Haas, Robert H.

Hoffman, Garlyn O.

Texas A&M University

-/2-6 7
7hf/- ~.
L-848
CONTROLLING DRIFT Of HERBICIDAL SPRAYS
Garlyn o. Hoffman and Robert H. Haas*
Spray drift is a major problem in applying
herbicides for brush or weed control. The problem
lS two-fold:
1. Drift from the spray operation may contami-
nate crops and animals on adJ cent areas.
2. Loss of spray material from the target area
may reduce the efficiency of the treatment.
From a safety standpoint, the first of these
problems is more serious. Safe application of
herbicides must limit the spray deposit to the
target area.
In Texas, the primary problem of herbicidal
drift is what happens to susceptible crop plants.
Sorne plants are extremely sensitive to the phenoxy-
type herbicides and will show effects from ex-
posure to minute amounts. Cotton, for example,
may deve10p characteristic 2,4-D symptoms at rates
of 1/1000 pound per acre and seed cotton yie1ds
may be reduced when sprayed with as little as
1/100 pound per acre. Herbicides may drift several
miles in sufficient quantities to cause effect on
susceptible crops if weather conditions are con-
ducive to drift and large acreage is treated.
WHAT HERBICIDAL SPRAY DRIFT IS
Spray drift is the movement of herbicides away
from the target area. The different kinds of
herbicidal movement are:
Vapor drift occurs when the herbicide vola-
tilizes after being deposited on the target area.
Vapor drift most like1y occurs when the ester
formulation of herbicides is used. These materials
are separated as "volatile" or "low volatile" formu-
1ations. Amine and salt formu1ations are referred
to as "non-volatile." These are relative terms and
even "non-volatile" herbicides will vaporize to
sorne degree under extreme weather conditions.
The "low volatile" esters generally vaporize slowly
when the temperature is below 90-100 degrees F.,
while the "volatile" esters vaporize readily at 65
·Extension range brush & weed control specialist and assist·
ant professor, Department of Range Science Texas A&M
University.
30.2
degrees F. The relative volatility of a herbicide
is related to its formulation, and "vapor drift" can
be controlled by using the proper herbicide formu-
lation.
Physical drift is defined as the movement of
droplets or partides away from the treated area
before they reach the intended target. Physical
drift results from wind carrying the small partides
away from the target area. AIso, temperature
inversion may prevent the sett1ing of very small
droplets and allow their movement away from the
target area under very calm conditions. Physical
drift can be reduced also by making the herbicide
application when the wind velocity is low, but
when there is no temperature inversion.
FACTORS AFFECTING PHYSICAL SPRAY DRIFT
Spray drift is determined primarily by meteor-
ological conditions and spray droplet size. The
important meteorological factors indude thermal
conditions, wind velocity and humidity. The de-
gree to which these forces exert their influence
depends on droplet size. Small droplets are easi1y
displaced. Table 1 shows the approximate dis-
tances that water droplets of different sizes may
be displaced while settling 10 feet in a 3 mph wind.
A greater fall distance or higher wind velocity
will markedly increase the drift distance. For
example, a 100 micron droplet falling 50 feet in
a 3 mph wind would be displaced 265 feet; in a
5 mph wind it would move 435 feet from the
point of re1ease. One micron is 1/25,400 of 1 inch.
Table 1. Approximate distances various size water droplets
would travel while settling 10 feet in an air flow averaging
3 mph.
Droplet diameter Description Distance
(microns) traveled
(feet)
1000 Moderate rain 5
500 light rain 7.5
300 10
200 drizzle 18
100 mist 50
50 200
15 coarse aerosol 2000
Evaporation of the droplets as they falI increases
the driftability of a spray. Droplet size affects
evaporation rate because the descent velocity and
surface to volume relationship both are functions
of droplet diameter. As the diameter of a droplet
decreases, the surface area to volume ratio increases
and evaporation proceeds at an increased rateo
The time of exposure to evaporative conditions
also increases with decreasing droplet size. A drop-
let 200 microns in diameter falIs an estimated
50 feet in 13 seconds, whereas a 50-micron droplet
takes 3.4 minutes to falI 50 feet. Measurements
of the lifetime of droplets in an atmosphere of
86 degrees F. and 50 percent relative humidity
have shown water droplets of 50 and 200 microns
in diameter to have a lifetime of 3.5 seconds and
56 seconds respectively.
Research with ground and aerial spray equip-
ment has shown that increases in spray pressure
result in a higher percentage of smalI droplets
in the spray. As pressure increased, the median
droplet diameter decreased with either a water
or o. 2 fuel oil carrier. Comparison of water
and fuel oil carriers in these tests shows that median
droplet diameters from water sprays were approxi-
mately 1.7 times as large as those from diesel oil
sprays applied under the same conditions.
Nozzle orifice size and design affects the spray
partic1e size. Larger orifices or nozzle tips that
produce solid jets generalIy produce coarse sprays.
Research with oil-base sprays from aircraft has
hown that as the nozzle orifice diameter is in-
creased the amount of drift susceptible spray is
reduced appreeiably. This is a reduction, not
elimination of smalI droplets.
With aerial application, the orientation of
nozzles relative to the thrust line of the aircraft
also markedly affects breakup of sprays. Orien-
tating the nozzles straight down or forward causes
the spray to be broken up more than if the nozzle
is oriented back and into the air stream. Orientat-
ing the nozzle so the spray is released in less
turbulent air aids in reducing droplet breakups.
The potential amount of spray drift varies
with methods of application. Ground application
generalIy is safer than aerial application because
the distance between the point of release and the
target is less and because coarse sprays can be
formed. Sprays from aircraft are drift susceptible
for thflse reasons: (1) spray from the nozzle is
subject to shear by high velocity air movement
past the nozzle tip, and (2) sprays usualIy are
released from considerable height aboye the target.
Sprays from fixed-wing aircraft commonly have a
droplet size distribution in which 50 percent or
more of the droplets are smaller than 200 microns
in diameter.
DRIFT CONTROL MATERIALS
During the past few years, the agricultural
chemical industry has developed various materials
and techniques for applying herbicides in low-drift
sprays. In general, the approach has been to reduce
drift by regulating the size of droplets by the
application of thickened sprays.
Invert emulsions. An invert emulsion is a
water-in-oil emulsion in contrast to the commonly
used oil-in-water emulsiono Use of invert emulsions
in the past has been limited, primarily because
of problems in handling and application. The
invert emulsion has a consistency similar to that
of mayonnaise and is commonly produced by mix...
ing the oil and water phases at the pump or at
the nozzle.
Mixing the herbicide with a bifluid system
has several distinct advantages: (1) premixing is
eliminated; (2) herbicide and water are metered
into the mixing chamber, thereby assuring a con-
sistent, correet ratio; and (3) herbicide formu-
lations may be designed to produce emulsions of
known and predictable viscosity. Limitations are:
(1) special equipment is required; and (2) only
those herbicides specially formulated to invert or
a specific invert additive can be used.
Invert emulsions have shown good potential
in reducing spray drift. The percentage of spray
volume contained by small droplets has been
greatly reduced. However, the invert is also subject
to factors such as nozzle tip design, orifice size
and nozzle orientation, alI of which affect the
degree of spray atomization. Emulsion thickness
also may markedly affect drift control. Thinner
emulsions produce more drift-susceptible droplets.
Invert emulsions may also be premixed and
applied through a conventional system. One such
method makes use of special equipment by apply-
ing the premixed invert from a centrifugal disc
apparatus designed to produce very coarse sprays.
Particulated sprays. A second drift control ma-
terial is a water-swelIable, water-insoluble polymer
that forms a "particulated" spray. Upon addition
of the proper amount to a water and herbicide
mixture, essentially all of the solution is absorbed
by the polymer partic1es which do not dissolve.
The swollen partic1e size is determined by the
size limit of ground dry polymer. The particulated
spray solution is quite viscous and similar to
that of a thin tapioca pudding. This material
offers the possibility of control over droplet size;
however, there is sorne aggregation of partic1es to
form large partic1es, and sorne small partic1es are
also formed from unabsorbed solution.
The volume of spray contained by smalI,
driftable partic1es is substantially lower with the
particulated spray than with conventional sprays.
Coverage aspects are similar to those of the invert
emulsiono
A disadvantage of this material is in premixing,
since a small error in measuring will result in a
poor mixture. If too much polymer is added, it
may be very difficult to pump. If too little is
added, there may be excessive free water, producing
small droplets when applied from fixed wing
aircraft. A limiting factor on the uses of herbi-
cides with the particulated spray is that only water
soluble herbicides may be used.
Other spray thickening agents. A third material
which shows promise for controlling drift is
hydroxyethyl cellulose. This material is a water
soluble polymer that increases the viscosity of the
spray solution according to the amount added.
Herbicides are m·ixed in the thickened solution
and the viscous mixture is sprayed through con-
ventional equipment. Hydroxyethyl cellulose of-
fers several advantages: (1) special equipment is
not required and (2) water soluble or emulsifiable
herbicides may be used. Herbicide mixtures can
be used and oil-in-water emulsions may be pre-
pared. The broad range of herbicides that can be
used with hydroxyethyl cellulose gives this material
broad potential use.
Other water soluble materials for thickening
spray solutions are on the market. Although these
materials have not been tested as thoroughly as
the hydroxyethyl cellulose, they are expected to
give similar results.
CHARACTERISTICS Of THICKENED SPRAYS
Thickened solutions tend to produce sprays
having a broad range of droplet sizes. Under
proper conditions they may provide a substantial
degree of drift control. At the same time, a sub-
stantial number of large droplets are formed.
These large droplets have a significant influence
on the distribution of spray volume and result in
a much lower number of droplets deposited per
unit area with a given volume of spray.
As droplet diameter is doubled, there is an
eight-fold increase in volume with a corresponding
reduction in cIrop numbers. The number of drop-
lets in a spray pattern may be especially important
with low-volume applications, even though require-
ments vary among plant species. Sorne evicIence
shows that a minimum number of droplets per unit
area is required for effective mesquite control.
Proper coverage for effective control requires at
least 72 droplets per square inch of leaf surface.
Research with invert emulsion sprays in field
tests has shown that droplet size and distribution
may be critical factors in obtaining maximum
effective spray coverage with thickened sprays.
Data in Table 2 show the range of droplet sizes
from several invert emulsion spray tests compared
with the conventional emulsiono If 200 microns
are used as a limit for effective drift control, 7.5
percent of the conventional spray volume is in
a very driftable formo The conventional spray
contains ten times as much volume in droplets
below 200 microns as the helicopter invert spray,
and more than seven times as much as the fixed-
wing invert spray.
Another significant characteristic of the spray
described in Table 2 is the effect of large droplets
on volume distribution. With the helicopter spray,
only 7 percent of the droplets were larger than
1000 microns but they accounted for 60 percent
of -rhe spray volume. This spray, when compared
with the conventional spray from the fixed-wing
aircraft, provided less than a third the number
of droplets per unit area. This could be a critical
factor in the effectiveness of a spray application.
In field applications it appears that twice the
volume per acre is required with thickened sprays
as with conventional sprays to obtain satisfactory
coverage for satisfactory control.
The drift control and droplet size range
characteristics shown for the invert emulsion sprays
are fairIy typical of other thickened sprays. Thus
far, a spray in which all droplets below 200 microns
were eliminated has not been observed. Reducing
the number of large droplets formed without
Jable 2. Relationship of droplet sin distribution to volume distribution on oil-in-water spray and two invert emulsion sprays.
Droplet size range (micronsl
Percent Less than 200- 600- Greater than Drops/sq. in.
Emulsion of total 200 600 1000 1000 at 5 gpa
Conventional Droplets 55.0 43.3 1.7 O 130
(fixed-wing) Volume 7.5 63.5 29.0 O
Invert Droplets 22.0 66.0 10.4 1.6 65
(fixed-wing) Volume 1.0 32.0 39.0 28.0
Invert Droplets 17.0 65.0 11.0 7.0 40
(helicopter) Volume 0.7 16.3 23.0 60.0
EMULSION TYPES
Cross Section of Typical Magnified Droplets
KEY
Water
Oil
Invert Emulsion
(Water-in-Oil)
Oil Film Surrounds
Water Particles
Both droplets are based on a ratio of one part of oil to 10 parts of water.
Conventional Emulsion
(Oil-in-WéJter)
Water Film Surrounds Oil
sorne increase in number of very small droplets
also has not been achieved.
Thickened sprays in general produce a more
variable spray deposit pattern than conventional
sprays. Measurements made at regular intervals
across spray swaths show more variation in the
spray deposit with thickened sprays. They are
also characterized by more abrupt cut-off points
at the edge of the spray swath.
Thickening agents offer considerable promise
for safer application of herbicides. Each of the
materials has certain advantages and limitations.
Successful use of any one requires a knowledge
of its specific characteristics. Drift control is ac-
complished by a reduction in the percentage of
small, drift susceptible droplets in the spray.
Nozzle tip design and orientation, solution vis-
cosity, airspeed and sprayer pressure appear to
be important factors in determining the success
of thickened sprays.
REDUCING SPRAY DRIFT BY REGULATING BREAKUP
Recent development of an air foil boom with
many solid stream orifices appears to hold promise
for regulating the size of droplets of conventional
spray solutions. The Micro Foil Boom used with
a helicopter, provides a spray having a minimum
of large and small droplets. Good coverage is
obtained with moderate volumes of application.
Special care is required to maintain the system
free of foreign matter which would plug the small
orifices. l.--he system has not been evaluated for
use on fixed-wing aircraft. Further evaluation is
necessary· to determine the potential of this equip-
ment for controlling drift of herbicide sprays
applied in rangeland brush and weed control.
Additional research is needed to develop methods
of regulating spray breakup with fixed-wing air-
craft application.
PRECAUTIONS
Successful use of thickened sprays depends on:
(1) the use of correct concentrations of the thicken-
ing agent, and (2) use of proper nozzle tips de-
signed and oriented to release the material in a
manner that subjects the solution to the least shear
forces at the nozzle. Applied under less than opti-
mum conditions, any one of the thickened solu-
tions can produce sprays containing considerable
drift susceptible droplets.
Herbicide applications should be made (1) in
slight wind conditions with a definite direction;
(2) no temperature inversion; (3) clear weather
conditions; (4) when spray doplets settle on target
area at a rapid rate; and (5) using low sprayer
pressure and large nozzle orifices.
PR-26D5} Possibilities for Controlling Drift with
Thickened Sprays} published by the Texas Agri-
cultural Experiment Station} Texas A&M Univer-
sity} 1968} lists 'references about drift control. This
information is available from your county Exten-
sion agent.
Cooperative Extension Work in Agriculture and Home Economics, Texas A&M University and the United States
Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, 1914, as
amended, and June 30, 1914.
10M-7-69


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Controlling Drift of Herbicidal Sprays.

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Controlling Drift of Herbicidal Sprays.

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