Download Resistant weed

AGR 3102
Principles of Weed Science
Herbicide
Muhammad Saiful Ahmad Hamdani
Unit 7 – Topics Covered
Herbicides:
• Selectivity
• Resistance
Herbicide Selectivity
• Herbicide selectivity is the basis for successful
chemical weed management in crops.
• Selectivity is accomplished primarily by 3 methods:
Selectivity by Placement
• Selectivity avoiding or minimizing contact between
the herbicide and the desired crop is called
selectivity by placement. Mainly for non-selective
herbicides.
• Example: apply glyphosate directly to the weed, not
exposing to the crop. Suitable esp. for big and tall
crops i.e. orchard, plantation, landscape.
Selectivity by Differences in Crop and Weed Growth Stage
• Selectivity as a result of some morphological and physiological
between crop and weed. Selectivity mainly for non-selective
herbicides.
• When crop is older than the weed, crop will have an advantage
on things such as:
- thicker cuticle and leaf wax: less herbicide penetration.
- more leaves: many angles - difficult to get a good
coverage; insufficient translocation of herbicide.
- higher growing point (meristem): growing point is
protected, less likely for foliar herbicides to reach
the growing point.
- deeper root: less absorption: insufficient
translocation.
• Q1: Why selective herbicides; i.e. ACCase herbicides
(graminicides) only kill grass weeds but not broadleaf
weeds?
• Q2: And why ACCase herbicides don’t have effect on
grass crops too?????
• Q3: How about PGRs, where grass weeds/crops are not
affected, but broadleaf crops/weeds are?
True Selectivity
• Selectivity because of the differences in the
biochemical process in plants. For selective herbicides.
• A1: Broadleaf weeds and crops/plants are not affected
by ACCase herbicides because their ACCase is less
sensitive to these herbicides.
• A2: Grass crops (corn, rice, wheat…) are
also not affected by ACCase herbicides
because they can rapidly metabolise
these herbicides into inactive product
(difference in metabolism rate between
grass crops and grass weeds).
• A3: As for the PGRs, broadleaf plants
are more sensitive to the changes in the
auxin level than the grass plants.
• Q4: How about dinitroanilines from
seedling growth inhibitors? They are
not metabolized by plants.
• A4: Difference in absorption of
dinitroaniline herbicides between
tolerance crops and susceptible weeds.
And of course, herbicide placement
plays an important part too…
Evolution of Herbicide Resistance in Weed
Species
• Herbicides have revolutionized weed control
practices all over the world.
• Farmers preferred herbicides over cultural and
mechanical control practices because of time and cost
efficiency, easy to apply, and, fast + effective weed
control.
• However, good things won’t last long.....
• Repeated use and strong reliance on herbicides led to
evolution of herbicide resistance in weed populations,
especially for herbicides with a single MOA.
What is herbicide resistance (HR)???
• The inherited ability of a plant to survive and
reproduce following exposure to a dose of herbicide
that would normally inhibit and kill wild type
individuals of the same population.
What is herbicide susceptibility???
• The degree to which a plant is subject to injury or
killed by a particular herbicide.
What is herbicide tolerance???
• The inherited ability of a species to survive and
reproduce following a herbicide treatment.
So is there any difference between HR and
herbicide tolerance???
• There is. No selection to make the plants
tolerant; those plants simply possess a natural
tolerance. Resistant (R) plants were originally
susceptible (S), but through continuous
exposure to herbicide, they become R.
• How plants evolved resistance???
- continuous selection pressure from the
HERBICIDE, naturally or induced via genetic
engineering by human.
Only R plants remain: control failure.
Types of Herbicide Resistance
1. Cross resistance
• Plant is resistant to two or more herbicides
having the same MOA.
• i.e. Lolium rigidum is resistant to diclofop
(WSSA Group 1/HRAC Group A; Chemical
Family APP; MOA ACCase biosynthesis
inhibitors) and clethodim (also WSSA Group
1/HRAC Group A; Chemical Family CHD; MOA
ACCase biosynthesis inhibitors).
2. Multiple resistance
• Plant is resistant to two or more herbicides
having different MOA.
• i.e. Eleucine indica is resistant to glyphosate
(WSSA Group 9/HRAC Group G; Chemical
Family Glycines; MOA EPSPS inhibitor) and
fluazifop-P-butyl (WSSA Group 1/HRAC
Group A; Chemical Family APP; MOA ACCase
biosynthesis inhibitors).
RESISTANCE
MECHANISMS
TARGET-SITE
RESISTANCE
NON-TARGETSITE RESISTANCE
GENE MUTATIONS
REDUCED
UPTAKE
OVEREXPRESSION/
OVERPRODUCTION
OF GENES
REDUCED
TRANSLOCATION
ENHANCED
METABOLISM
SEQUESTRATION
Evolved herbicide resistance
patterns
• Glyphosate is a very effective herbicide
• Broad weed spectrum
• Systemic activity
• Glyphosate has favorable toxicological and
environmental properties
• So the adoption begins….. What happened
not so long after?
Weeds are still winning…
IF weeds could talk…
“GM crops? It was fun, nice try
human…” (Mr. and Mrs. Resistant
Weed Species, 1995-now)
Herbicide Resistance Management
Divided into 2 types:
1. Prevention and Delaying Resistance
• Use herbicides wisely
Where available, herbicide applications should be
based on economic thresholds – control when
necessary (critical control period). And don’t cut the
rates.
• Herbicide Rotation
Don’t stick to a single herbicide or herbicides with a
single MOA.
• Crop Rotation
Can manipulate planting time, spectrum of weed
infestation, cultivation techniques, choice of
herbicide with different mode of action, different
stage and different way of application.
• Integrated Weed Control Methods
Mechanical weeding eliminates the weed
plants before the seed set. Mulching will
simultaneously cover weed seeds and stop
from germinating.
2. Post-evolution
• Crop rotation and herbicide rotation will keep
the resistant population down.
• Maximize crop competition (row spacing,
proper fertility, optimum planting dates,
water management etc.).
• Ensure clean and certified seed is planted
each season.
• Clean farm machinery is important to avoid
movement of seeds from HR weed spp.
We have always had weeds,
and…..
We always will have weeds,
so…..
Deal with it…