Ag Engineer Outlines Strategies for Reducing Spray Drift
Drifting spray wastes money, reduces the effectiveness of pesticides and
can cause serious damage to surrounding crops, trees, wildlife and water
supplies. Keeping pesticide applications on target is a key to having the
maximum impact on weeds, insects and diseases while minimizing costs, notes
a North Dakota State University agricultural engineer.
"Probably the most important threat from spray drift is the
potential damage to other crops in the area," says Vern Hofman of the
NDSU Extension Service. "Some crops, as well as trees and other native
vegetation are extremely sensitive to herbicides. An unintended application
from drift can have devastating results."
Major factors that contribute to pesticide drift are droplet size,
equipment, application methods, and environmental conditions, Hofman notes.
Applicators need to consider all those factors and make appropriate
adjustments to minimize the potential for drift.
Droplet Size is Important
Atomizing the spray solution into very small droplets may increase
coverage, but applicators need to consider the potential for evaporation,
drift out of the target field, canopy penetration and where the small drops
will be deposited. The smaller the drop the greater the risk of drift,
Drops less than 100 microns (about the thickness of human hair) lose
their velocity soon after leaving the spray nozzle. They are in free fall
within a few inches (a 50 micron drop loses itís velocity in 3 inches)
from the nozzle and evaporate rapidly. Rather than reaching the target, the
pesticide contained in water droplets become very small aerosols, which will
remain in the air until picked up in falling rain.
Drops over 150 microns resist evaporation much more because of their
larger surface area. The potential for drift rapidly decreases with these
"In reality, a range of droplet sizes is needed to deposit
pesticides on the wide variety of plant types, sizes and shapes that are in
the field," Hofman says.. The following suggests how different size
spray drops vary in effectiveness:
- Very fine droplets measuring less than 120 microns are collected
efficiently by insects and needles on coniferous plants, but tend to remain
in the airstream and are carried around the stems and leaves of weeds.
- Fine and medium droplets measuring between 120 and 350 microns deposit
more efficiently on stems and narrow vertical leaves such as grasses when
there is some air movement.
- Coarse and very coarse droplets measuring more than 350 microns deposit
most efficiently on large flat surfaces such as broadleaf weeds.
"To effectively control weed and insect pests, the actual range of
droplet sizes depends on the specific pesticide being used, the kind and
size of the target plant, and weather conditions," Hofman says. A few
nozzles are specifically designed to reduce drift by reducing the amount of
small, driftable droplets in the spray pattern.
Insecticides and fungicides generally require smaller droplets than
herbicide applications to obtain adequate coverage, he says. For foliar
herbicides, research suggests that droplet sizes in the range of 100 to 400
microns do not significantly differ in weed control effectiveness, unless
application volumes are extremely high or very low. Exceptions to this
guideline may exist for specific herbicides.
Equipment and Application Methods Also Play a Role
"Reduce drift by mounting the spray boom closer to the ground while
being careful not to disrupt the uniformity of the spray pattern. Wind speed
and drift increase with height," Hofman notes. The correct spray height
for each nozzle is determined by the nozzle spacing and the spray angle.
Wide-angle nozzles can be placed closer to the ground than narrow-angle
nozzles. Nozzles spaced 20 inches apart should be 18 inches above the target
for 80 degree tips and 15 inches for 110 degree tips.
However, wide-angle nozzles also produce smaller droplets, offsetting some
of the advantages of a lower boom height.
"Applicators are also advised to use the lower end of the nozzle
pressure operating range if the pesticide label allows. Higher pressures
generate more small droplets," Hofman notes. Try not to use pressures
that exceed 40 pounds per square inch (psi). Extended-range nozzles are
capable of operating at 15 to 20 psi while providing a uniform spray
pattern. "Remember that flow rate will go down as pressures are
reduced, so the sprayer will need to be recalibrated," he says.
An increase in nozzle size will create larger droplets, which are less
likely to move off-target. If you use nozzles that put out 5 to 10 gallons
per acre (GPA), increase to nozzles that put out 10 to 12 GPA, Hofman
Some applicators are reducing the spray volume of foliar herbicides from
the commonly used 7 to 10 GPA to 5 GPA or less. "When you reduce spray
volume, the herbicide concentration must increase to maintain the same dose
of active ingredient," he notes. "But as spray volume is reduced,
the droplet size will decrease, increasing the potential for drift.
Pesticide labels specify application rates which should be followed as the
label is the law."
Research has also shown that control of some broadleaf weeds with contact
herbicides declines as spray volume is reduced. However, reduced volumes
usually have little effect on weed control with most herbicides, as long as
the chemical is applied properly. It is best to follow chemical label
recommendations on application rates.
"To compensate for reduced spray volume, some applicators increase
spray pressure from 30 to 40 psi to 60 to 80 psi," Hofman says.
"They believe they can drive small droplets into the crop canopy to
increase coverage. The actual result is the opposite. Small drops will
quickly lose their velocity and evaporate before they reach the plant. In
addition, small droplets have less momentum and insufficient energy to be
driven into a plant canopy. Larger spray drops will maintain their velocity
and are able to be driven into the plant canopy."
Increased pressure should not be used as a substitute for spray volume,
Hofman says. It is recommended to maintain pressures below 40 psi, and if
you need increased coverage, increase spray volume (GPA).
Newer drift-reducing nozzles are becoming popular. All of them contain a
pressure reducing chamber in the nozzle so the spray drop produced is larger
with fewer fine drops. Research studies show excellent results in reducing
drift, Hofman notes.
The latest addition to this group of nozzles is the "air
induction" type. These nozzles pull air into the nozzle and introduce
air bubbles into the spray drop increasing the drop size. When the drop hits
the target it explodes spreading spray mix over a larger area of the plant
leaf. "This type of nozzle is excellent for systemic type
herbicides," Hofman says. "It should not be used for contact type
herbicides, which require smaller drops for good coverage."
Climatic Conditions are Another Key
"Wind speed and direction, temperature, relative humidity, and
atmospheric stability all affect spray drift. Wind speed is usually the most
critical meteorological condition. The greater the wind speed, the farther
small droplets will be carried," Hofman says.
"There is no maximum wind speed to serve as a guideline in all
situations, but try to spray when the wind speed is less than 10 miles per
hour," he says.
To minimize the damage done by drift, applicators should determine if
sensitive crops are downwind. To greatly reduce damage to sensitive plants,
leave a buffer zone at the downwind edge of the spray area. After the wind
has died down or changed direction, spray the buffer zone. The size of the
buffer zone is determined by the pesticide being sprayed and the sensitivity
of the adjacent crop.
Temperature and humidity affect the amount of drift that occurs through
evaporation of spray particles. Although some spray is lost through
evaporation under all atmospheric conditions, losses are reduced
significantly in cool, damp conditions, Hofman notes.
Temperature also influences atmospheric stability, as well as the
presence of air turbulence and inversions. An inversion can occur when the
air is very calm, with very little mixing. An inversion contains cool dense
air near the earthís surface with warmer air at higher elevations. This
condition makes it easy for small spray drops to remain suspended in the air
and move slowly downwind.
"That means extremely calm conditions can pose a significant risk
for pesticide drift; wind doesnít always have to be a factor," Hofman
Inversions often occur in early morning or late evening. "You can
recognize an inversion by observing a column of smoke. If the smoke does not
dissipate, or if it moves downwind without mixing vertically, conditions are
not good for spraying," he notes.
"The best way to avoid the kind of drift associated with these
atmospheric conditions is to eliminate the formation of very small droplets
in the spray. Once youíve eliminated those very small droplets, youíve
drastically reduced the effects of weather stability factors on drift
potential," Hofman says. "Larger spray drops will resist
atmospheric conditions much better than smaller drops."
Source: Vernon Hofman, (701) 231-7240, firstname.lastname@example.org
Editor: Tom Jirik, (701) 231-96290, email@example.com