What
is a Herbicide?
Herbicides
are substances that are used for
killing all types of plants, especially noxious weeds. Herbicides
can
be put
into 8 different groups including one group that is "ungrouped."
Herbicides
are chemicals that kill plants or inhibit their growth.
Herbicides
doings
are varied and theoretically as numerous as the processes essential to
plant
life. Herbicides are most often and most effectively used
together with
good
cultural practices in a turf weed management program. The choice
of the
best
specific combination varies with agronomic, ecological and economic
factors.
How
Do Herbicides Really Work?
They
work in many different ways. In order to be
effective, herbicides must first be applied to a plant. After the
herbicide is
applied, it must then be retained on the leaf's surface. Then it
must
move
through the water filled space that is around the cell. Once the
herbicide has
moved through the water filled cell or space, it must then enter a new
cell while
passing
through a lipid like membrane. When it has passed through that
cell the
herbicide will usually reach its target. The target is usually an
enzyme.
Finally when the herbicide reaches the target it will bind to and
inhibit the
target enzyme. The herbicide must take a different
approach. It must
first
target the weed and not the plant or crop. Then the crop and not
the
weed
itself must break it down.
Why Do Herbicides Kill Plants?
They kill the
plants by causing a build up of
toxic substances where the toxic compounds usually stay at low
levels. By
inhabiting
the target site the herbicide causes substances to build up and then
damage the
plant. This is how herbicide glyphosate works. In other
cases the
death of the
target plants seems to occur from deregulation of the very carefully
controlled
process of cell growth. This is how herbicides such as 2, 4-D
work; the
plants just grow them selves to death. Herbicide resistant plants
may
help a
farmer reduce expenses indirectly. Although seeds are a small
part of
total
farm costs a farmer may save money if the new seeds are paired with
herbicides
that are cheaper because their patents have expired and have to compete
with
generic versions. The biggest potential lies with designer seeds
matched with
new more efficient proprietary chemicals, according to Charles
Benbrook,
executive director of the Board of agriculture at the National Academy
of
Sciences. Herbicides resistant seeds may also help to solve the
problem
of
“carryover” in which the chemical lingers in the soil and is toxic to
the next
crop to be planted. The new plants will almost certainly be
designed
with
multiple resistance's to give the grower even more choices to match his
needs.
For chemical companies herbicide resistance is a smart way to protect
their
current markets or expand into new ones. Most of the major
herbicide
manufactures are engaged in some research to develop herbicide
resistant
plants. The volume of herbicide use in the U.S.
is actually declining because
the most recent weed killers are more potent. Some types of
herbicides
are
soil applied herbicides.
Herbicide
Use in Forestry
The
herbicide uses in forests are in some instances, a cost
effective tool for the control of vegetation. Landowners use them
in
several
different ways to increase the forest productivity. Forestry
herbicides
can
help prepare sites for tree planting, reduce unwanted vegetation, and
provide
conditions for prescribed fires. They will also reduce
competition form
herbaceous weeds so that newly planted trees are given a boost for
survival and
reproduction. They may be applied to improve the growth rates
in
established
stands by selective removal of non crop trees. The vast majority
of
herbicides
are used for growing pines although some are applied for herbaceous
control and
timber stand improvement in hardwood forest.
Toxicity
Many people have the
misconception that all compounds whose name ends in “ cide” such as
insecticide, rodenticide, or fungicide can be lumped together as danger
highly
toxic chemicals, and unsafe at any application level. This is
simply
not the
vast majority of agriculture pesticides and is certainly not true of
forestry
herbicides. Table 1 provides the acute toxicity of the active
ingredient in
several forestry herbicides for comparison
some other
common chemicals. The table lists the
LD50, which is a rating system for chemical toxicity. A low LD50
indicates that
a small amount of chemical is toxic and is a more dangerous
substance.
On the
other hand, the larger the LD50 the less toxic the chemical is.
All of
the
forestry herbicides have active ingredients that are less toxic than
caffeine.
And, the active ingredient is diluted to make the herbicide product
sold on the
market. All over the counter formulations of the products listed
in
table 1
have LD50 above 1700 mg/kg and so are therefore less toxic than
aspirin.
Trade Name
|
Active Ingredient
|
LD50 of Active Ingredient mg/kg
|
Arsenal
|
Imazpyr
|
5000
|
Garlon
|
Triclopyr
|
630
|
Oust
|
Sulfomenturon
methyl
|
5000
|
Roundup
|
Glyphosate
|
4320
|
Tordon
|
Picloram
|
8200
|
Velpar
|
Hexazinone
|
1690
|
Weedone
|
2,
4-D
|
375
|
For
Comparison
|
table
salt
|
3750
|
|
Aspirin
|
1700
|
|
Malathion
|
370
|
|
Caffeine
|
200
|
Literature Cited
McNabb, Ken, Environmental Satey of Forestry
Herbicides, March, 1997.
Alberta Agriculture, Food and Rural Development,
How Herbicides Work,
Mechanisms
of Action, copy write 1996, 56 pgs.
Links
http://www.aces.edu/pubs/docs/A/ANR-0846/
http://www.weeds.crc.org.au/weed_management/chemical_control.html
http://mtwow.org/How-do-herbicides-work.html
http://infotrac.galegroup.com/itw/infomark/19/166/106605845w3/purl=rc1_STO_0_A4461282&dyn=3!xrn_1_0_A4461282?sw_aep=mtlib_2_1527
http://www.uri.edu/ce/factsheets/prints/herbicidesprint.html
http://www.uri.edu/ce/factsheets/sheets/herbicides.html
http://www.biotech-info.net/more_herbicide.html
http://www.pmac.net/blunting.htm
http://www.weeds.iastate.edu/reference/moa.pdf
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74121.html
By:
Chris Henderson
2/8/2006
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