Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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The effectveness curve stays telatively flat at low canopy densities because most herbcide applications involve an
excess, or -overkill." factor. Conweqiently. a certain amount of interc;eption by the upper canopy can be tolerated.
The point at which a further increase in tree %anopy will result in a sharply reduced kill of understory vegetation will
depend on the kind and amount of herbicide, and the number of droplets applied per acie over the canopy.
particular data in Figure 36 were obtained from applications of 2.4-D:2.4.5-T mixtures at does rates ranging fiwi 4.0 to
8.4 pounds. acid equivalent, per acre and at volumes of 10 gallons per acre.
The importance of applying an adequate number of droplets in order to overcome the effects of canopy interception
cannot be overemphasized.
We have observed the benefits from adequate droplet coverage in many situations, even in
spraying shrub types. For example. spraying dense stands of an easily killed shrub species in Hawaii produced good kill of
overstory vegetation about IS feet in height regardless of spray volume applied. But small understory plants of the same
species were not effectively desiccated when low volumes were applied. The range in percent effectiveness for three
different volumes were:
The
Ratio of
Volume Desiccation Effectiveness understory to
applied Overstory Understory overstory
(gal./acre) (percent) (percent)
5 100 97 .97
3 91 75 .82
S86 32 .37
Herbicide dose was the same for the 5-gallon and I-gallon applications. The I-gallon volume delivered about 100
droplets per square inch on the overstory plants, but coverage of understory plants was inadequate. The herbicide dose
was tripled in the 3-gallon application, but this did not overcome canopy interception to the degree obtained by the
greater number of droplets in the 5-gallon application at one-third the dose rate.
In summary, dense overstory canopies must be reduced in order to obtain adequate desiccation of understory
vegetation with foliar sprays. The canopy can be reduced by a defoliation spray, followed by a desiccation spray as soon
as most leaves have dropped from the upper canopy. The desiccation spray, should be applied at a volume of at least 5
gallons per acre even though upper canopy density has been reduced ahead of desiccation spraying.
An alternate to foliar spraying is aerial application of the herbicide in a solid form that will penetrate dense canopy.
Herbicides in a granular or pellet form will reach the soil surface with little canopy interception.
T
Types of Desiccants
Desiccants suitable for aerial application are of two kinds:
1. Systemic herbicides which enter the plant, move through the vascular system, and kill tissue away from the point
of contact.
2. Contact herbicides which kill only the living tissue that is directly contacted.
Contact herbicides are applied as sprays which usually dry leaves within a few days or weeks, depending on the plant
species and the weather. The contact sprays are particularly effective on leafy herbaceous plants, such as grass and weeds.
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