Lecture 4: Chemical Control of Weeds Flashcards
The global pesticide market was values at $___ in 2012 with the total value expected to reach ___ in 2017
$46.1 billion
$65.3 billion
Synthetic pesticides were worth a total of $__ in 2012 and ___ in 2017
$44 billion in 2012
$61.5 billion in 2017
Biopesticides were worth a total of $__ in 2012, and will surpass ___ in 2017
$2.1 billion in 2012
$3.7 billion in 2017
pesticide use is greatest in
developed countries
herbicides are chemicals used to control
weeds
herbicides make up __% of pesticides sales worldwide
45%
the greatest use of herbicides is in
developed countries
two type of herbicides:
contacts or systematic
2 type of treatment by herbicides
Pre-emergence or post-emergence of crops etc
classification of herbicides:
- type of herbicide
- type of treatment
- chemical nature
- mechanisms of action
contact herbicides:
- only kill the portion of green tissue contacted (they are not taken up by the plant)
- uniform spray converge is required to kill the plant
contact herbicides: how do they kill the plant?
due to their phytotoxicity (usually by rupturing cell walls)
contact herbicides are used against:
annual weeds only - do not kill perennial weeds
contact herbicides e.g.:
- Paraquat for non selective control
- Methyl arsenic acid for control of grasses
Systematic herbicides:
-enter the plant and are translocated in the phloem and/or the xylem to other parts of the plant
systematic herbicides are applied as
either a foliar application or as a soil drench
systematic herbicides require how long to achieve good control
days to weeks
systematic are more effective against annual/ perennial weeds?
PERENNIAL
examples of systematic herbicides:
- Glyphosate for non selective weed control
- Atrazine for certain annual grasses & broad-leaved weeds
for systematic herbicides to be an effective weed management tool it must:
- contact the plant surface
- remain on the plant surface long enough to be absorbed
- reach a cellular site and disrupt a life supporting process (e.g.amino acid synthesis) or structure (e.g. membranes)
- Degrade into non-toxic metabolites
contact herbicide positives:
-kill the plant quickly & they are not very persistent in the soil
contact herbicide negatives:
- have to be sprayed frequently often at quite high doses to kill weeds
- only annual weeds are controlled
systematic herbicide positives:
- do not have to cover the whole plant therefore they can be applied at lower doses
- can control perennial weeds
systematic herbicide negatives:
- must be applied carefully so that mist doesnt affect the crop plants
- tend to be more persistent in the soil
chemical structure of herbicides:
organic & inorganic compounds, some of which occur naturally & some of which are synthesised in labs by chemical companies
type of treatment: total weed control-
herbicides are applied with the intention of killing all plants present e.g. on railway tracks or in industrial areas
type of treatment: selective weed control:
- application of a herbicide is meant to kill only the weed species
- crop plants should not be harmed agricultural situations
mode/mechanism of action of systematic herbicides:
- inhibitors of normal growth (auxin mimics)
- photosynthetic inhibitors
- amino acid synthesis inhibitors
- inhibitors of cell division
- inhibitors of pigment synthesis
- inhibitors of lipid biosynthesis
inhibitors of normal growth (auxin mimics) are used primarily for controlling:
broadleaf perennial weeds in cereal crops e.g. picloram (cordon) fluroxypyr (starane)
inhibitors of normal growth (auxin mimics) are absorbed where
through both roots & foliage and are translocated in the xylem & phloem
inhibitors of normal growth (auxin mimics) have what effects on the plant
- growth hormones regulate cell elongation, protein synthesis, and cell division
- injury symptoms on susceptible plants include growth & reproduction abnormalities
photosynthetic inhibitors:
disrupt photosystem || or photosystem 1 e.g. simazine or paraquat
- plant but be undergoing active photosynthesis for the herbicide to be effective (tf more effective on sunny days)
- injury symptoms may be sdelyeaed under cloudy conditions and become evident after the first sunny day after application
- causes build up of e- in the leaves
amino acid synthesis inhibitors: act by..
inhibiting an enzyme (acetolactate synthase -ASL) involved in a.a synthesis
a.a synthesis inhibitors act over..
a number of days - weeks
typical symptoms of a.a synthesis inhibitors:
chlorosis, purple colouration around the veins, secondary root proliferation
a.a synthesis inhibitor example:
probably most famous herbicide - Glyphosate
production of a herbicide:
- chemicals (with specific or random structure) are produced in an chemistry lab
- applied to a small number to TEST weed species as a spray or systematic application -PRIMARY SCREEN
- any chemical which kills the plant goes to a 2nd CREEN (adult & seedling, stage, different concentrations etc.)
- tested for toxicity on crop species animals & Humans
- tested for persistence in the soil
- long term field trials & more thorough tests on possible toxic side effects
production of a herbicide takes __ years and costs how much
10-15 years from discovery to market & costs $10-15M
advantages of herbicides:
- they control multiple weed species
- some can move into roots and control perennial weed species
- some can move into roots & control perennial weed species
- some possess residual soil activity and can control weeds over a long period of time
- many cause little or no injury to the crop plant (As many are selective)
- They can be applied over a large area rapidly
- they control weeds quickly
disadvantage of herbicides:
-may be toxic to humans & Other animals
-may negatively impact the soil ecosystem
-may leach into the water supply
may persist in the soil for too long and damage a susceptible crop
improper application may result in injury to the crop
-repeated use of a herbicide may lead to herbicide resistant weeds
integrated weed management strategies involves:
- carefully managed use of different control tactics - (e.g. biological, cultural, chemical or GM )
- there are limitations to integrating these methods
- need to understand the biology of the weed, the biological control agent and the chemical herbicide to determine the best time to spray
e.g. of integrated weed management:
Control of the Water Hyacinth (Eichornia crassipes)
Water Hyacinth (eichornia crassipes) facts
- LAKE VICTORIA
- native to S. America & was introduced into florida in 1880’s
- very high growth rate - population doubles in 12 days
- prevents sunlight & oxygen getting into the water
- crowds & shades out submersed plants & reduces biological diversity
- prevents fishing & blocks waterways
- until the introduction
Water Hyacinth (Eichornia crassipes) is now controlled by a combination of
- classical biocontrol agents
- mycoherbicide (innundative control)
- occasionally a chemical herbicide
- mechanical clearance
classical biocontrol control agents used to control Water Hyacinth (Eichornia crassipes)
- Water hyacinth weevils (Neochetina spp)
- -both adults & larvae feed on various parts of the plants
- Water hyacinth moth larvae (Sameodes spp)
Mycoherbicide used to control Water Hyacinth (Eichornia crassipes)
-composed of spores of the fungus Cercospora rodmanii
• The pathogen exists at low levels in the population but because it is spread by rain splash it never reaches epidemic proportions.
• Therefore water hyacinths are inoculated with spores ro create an epidemic
• These control measures are used in Florida, Australia, Fiji, Honduras, India, Malaysia, S. Africa, Thailand (and other countries).
