Section V. HERBICIDES
Herbicides are chemicals used to kill or control the growth of weeds. A weed
may be defined as any noxious plant, that is, one whose presence is unwanted and
which interferes with the growth of desirable plants. Therefore, the term is a relative
one. Some plants that may be desirable in one place may be very noxious in others.
Examples are Bermuda grass, Johnson grass, oak trees, persimmon trees, and many
kinds of vines. Weed control is an Army Engineer Corps, rather than an Army Medical
Corps, responsibility. Therefore, Army Medical Department interest in herbicides is
confined primarily to the areas of toxicity and environmental impact.
1-14. CLASSIFICATION OF HERBICIDES
Herbicides may be classified as to their selectivity (principal use) or to their
Classification According to Selectivity.
(1) Selective herbicides. Selective herbicides kill certain weed species
without seriously injuring the desirable plants among which they are growing. The
reasons for selectivity in some combinations of weeds and desirable plants are known.
Annuals (seed-bearing plants which mature in one season) growing among perennials
(plants that live more than 2 years) can be killed by sprays from which the perennials
can recover. The reasons for selectivity in other situations are not known. Certain
herbicides kill broad-leaved weeds, others kill grasses, and some kill both.
(2) Nonselective herbicides. Nonselective herbicides kill vegetation with
little discrimination. Some species of plants, however, escape. Some are resistant;
some have roots that extend below the depth of chemical penetration; and some
shallow-rooted plants reinfest after the chemical has leached below the surface layer.
(1) Contact herbicides. Contact herbicides kill tissues that are wetted with
spray. Whether the plant dies or recovers depends upon whether it has a protected
growing point. Perennials usually have underground buds that will regrow.
(2) Growth regulator herbicides. Growth regulator herbicides act like plant
hormones. They are absorbed through the leaves, stems, or roots and are translocated
through the vascular system to other parts of the plant. They accumulate mostly in
areas of rapidly dividing cells, upsetting the normal metabolism of the plant and causing
death of the cells.