Managing Glyphosate-Resistant
Weeds in Glyphosate-Resistant
Crops
Dr. Bill McCloskey
Extension Weed Specialist
* RR Flex cotton
* RR alfalfa
* RR Corn
* Tree crops – lemons, pecans, pistachios, etc.
* General farm weed control/sanitation
* Over-reliance on glyphosate?
* Problem of herbicide resistant weeds
*
Desert Ag Conference – May 2012
2
*  July 2012: field after 3 sprays of 44 oz/A (1.5 lb ae
glyphosate/A). Difficulty controlling Palmer
amaranth in 2011; cotton/wheat double crop
3
*
4
*
5
Weeds too large
when spray by
grower
*
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Treatment
% Resistant
Weeds in
Population
.0001
Excellent
1st Application
.00143
Excellent
2nd Application
.0205
Excellent
3rd Application
.294
Excellent
5th Application
4.22
60.5
R4"0$-@&<$Y"&Z"J"8$("&
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Weed
Control
0 Application
4th Application
Year 2
0
1
Excellent
Failure
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R)%-4)8&$'&'+88&"X("88"%-9&6*-&&
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Treatment
% Resistant
Weeds in
Population
.0001
Excellent
1st Application
.00143
Excellent
2nd Application
.0205
Excellent
3rd Application
.294
Excellent
5th Application
4.22
60.5
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Weed
Control
0 Application
4th Application
3
Year 5
4
Excellent
Failure
I"46$($0"&4"'$'-1%("&(1%%)-&6"&4":"4'"0&$%&
seed pool or seed bank in soil
1&741(+(18&+C"&F41C";&S%&C1%2&(1'"'9&-5"&
'""0&7))8&$'&*%8$Y"82&-)&(51%#"&61(Y&
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..&
Robust annual –
reproduces by seed
Palmer amaranth
seed production –
600,000 to
1.6 million seeds
per large plant.
An infestation of
1.6 plants/ft of
crop row can
produce 600 million
seeds per acre.
12
Male Palmer amaranth Male flowers with yellow
anther sacs
*
Female
Palmer
amaranth
flowers
13
*
14
*
15
Male Palmer amaranth Male flowers with yellow
anther sacs
Female
Palmer
amaranth
flowers
16
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[Click to close.]
and
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.,&
* Apply integrated weed management practices
including sanitation, tillage and crop rotation.
* Use multiple herbicide modes-of-action with
overlapping weed spectrums in rotation, sequences,
or mixtures.
* Use the full recommended herbicide rate and proper
application timing for the hardest to control weed
species present in the field.
* Scout fields after herbicide application to ensure
control has been achieved. Avoid allowing weeds to
reproduce by seed or to proliferate vegetatively.
* Monitor site and clean equipment between sites.
*
18
Herbicides
Mechanical
Cultural
Multiple herbicides
with different
mechanisms of action
Tillage
Crop rotation
Mixes
Pre-plant
Plant population
Sequences
In-crop cultivation
Row spacing
Across seasons
In-row weeding
Planting date
Post-harvest
Fertilizer placement
Hand-rogueing before
seed set
*
19
Cover crops
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KN&
Herbicides
Mechanical
Cultural
Multiple herbicides
with different
mechanisms of action
Tillage
Crop rotation
Mixes
Pre-plant
Plant population
Sequences
In-crop cultivation
Row spacing
Across seasons
In-row weeding
Planting date
Post-harvest
Fertilizer placement
Hand-rogueing before
seed set
*
24
Cover crops
*
*
*
!"#$%$&&'"#()*)+,'-.#)/-,#,$01)234-56
23)4056,'7
-+./056,'7
23)40/,1*
21,53+08
-+./0/,1*
9:
'()*+(,
!
"!
#!
$!
%!
&!!
*
* Roundup Ready Flex Cotton – glyphosate
* Roundup Ready Flex Pima Cotton
* Liberty Link Cotton – glufosinate
* GlyTol: Glufosinate and Glyphosate resistant cotton
* Dicamba, glyphosate and glufosinate 3-gene stacked
cotton varieties
*
Preplant
Early POST
topical
Mid POST
Post-direct
Layby
PD broadcast
Tillage
Dual Magnum
Glyphosate
Glyphosate
Pendimethalin
Prowl H20
Staple LX
Staple LX
Trifluralin
Glyphosate
Liberty/Ignite 280
Liberty/Ignite 280
Prometryn
Staple LX
Sandea
Prowl H2O*
Diruon
Liberty/Ignite 280
Prometryn*
Prometryn*
Diuron*
Diuron*
Layby Pro*
Layby Pro*
Goal, GoalTender* Goal, GoalTender*
*Preemergence soil activity
Aim
Aim
Chateau*
Chateau*
ET
ET
Herbicides
Mechanical
Cultural
Multiple herbicides
with different
mechanisms of action
Tillage
Crop rotation
Mixes
Pre-plant
Plant population
Sequences
In-crop cultivation
Row spacing
Across seasons
In-row weeding
Planting date
Post-harvest
Fertilizer placement
Hand-rogueing before seed set Cover crops
*
30
*
*
Before cultivation
After cultivation
*
*
*
Key components:
Disk stabilizers to stop implement drift
Adjust sway blocks to allow implement to run at
correct depth. Implement must not rotate with respect
to tractor.
*
* Cotton must be 12 inches
tall with bark on the
lower stem to use
precision cultivation with
in-row weeding. Crop
must have a well
developed tap root.
* Can close cultivate on
smaller row crops
without in-row weeding.
* Crop stem damage
from moving soil
remains a limitation.
* Zero tolerance for Palmer amaranth flower (pollen) and
seed production = SANITATION!
* Remove weeds before they produce viable seed.
*
37
Herbicides
Mechanical
Cultural
Multiple herbicides
with different
mechanisms of action
Tillage
Crop rotation
Mixes
Pre-plant
Plant population
Sequences
In-crop cultivation
Row spacing
Across seasons
In-row weeding
Planting date
Post-harvest
Fertilizer placement
Hand-rogueing before
seed set
*
38
Cover crops
*
*  Eliminate weed competition
to optimize alfalfa stand
*  As a broadcast planted
crop, alfalfa is fairly
competitive against
weeds particularly if the
weeds are injured by
herbicides
*  Herbicide use can stunt or
injury alfalfa seedlings
*  Weed competition also
stunts alfalfa seedlings
*
*
41
* Alfalfa production (RR and conventional varieties)
* Plant in fall (mid-October through November)
* Enables 2,4-DB applications (e.g. Butyrac 200) due to
lower temps (temperature cut-off of 90 F)
* Raptor and pursuit alone or tank mix better on winter
weeds, possible resistance problem
* Trifluralin granules or water run Prowl H2O
* Treat berms with residual herbicide or use tillage to
keep them clean
* (Roundup where susceptible weed populations occur)
*
42
*
43
*
44
*
•  Center pivot irrigation
•  Row spacing: 30 inch
•  Plot size: 20 x 30 feet
•  Herbicides applied at 3 leaf (5 in) and 9 leaf (30 in) growth
stages
*
*
* Any weed management program that use soil-applied
herbicides followed by postemergence herbicides have
less variability in weed control, crop yield, and net
return compared with total postemergence programs.
*  Weed Management
* Herbicide – proactive and reactive
* Utilize multiple herbicide modes of action, including
those with residual effects, before applying
glyphosate and/or tank mixing another herbicide with
glyphosate.
* Apply herbicides at the recommended stage of weed
growth as stated on the label.
* Conventional herbicides can and should still be part of
the overall weed management system in glyphosateresistant cropping systems.
* Since glyphosate resistance may be controlled by
more than one gene, it is important to use full label
glyphosate rates.
*  Resistant Management Strategies and Practices
* Glyphosate-resistant crops are a powerful tool to
manage weeds but must be used as part of an integrated
pest management (IPM) program.
* IPM principles and practices include field scouting or
other detection methods, weed identification,
population monitoring, and treatments.
* Weed IPM programs in glyphosate-resistant crops need to
consider all available weed management options
appropriate to the system.
*  Summary