Quantcast Modes of Action of Herbicides

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AFM 91-19 / TM 5-629 / NAVFAC MO-314
24 May 1989
equivalent is used to express the active chemical
are various obstructions to herbicidal action
in dinitrophenol derivatives.
along the critical path from the point of applica-
b. Most herbicides are purchased as commer-
tion to the arrival of the herbicide at its site of
action in the plant. These obstructions differ
cial formulations that can be (1) dissolved,
emulsified, or suspended in a liquid carrier;
among plant species and are part of the basis for
(2) distributed dry as granular products or pel-
selectivity. There are also varying plant re-
lets by a spreader or by hand; or (3) injected
sponses at the sites of action that form a basis
into soil for vaporization and fumigation. Often
of selectivity. For instance, in some species the
an emulsifier, spreader, sticker, or other surfac-
breakdown of the herbicide by the plant may
tant is added to facilitate dilution and adhering
keep pace with herbicide accumulation so that
capacity to increase wetting by the diluted
lethal concentrations are not reached, whereas in
other species lethal concentrations are reached
and kill the plant.
c. Granules and pellets are prepared in several
ways. Some herbicides are impregnated on gran-
(2) Some of the barriers or obstructions
ules of clay, vermiculite, or crop residues, such
that prevent a herbicide from killing plants
as ground corncobs. This is accomplished by
differ among plant species and may be encoun-
spraying, dipping, or exposing the granules to
tered at each of four steps in a critical path that
the herbicide, Pellets and granules are also
a herbicide travels after leaving its container.
prepared by mixing a herbicide with finely
The steps of the critical path are:
ground clay or fertilizer salts and forming parti-
(1) Achievement of herbicide surface contact
cles by extrusion or prilling.
with the plant or plant parts; (2) penetration or
entry into the plant; (3) translocation to a site
d. Each herbicide, whether used in spray or
granular form, is most effective if used by a
of toxic action; and (4) disruption of some vital
certain application technique under specific cli-
mate and soil conditions. Recommendations pre-
(3) Nonselective herbicides kill vegetation
pared by weed research specialists in state agri-
with little discrimination. A limited number of
cultural experiment s t a t i o n s a n d b y t h e
species, however, are physiologically resistant to
individual manufacturers of herbicides outline
the chemical, and some of these escape. Thus,
these necessary conditions and techniques.
there is no herbicide known that is completely
nonselective. Some escapees are perennials that
have part of their root systems below treated
1-7. Modes of Action of Herbicides.
layers of soil; others are annuals and shallow-
The actual mechanisms by which plants may
rooted perennials that reinfest the area after the
be killed are so numerous, and plant functions
chemical has leached below the surface layer.
are so intimately interconnected, that there can
b. Contact versus Translocated Herbicides.
be no single theory on herbicidal action. The
These are distinctly different in use and the
physiological responses of plants that may occur
types of weed that they will kill.
after a herbicide reaches a site of action include:
(1) Contact herbicides kill all tissues that
changes in respiration, nutrient uptake, and
are contacted by the spray. Whether the plant
carbohydrate utilization; disturbances in potas-
dies or recovers depends on whether it has a
sium metabolism; abnormal cell production; ab-
protected growing point. Also, many perennials
normal phosphatase activity; blockage of photo-
have underground buds that are not contacted
synthesis; reduction in vital leaf area; arrested
by the herbicide and that are capable of generat-
cell division; and production of metabolites
ing new plants.
injurious to the cell.
a. Selective Versus Nonselective Herbicides.
(2) Translocated chemicals are absorbed by
the leaves and stems, or by the roots, and move
Selective herbicides kill certain plant species
through the vascular system to the leaves, buds,
without seriously injuring other plants among
and root tips. Translocated herbicides, when
which they are growing. Herbicides that selec-
absorbed by the leaves and stems, commonly
tively kill crabgrass or dandelions in a lawn are
move in the plant's phloem (food-conducting
examples. The reasons that herbicides are selec-
tissue) with the food materials manufactured by
tive in some combinations of weeds and desir-
the leaves and stems. When absorbed by the
able plants are known. The reasons they are
roots, they move in the xylem (water-conducting
selective in other situations are unknown.
tissue). The growth-regulator type of transloca-
(1) Selectivity is caused by the differing
ted herbicide is a synthetic compound that
responses of plant species to a herbicide. There


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