AGR 3102Principles of Weed ScienceHerbicideAGR 3102Principles of Weed ScienceHerbicide
Muhammad Saiful Ahmad HamdaniMuhammad Saiful Ahmad Hamdani
Unit 7 – Topics CoveredUnit 7 – Topics Covered
Herbicides:
•Selectivity
•Resistance
Herbicide Selectivity
•Herbicide selectivity is the basis for successfulchemical weed management in crops.
•Selectivity is accomplished primarily by 3 methods:
Selectivity by Placement
•Selectivity avoiding or minimizing contact betweenthe herbicide and the desired crop is calledselectivity by placement. Mainly for non-selectiveherbicides.
•Example: apply glyphosate directly to the weed, notexposing to the crop. Suitable esp. for big and tallcrops i.e. orchard, plantation, landscape.
Selectivity by Differences in Crop and Weed Growth Stage
•Selectivity as a result of some morphological and physiologicalbetween crop and weed. Selectivity mainly for non-selectiveherbicides.
•When crop is older than the weed, crop will have an advantageon 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 broadleafweeds?
•Q2: And why ACCase herbicides don’t have effect ongrass crops too?????
•Q3: How about PGRs, where grass weeds/crops are notaffected, but broadleaf crops/weeds are?
True Selectivity
•Selectivity because of the differences in thebiochemical process in plants. For selective herbicides.
•A1: Broadleaf weeds and crops/plants are not affectedby ACCase herbicides because their ACCase is lesssensitive to these herbicides.
•A2: Grass crops (corn, rice, wheat…) arealso not affected by ACCase herbicidesbecause they can rapidly metabolisethese herbicides into inactive product(difference in metabolism rate betweengrass crops and grass weeds).
•A3: As for the PGRs, broadleaf plantsare more sensitive to the changes in theauxin level than the grass plants.
•Q4: How about dinitroanilines fromseedling growth inhibitors? They arenot metabolized by plants.
•A4: Difference in absorption ofdinitroaniline herbicides betweentolerance crops and susceptible weeds.And of course, herbicide placementplays an important part too…
Evolution of Herbicide Resistance in WeedSpecies
•Herbicides have revolutionized weed controlpractices all over the world.
•Farmers preferred herbicides over cultural andmechanical control practices because of time and costefficiency, easy to apply, and, fast + effective weedcontrol.
•However, good things won’t last long.....
•Repeated use and strong reliance on herbicides led toevolution 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 andreproduce following exposure to a dose of herbicidethat would normally inhibit and kill wild typeindividuals of the same population.
What is herbicide susceptibility???
•The degree to which a plant is subject to injury orkilled by a particular herbicide.
What is herbicide tolerance???
•The inherited ability of a species to survive andreproduce following a herbicide treatment.
So is there any difference between HR andherbicide tolerance???
•There is. No selection to make the plantstolerant; those plants simply possess a naturaltolerance. Resistant (R) plants were originallysusceptible (S), but through continuousexposure to herbicide, they become R.
•How plants evolved resistance???
-continuous selection pressure from theHERBICIDE, naturally or induced via geneticengineering by human.
Only R plants remain: control failure.
1.Cross resistance
•Plant is resistant to two or more herbicideshaving the same MOA.
•i.e. Lolium rigidum is resistant to diclofop(WSSA Group 1/HRAC Group A; ChemicalFamily APP; MOA ACCase biosynthesisinhibitors) and clethodim (also WSSA Group1/HRAC Group A; Chemical Family CHD; MOAACCase biosynthesis inhibitors).
Types of Herbicide Resistance
2.Multiple resistance
•Plant is resistant to two or more herbicideshaving different MOA.
•i.e. Eleucine indica is resistant to glyphosate(WSSA Group 9/HRAC Group G; ChemicalFamily Glycines; MOA EPSPS inhibitor) andfluazifop-P-butyl (WSSA Group 1/HRACGroup A; Chemical Family APP; MOA ACCasebiosynthesis inhibitors).
RESISTANCE
MECHANISMS
TARGET-SITE
RESISTANCE
GENE MUTATIONS
OVEREXPRESSION/
OVERPRODUCTION
OF GENES
NON-TARGET-
SITE RESISTANCE
REDUCED
UPTAKE
REDUCED
TRANSLOCATION
ENHANCED
METABOLISM
SEQUESTRATION
Herbicide
Target molecule
Resistanceweed
Herbicide
1. Target site mutation
Target molecule
Target molecule
Target molecule
Major herbicide resistancemechanism
Susceptibleweed
2. Herbicide metabolism
Herbicide
Target molecule
Target molecule
Herbicide
Resistant weed
Evolved herbicide resistancepatterns
•Glyphosate is a very effective herbicide•Glyphosate is a very effective herbicide
•Broad weed spectrum•Broad weed spectrum
•Systemic activity•Systemic activity
•Glyphosate has favorable toxicological andenvironmental properties•Glyphosate has favorable toxicological andenvironmental properties
•So the adoption begins….. What happenednot so long after?•So the adoption begins….. What happenednot so long after?
IF weeds could talk…IF weeds could talk…
“GM crops? It was fun, nice tryhuman…” (Mr. and Mrs. ResistantWeed Species, 1995-now)“GM crops? It was fun, nice tryhuman…” (Mr. and Mrs. ResistantWeed Species, 1995-now)
Weeds are still winning…Weeds are still winning…
Herbicide Resistance Management
Divided into 2 types:
1.Prevention and Delaying Resistance
•Use herbicides wisely
Where available, herbicide applications should bebased on economic thresholds – control whennecessary (critical control period). And don’t cut therates.
•Herbicide Rotation
Don’t stick to a single herbicide or herbicides with asingle MOA.
•Crop Rotation
Can manipulate planting time, spectrum of weedinfestation, cultivation techniques, choice ofherbicide with different mode of action, differentstage and different way of application.
•Integrated Weed Control Methods
Mechanical weeding eliminates the weedplants before the seed set. Mulching willsimultaneously cover weed seeds and stopfrom germinating.
2.Post-evolution
•Crop rotation and herbicide rotation will keepthe resistant population down.
•Maximize crop competition (row spacing,proper fertility, optimum planting dates,water management etc.).
•Ensure clean and certified seed is plantedeach season.
•Clean farm machinery is important to avoidmovement of seeds from HR weed spp.
We have always had weeds,and…..We have always had weeds,and…..
We always will have weeds,so…..We always will have weeds,so…..
Deal with it…Deal with it…