Transcript To the weed

AGR 3102 (1)
Principles of Weed Science
Herbicides
Muhammad Saiful Ahmad Hamdani
Week 7 – Topics Covered
Herbicides:
Introduction to herbicides
History, nomenclature, toxicity,
classification
Introduction to Herbicides
• Any chemical agent that kills or greatly
inhibits plant growth.
• “Plant killer". Herbicides specifically
cause phytotoxicity to plants.
• Phytotoxicity: the degree of toxic effect by
a compound on plant growth.
Why herbicides are less harmful to us and
animals?

Herbicides target highly specific biological or biochemical
processes within plants (i.e. PS, branch-chain amino acids
synthesis, plant enzyme & hormone, etc).

At label rate, have very little impact on non plant or nonphotosynthetic organisms.

Still, not safe to intentionally ingest herbicides (still have toxic
effect, different formulation, mixed with other potentially
dangerous chemicals too i.e. adjuvant).

Paraquat is exceptional!!!
1. History of Herbicides

1890 to 1900 - sodium arsenite to control
aquatic plants in waterways in the US.

1925 - sodium chlorate (direct soil application)
was first used for killing weeds.

1934 - sodium nitrocresylate, as the first
selective weed killer in France.

1940s – first organic herbicide 2,4-D was
commercialized providing selective control of
certain broadleaf weeds in grass crops.

1960's – pre-emergence herbicides trifluralin
and atrazine introduced.

1974 - glyphosate (Roundup) available - among
the world’s most important herbicides S.B.Powles. Control broad-spectrum of weed
spp.

As in 2011, more than 200 herbicides from 22
mode-of-actions available in the market.
Why Herbicides Are Important Component
in Agriculture???
Pesticide expenditure totaled more than $39.4
billion in 2007.
 Expected to increase 2.9% per year to $52
billion in 2014.
 Herbicides make up the largest number of
products and are the most used in the
agricultural sector, due to their extensive use.
 Worldwide, expenditure on herbicides account
40% of all pesticide expenditures (Grube et al.
2011).


Malaysia: 75-80% herbicides use & 70-75% trade
compared to other pesticides (FAO, 2013).
Examples of herbicide in Malaysia
15
Why Herbicide Use is Always Higher Than
Other Pesticides?
We human don’t want crops and weeds to
co-exist
Herbicide Nomenclature
1.
Active ingredient (AI) - the chemical in the herbicide
formulation primarily responsible for its
phytotoxicity. Kills/injures the plant/weed.
2.
Trade name - a trademark or other designation by
which a commercial product is identified.
Registration no., registrant/distributor included.
3.
Common name - each herbicidal chemical has one
common name assigned to it. In many cases, the
common name is a simplified version of the chemical
name.
4. Chemical name - the systematic name of a chemical
compound according to the rules of nomenclature of
the International Union of Pure and Applied Chemistry
(IUPAC), Chemical Abstracts Service (CAS) or other
organization. Represents AI.
5. Toxicity class – how dangerous of a herbicide to
animals/humans.
6. Formulation & type of herbicide – form/condition of
the herbicide. Mixing instructions & procedures prior
application included.
Common name
Herbicide Toxicity Class
 Acute toxicity (death) studies of a herbicide
on non target population (non
photosynthetic)
 Experiments on animals, via oral, inhaled, or
external application of a herbicide.
 Test was created by J. W. Trevan in 1927.
Source: Richards, Kerry Penn State University Extension Service
Source: Richards, Kerry Penn State University Extension Service
LD50
Lethal Dose = Amount of
chemical it takes to kill 50% of
an experimental population

LD50 value measurement: mg chemical/kg of
test population

Draw back: not address issues of other
potential harm of the agent, such as
bioaccumulation, issues of carcinogenicity,
mutagenic effects, or the impact on
reproduction.
Source: Richards, Kerry Penn State University Extension Service
Class
LD50
Solid
LD50
Solution
Colour Symbol
Code
Hazard
Statement
Ia
<5
<20
Very Toxic
Ib
5-50
20-200
Toxic
II
50-500
200-2000
Harmful
III
5005000
200010000
Caution
IV
>5000
>10000
NA
Table 2. Comparison of oral LD50 values for commonly
used herbicides and consumer goods.
Herbicide
LD50
Common consumer
chemicals
LD50
Paraquat (Gramoxone) ~100
Nicotine
9-10
Pinoxaden (Axial)
>3100
Bleach
192
2,4-D
666
Caffeine
192
Atrazine
3090
Household ammonia (10%)
350
Glyphosate (Roundup)
4900
Table salt
3000
Glufosinate
>430
Codeine (pain and cough
relief)
427
Source: Fishel et al. 2009
Important!!!

Some herbicides such as paraquat are
extremely harmful and must be handled with
great care.

However, just because some herbicides are less
toxic does not mean that any herbicides should
be handled carelessly.

Using a herbicide in accordance with the
product label will not often result in personal
injury or cause for alarm.
Herbicide Classification
 Classified
based on different activity,
mode of action, residual activity,
placement, time of application, methods
of application, & type of vegetation
controlled (selectivity).
A. Selectivity: Selective vs. Non-selective
1. Selective: toxic to some species and much less
toxic to others at the label rate. Kills weeds but
not the crop at label rate.
2. Non-selective/broad-spectrum: kills or severely
damages all or most species. A non-selective
herbicide kills weeds and potentially crops.
Must avoid contact with crops!!!

A selective herbicide can be made nonselective by applying improperly or by applying
high rates. Selectivity is usually rate
dependent.
Herbicide Selectivity
Selective: A herbicide that is more effective in
controlling some plant species than others.
No
activity
No
activity
Activit
y
Non Selective/ Broad Spectrum: A herbicide that
controls many plant species.
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B. Activity: contact vs. systemic
1. Contact herbicides
a. Effective only on tissue contacted by herbicide.
b. Very little movement within the plant.
c. Full spray coverage is crucially essential.
d. Usually exhibit acute effects.
e. Effective on annuals, less for creeping
perennials.
2. Systemic herbicides
a. Absorbed by roots or above-ground
parts, translocated within plant.
b. Good spray coverage is necessary,
but less critical than contact
herbicides.
c. Usually exhibit chronic effects.
d. Effective on annuals, as well as on
perennials.
Effect of different activities of herbicide
on Asystasia spp. in rubber plantation 3 weeks after spraying
Glyphosate: systemic
Paraquat: contact
C. Residual activity: Residual vs. Non-residual
1. Residual: Herbicide remains in active form in soil
& retains activity on weeds growing after
application.
2. Non-residual: Has activity only on weeds present
at time of application. Become invalid upon
reaching soil. Does not provide control of weeds
that emerge after application.
D. Time of application (when herbicide is applied)

Pre-plant: applied to soil prior to planting the
crop. Incorporate or not.

Pre-emergence: applied to the soil prior to
emergence of the weed.

Post-emergence: applied after emergence of
the crop or/and weed.
Time of Applications
Preplant
Preemergence
Postemergence
To the crop & weed
After crop is sown
but before crop or
weeds emerge
OR
Before the crop
is planted
To the weed
To control
emerging weeds
after crop emerges
After crop and/or
weeds emerge
= Weed Seed
= Crop Seed
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E. Placement of herbicide
Two primary placement options for herbicides:
Soil Application
Foliar/Plant Application
F. Methods of application

Refer to the ways herbicides can be
applied.

Several methods depending on
herbicide, cropping system,
crop/plant, & area:
1.
•
•
•
Banding:
Application directly to a strip over or along a
crop/crop seeds row.
Pre-emergence and post-emergence herbicides.
Targets weeds underneath/in crop canopy.
2. Broadcast/overall:
• Uniform application to entire area/field/farm.
• Pre-emergence and post-emergence
herbicides.
• Needs high amount of herbicides.
3. Directed:
• Spray to the base of the crop, avoiding contact
with foliage and green stems.
• Post-emergence.
• Need shield (hooded spray) in narrow rows.
4. Spot treatment:
• Applying small amounts of herbicides to
individual or small patches/groups of weeds.
• Post-emergence.
• Suitable in small areas.
5.
•
•
•
•
Wiping technique:
Variation to spot treatment.
Mainly used in ornamental areas.
Just wipe the weeds when they are taller than
the ornamentals.
Use rope, paint roller, or sponge type devices
instead of nozzle.
Conclusion

Herbicides = kill unwanted plant if use properly &
appropriately. Kill crops, beneficial plants & even us if
misuse.

Most used compared to other pesticides.

a.i. kills the weed, LD50 indicates the toxicity level to
humans and animals, the lower LD50, the higher
toxicity.

Can be distinguished by different classifications.

Know herbicide classification, can decide the most
appropriate herbicide to apply & how to apply =
avoid/minimise risk to us, crops and environment.