide Resistance vs Tolerance
Resistance: herbicide used to kill that weed; it no
longer does; we have selected for
resistance.
Example: Hoelon and ryegrass
Tolerance: the herbicide never did kill that species.
Example: 2,4-D and ryegrass
Cause of Herbicide Resistance
• Herbicides do not create resistance.
• Herbicides can select for resistant individuals already in the population.
Resistant individuals initially rare
Initial frequency of resistance: 1 in 100 thousand
1 in 1 million
1 in 10 million
1 weed/4.3 square feet = 10,000 weeds/acre
= 1,000,000 weeds/100 acres
Frequency of 1 in 1 million equal to one resistant
plant per 100 acres
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X Hoelon-susceptible biotype
Hoelon-resistant biotype, Union Co.;Treated with Hoelon Group 1, ACCase inhibitor
Hoelon-resistant biotype, Union Co.; Treated with Axial Group 1, ACCase inhibitor
Example of Cross Resistance
Italian ryegrass
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32XHoelon-susceptible biotype
Hoelon-resistant biotypeTreated with Hoelon ACCase inhibitor, Group 1
Hoelon-resistant biotypeTreated with Osprey ALS inhibitor, Group 2
0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X
Example of Multiple Resistance:
Italian Ryegrass
Quiz
Multiple resistance is greater concern
than cross resistance.
Why?
Discussion Topics
What is resistance?
What causes resistance?
What is the extent of resistance?
What species have resistant biotypes?
ACY 2008
0 10 20 30 40 50
No. species with resistant botypes
ALS inhibitors
PS II inhibitors (triazines)
ACCase inhibitors
Glyphosate
DNA’s
Ureas/amides
Auxins
Arsenicals
Thiocarbamates
PPO inhibitors
Carotenoid biosyn. inhibitors
Nitriles
Herbicide Resistance in US by Site of Action*
Paraquat
HPPD inhibitors
*Current as of Dec. 6, 2010
Resistance reported with 14 SOAs and 76 species;
128 species by SOA combinations, due to multiple resistance
0
2
4
6
8
10
12N
o.
resi
stan
t b
ioty
pes
1998 2000 2002 2004 2006 2008 2010
Glyphosate-resistant biotypes in U.S.
0
2
4
6
8
10
12N
o.
resi
stan
t b
ioty
pes
1998 2000 2002 2004 2006 2008 2010
Glyphosate-resistant biotypes in U.S.
Glyphosate-Resistant Weeds in the US
7 Broadleaf weeds 5 Grasses
Palmer amaranth Italian ryegrass
Tall waterhemp Rigid ryegrass
Common ragweed Johnsongrass
Giant ragweed Goosegrass
Hairy fleabane Annual bluegrass
Horseweed
Kochia
Examples of glyphosate multiple resistance in US
Palmer amaranth glyphosate and ALS inhibitors
Horseweed glyphosate and ALS inhibitors
glyphosate and paraquat
Common ragweed glyphosate and ALS inhibitors
Giant ragweed glyphosate and ALS inhibitors
Hairy fleabane glyphosate and paraquat
Tall waterhemp glyphosate, ALS inhibitors, and PPO inhibitors
Glyphosate-
Resistant Weeds
in NC
Discussion Topics
What is resistance?
What causes resistance?
What is the extent of resistance?
What species have resistant biotypes?
Impacts of herbicide resistance?
Impacts of Herbicide ResistanceIncreased cost of weed management; reduced profitabilityIncreased complexity of weed managementFewer herbicide optionsFewer crop optionsCompromise conservation tillage
Impacts of Herbicide Resistance
Increased cost of weed management; reduced profitability
Increased complexity of weed management
Fewer herbicide options
Fewer crop options
Impacts of Herbicide ResistanceIncreased cost of weed management; reduced profitabilityIncreased complexity of weed managementFewer herbicide optionsFewer crop optionsCompromise conservation tillage
Impacts of Herbicide Resistance
Increased cost of weed management; reduced profitability
Increased complexity of weed management
Fewer herbicide options
Fewer crop options
Compromise conservation tillage
Discussion Topics
What is resistance?
What causes resistance?
What is the extent of resistance?
What species have resistant biotypes?
Impacts of herbicide resistance?
Basics of resistance management
How does herbicide resistance develop?
Prerequisites for Resistance
1. At least one resistant plant
must be present
2. Selection pressure must be
put on that resistant plant
Herbicide Resistance Management
The focus must be on
reducing selection pressure.
Diversity in management options reduces
selection pressure on herbicides.
Weed Management in Cotton: Before RR
1. Conventional tillage
2. PPI and PRE herbicides
3. Early POST or POST-directed herbicides
4. Late POST-directed herbicides
5. Cultivation
Weed Management in Cotton: with RR
1. No-till or strip-till
2. No cultivation
3. No PPI herbicide; little to no PRE
4. Multiple in-crop Roundup applications
5. Very little other chemistry
Herbicide Resistance Management (Reducing selection pressure)
1. Minimize seed bank; avoid seed production
Impact of Crop Rotation and Good Control in Rotational Crop (whole field in soybean in 2009; all treated alike in 2009)
Carthage, NC 2009
Impact of Crop Rotation and Good Control in Rotational Crop (whole field in soybean in 2009; all treated alike in 2009)
Carthage, NC 2009
Soybean2008
Tobacco2008
Herbicide Resistance Management (Reducing selection pressure)
1. Minimize seed bank; avoid seed production
Think whole-farm
Good control in all crops in rotation
Avoid seed production on ditchbanks, turnrows
Avoid seed movement
Herbicide Resistance Management
1. Minimize seed bank
2. Do not depend on a single herbicide (or a single SOA)
a. Multiple SOA’s within a crop, starting with a PRE or Preplant
b. Crop rotation, multiple and different SOA’s
Herbicide Resistance Management (Reducing selection pressure)
Herbicide Resistance Management
1. Minimize seed bank
2. Do not depend on a single herbicide (or SOA)
a.Multiple SOA’s within a crop, starting with a PRE.b.Crop rotation, with different SOA’s
3. Use full rates, including sequentials and tank mixes
Herbicide Resistance Management (Reducing selection pressure)
Herbicide Resistance Management
1. Minimize seed bank
2. Do not depend on a single herbicide (or SOA)
a.Multiple SOA’s within a crop, starting with a PRE.b.Crop rotation, with different SOA’s
3. Use full rates, including sequentials and tank mixes
4. Timely POST application
Herbicide Resistance Management (Reducing selection pressure)
This is not a 4-inch pigweed!
Herbicide Resistance Management
1. Minimize seed bank
2. Do not depend on a single herbicide (or SOA)
a.Multiple SOA’s within a crop, starting with a PRE.b.Crop rotation, with different SOA’s
3. Use full rates, including sequentials and tank mixes
4. Timely POST application
5. Start clean
Herbicide Resistance Management (Reducing selection pressure)
1. Minimize seed bank
2. Do not depend on a single herbicide (or SOA)
a. Multiple SOA’s within a crop, starting with a PRE.b. Crop rotation, with different SOA’s
3. Use full rates, including sequentials and tank mixes
4. Timely POST application
5. Start clean
6. Take advantage of non-chemical, cultural practices where applicable (tillage, narrow rows, planting dates, cover crops, etc.)
Herbicide Resistance Management
&
Discussion Topics
What is resistance?
What causes resistance?
What is the extent of resistance?
What species have resistant biotypes?
Basics of resistance management
Management of glyphosate-resistant weeds in NC
Glyphosate-
Resistant Weeds
in NC
96 oz Weathermax Untreated
Martin Co., NC 2006
Distribution of Glyphosate-Resistant Common Ragweed
Grower Applied Glyphosate; Dead and Live Plants;Union Co., NC. Spring, 2009.
Johnston Co.susceptible
Stanley Co.
Union Co.
Edgecombe Co.susceptible
Glyphosate rates, left to right, where 1X = 860 g/ha or 21.8 fl oz PowerMax
0.063X, 0.125X, 0.25X, 0.5X, 1X, 2X, 4X, 8X
1X
1X
8X
8X
Dec. 2009
Distribution of Glyphosate-Resistant Italian Ryegrass
Burndown for Glyphosate-Resistant Ryegrass
• If you suspect glyphosate resistance:
– Minimum of 2 pt/A Gramoxone applied twice.
– Better control if mixed with a Photosystem II inhibitor:
atrazine for corn; Direx for cotton;
linuron or metribuzin product for soybean
Gramoxone 3 pt Mar. 9 PowerMax 22 oz Mar. 9
Gramoxone 2 pt Mar. 9
Gram 2 pt + atrazine Mar. 24
PowerMax 22 oz Mar. 9
PowerMax 22 oz + atrazine Mar. 24
Untreated
C. Simpson Farm
Apr. 10, 2010
Union County
Glyphosate-Resistant Horseweed
Initially found in 2003; resistant biotype spread quickly beginning in
2008; assume all horseweed in eastern NC is resistant.
Wind-borne seed; easily moves long distances.
Problem in no-till. Postemergence options limited. Focus on good burndown program. Photo by R. Hayes
Burndown Program for
Glyphosate-Resistant Horseweed
Two components of burndown program:
1. Something to kill emerged horseweed
2. Something to give residual control
0
5
10
15
20
25
30
35
40
% e
mer
gen
ce
Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug
Burndown Programs for
Glyphosate-Resistant Horseweed
Two components of burndown program:
1. Something to kill emerged horseweed*
2. Something to give residual control
Programs:Glyphosate + Clarity (1/2 pt)** + residual herbicide
Glyphosate + 2,4-D (2 pt)** + residual herbicide
*Best if applied while weed still in rosette state
__________________________________
** Waiting intervals required
Edgecombe Co., 2008
GR Horseweed
Roundup at burndown
Roundup + 2 pt 2,4-D
+ 2 oz Valor
at burndown
In-Crop Control of Glyphosate-Resistant Horseweed
Cotton: Ignite under hood (any variety)Ignite overtop tolerant variety
Corn: Dicamba or 2,4-DIgnite on LL hybrid
Soybean: FirstRate, horseweed < 6 inches
Ignite overtop LL variety
Glyphosate-Resistant Palmer Amaranth
• Found in NC in 2005; now common • Spreading rapidly
- Equipment- Pollen flow
• Best to assume it is all glyphosate-resistant• ALS resistance also common in Palmer
amaranth; multiple resistance in some populations
GR Palmer amaranth distribution
Palmer amaranth sampling, fall 2010
Palmer present
No Palmer found
Not surveyed
What Makes Palmer Amaranth Such a Bad Weed?
• Heat and drought tolerant
• High photosynthetic capacity
• Rapid growth; large stature
• Very prolific
• Resistance trait transferred via pollen
4” in 52 hrs
Can have thousands of seed 99% control is a disaster
per square yard in seed bank;
hundreds emerge per square yard
Untreated Atrazine-based program
GR Palmer Amaranth Control in RR CottonSoutheast US Recommendations
• Avoid cotton in badly infested fields; rotate out of
cotton for a year or more and strive for good control
in rotational crop; reduce seed bank
Palmer Amaranth Programs for Corn
POST
PRE RR Corn LL Corn Conventional
Bicep Halex GT + atrazine Ignite + atrazine Capreno + atrazine
Guardsman Max Roundup + atrazine Clarity + atrazine
Harness ExtraRoundup + Dual II
Magnum* Laudis + atrazine
Lariat Roundup + Warrant*
Lexar Roundup + Clarity
*No activity on emerged glyphosate-resistant weeds.
Palmer Amaranth Programs for Soybeans
PPO PRE’s Non-PPO PRE’s POST
Authority MTZ + grass herbicide
Boundary Roundup + Blazer
Envive + ProwlCanopy +
grass herbicideRoundup + Cobra
Prefix* Roundup + Flexstar*
Valor + Prowl R’up + Harmony SG**
Roundup + Prefix*
Roundup + Pursuit**
*Contains fomesafen. Only one application per year.
**ALS inhibitor.
Glyphosate-resistant Palmer amaranth, Wayne Co., NC 2008;
PRE herbicide followed by timely glyphosate tank mix
GR Palmer Amaranth Control in RR CottonSoutheast US Recommendations
• Avoid cotton in badly infested fields
• Aggressive Preplant and/or PRE herbicide program.
Get it PRE or you don’t get it. No salvage POST
options in RR or conventional cotton.
Hedging Against Lack of Timely Rain on PRE Herbicides:Palmer Amaranth in Cotton or Soybeans
• In conventional tillage, incorporate a yellow herbicide.
• In no-till cotton or no-till full-season soybeans, put residual with
preplant burndown. Paraquat if needed at planting.
• In double-crop soybeans, pray for rain.
Preplant/PRE herbicides for Palmer amaranthin Cotton
Preplant PPI no-till conv. till PRE
Valor Treflan Cotoran Diuron + Staple
Direx Prowl Diuron Reflex + Staple
Reflex Reflex + Cotoran
Staple Reflex + diuron
Prowl Prowl + Reflex
Prowl + Cotoran
Prowl + diuron
• Avoid cotton in badly infested fields
• Aggressive Preplant and/or PRE herbicide
program.
• Dual Magnum or Warrant early POST
• Residual lay-by
GR Palmer Amaranth Control in RR CottonSoutheast US Recommendations
Weed control with
glufosinate-based systems
Weed Management in Glufosinate-Tolerant Cotton• Weaknesses of Ignite: grasses, pigweed sp., nutsedge, perennials
• Ignite not as effective as glyphosate on grasses, perennialweeds, or glyphosate-susceptible Palmer, but it will control glyphosate-resistant Palmer if properly timed
Left: 29 oz Ignite Right: 22 oz Powermax
GR Palmer amaranth,14 days after treatment
• Timing is critical!! 3 to 4 inches for consistent Palmer control.
• Use additional chemistry, similar to RR program
Better control, Ignite timing somewhat less critical Absolutely must avoid Ignite resistance
Strong PRE and/or preplant program for residual control
Dual or Warrant POST for additional residual control
Residual lay-by
Weed Management in Glufosinate-Tolerant Cotton
1. Minimize seed bank
2. Do not depend on a single herbicide (or SOA)
a. Multiple SOA’s within a crop, starting with a PRE.b. Crop rotation, with different SOA’s
3. Use full rates, including sequentials and tank mixes
4. Timely POST application
5. Start clean
6. Take advantage of non-chemical, cultural practices where applicable (tillage, narrow rows, planting dates, cover crops, etc.)
Herbicide Resistance ManagementHerbicide Resistance Management (Reducing selection pressure)
Harrington Seed DestructorRay Harrington
Western Australia
No Deep Tillage Deep Turn
No Herbicide Treatment
Number of Palmer Amaranth Plants During Early Season. Macon Co., Georgia 2008.*
0
20
40
60
80
100
81 b
No deep turn
No Cover
Deep turn
No cover
Rye 7 foot when killed, rolled, planted.
Deep turn 12 inch in previous fall.
Herbicide program: Direx + Reflex + Staple PRE; RU + Parrlay POST, Direx + MSMA at layby.
per
cen
t
63 c
80 b
98 a
GR Palmer amaranth control at harvest with Roundup-based systems. Macon Co., GA 2009.
No deep turn
Rye Cover
Deep turn
Rye Cover
0
700
1400
2100
1627 b
No deep turn
No Cover
Deep turn
No cover
Rye 7 foot when killed, rolled, planted.
Deep turn 12 inch in previous fall.
Herbicide program: Direx + Reflex + Staple PRE; RU + Parrlay POST, Direx + MSMA at layby.
1298 d1524 bc
2078 a
Seed cotton yield (lb/A) with Roundup-based systems. Macon Co., GA 2009.
No deep turn
Rye Cover
Deep turn
Rye Cover
Staple + Reflex + Direx PRE
Roundup WMax + Parrlay POST
Direx + MSMA Layby
No deep tillageNo cover crop
Deep tillageNo cover crop
Palmer amaranth response to deep tillage. Macon County, GA. 2009.
Staple + Reflex + Direx PRE
Roundup WMax + Parrlay POST
Direx + MSMA Layby
No deep tillageNo cover crop
No deep tillageRye cover crop
Palmer amaranth response to cover crops. Macon County, GA. 2009.
Staple + Reflex + Direx PRE
Roundup WMax + Parrlay POST
Direx + MSMA Layby
No deep tillageNo cover crop
Deep tillageRye cover crop
Palmer amaranth response to deep tillage and cover crops. Macon County, GA. 2009.
Seed concentrated on surface in no-till system.
Palmer amaranth seed will not emerge from deeper than about 1 inch.
Seed viability reduced 80% 30 months after burial.
Fall plowing every 3 to 5 years, followed by cover crop established in same fall, then no-till for 3 to 5 years.
Staple + Reflex + Direx PRE
Roundup WMax + Parrlay POST
Direx + MSMA Layby
No deep tillageNo cover crop
Palmer amaranth response to deep tillage and cover crops. Macon County, GA. 2009.
Staple + Reflex + Direx PRE
Ignite + Parrlay POST
Direx + MSMA Layby
No deep tillageNo cover crop
Percent reduction in Palmer emergence by cover crops without herbicides. GA and NC. 2007. 42 DAP.*
0
20
40
60
80
100
59,895 lb/A 66,429 lb/A 68,607 lb/A 96,921 lb/A
*Compared to the no cover conventionally tilled system.
38 b
53 ab40 b
57 a
wheat biomass rye biomass