Quantcast Section A -- Selecting Herbicides

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AFM 91-19 / TM 5-629 / NAVFAC MO-314
24 May 1989
Chapter 2
Section A -- Selecting Herbicides
b. One of the most important properties of
granular and pelletted herbicides is their physical
selectivity. The particles tend to bounce off dry
2-1. Analyzing the Situation:
foliage and other plant parts to the soil or to
a. Chemical control of a mixed population of
settle to the bottom of ponds, lakes, and other
weeds and brush requires a herbicide or a
bodies of water. This enhances the chemical
mixture of herbicides to which the many species
selectivity of the herbicides when used on grow-
on a site are susceptible. Therefore, it is impor-
ing crops or on submersed aquatic weeds. Gran-
tant to know the kinds of weeds that are present
ules and sprays of many herbicides are equally
and the herbicides that will control them. Identi-
effective on germinating weed seed in soil.
fying the main species in the population is the
However, when a herbicide is applied as a spray,
first requirement.
it is often intercepted by foliage, and its effec-
b. The response of a plant to an application
tiveness in killing germinating weed seeds in the
of a herbicide depends not only on the species
soil is reduced. Granular herbicides frequently
but also on the age of the plant, the rate of
have been used because of the scarcity of clean
application, and the soil and climatic environ-
water supplies for spraying in many areas and
ment in which the plant is grown. Seedlings are
the need for extra labor and equipment for
killed most easily-even seedlings of some resis-
water hauling.
tant species are killed. Many plants become
c. Granular herbicides are of special interest
more tolerant to a herbicide as they grow older.
for ornamental plantings because their physical
Some perennials are most easily killed when in
selectivity helps to broaden the use of a few
bloom. Woody plants may be more susceptible
effective herbicides to cover a relatively large
to one method of application than to another. A
number of plant species. That is, sprays of some
weed may be more susceptible to a herbicide if
herbicides would injure plant foliage whereas the
treated at the optimum time, with the optimum
herbicide in granular form does not. Therefore,
rate, and under the optimum environmental
they have been used rather extensively in horti-
conditions, but may be more tolerant under
cultural plantings after clean cultivation. Gran-
conditions that are less favorable for control.
ules also fill a specific need in transplanted
c. Selection of herbicide, in addition to con-
ornamental crops where preemergence herbicide
trolling the weeds, must also fulfill other crite-
treatments cannot be used. As a matter of
ria. Is it cost effective? Does the herbicide kill
convenience, granular herbicides have also been
all the vegetation, or does it selectively leave
used extensively in preplant soil-incorporated
some types of plants in the area? Methods of
treatments and preemergence treatments of orna-
application, modes of action of the herbicide,
mental plantings.
and effects on the ecology of the area are
d. Each herbicide, whether used in spray
important considerations. What are the hazards
or granular form, is most effective if used
of drift or runoff water from treated areas to
by a certain application technique under spe-
nearby desirable plants or other biota?
cific climate and soil conditions. Recommen-
dations prepared by weed research special-
2-2. Choosing a Formulation. Formulations
ists in state agricultural experiment stations and
make an important contribution to the efficacy
by the individual manufacturers of herbicides
of herbicides and to their ease of use in the
outline these necessary conditions and tech-
a. Emulsifiable concentrates and wettable
powder formulations are readily dispersed in a
water carrier, but require agitation during the
2-3. Comparing Costs. The carrier components
spraying operation. Water-soluble salts, water-
contained in herbicide formulations, such as
miscible liquids, and water-soluble powders nor-
emulsifiers, solvents, and other adjuvants, often
mally do not require agitation. Granular materi-
improve mixing, spraying, and weed-control re-
als are applied with granular pesticide spreading
sults, but the cost of the herbicide largely
equipment, modified fertilizer spreaders, or by
depends on the amount of phytotoxic chemical
that it contains.


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