Dynamica en Residu’s van Pesticiden
Drift van pesticiden
9. Drift van pesticiden
1. INTRODUCTION: AGRICULTURAL SPRAY APPLICATIONS .................................. 2
2. DEFINITION OF DRIFT........................................................................................ 5
3. FACTORS THAT AFFECT DRIFT.......................................................................... 7
3.1 DROPLET SIZE DISTRIBUTION ..................................................................... 7
3.1.1 Nozzle type and size ................................................................................ 7
3.1.2 Spray pressure ........................................................................................ 9
3.1.3 Physicochemical properties of the spray liquid ........................................ 9
3.2 WEATHER CONDITIONS ............................................................................. 12
3.2.1 Wind ..................................................................................................... 12
3.2.2 Temperature and Air Humidity .............................................................. 12
3.2.3 Atmospheric Stability ............................................................................ 13
3.3 SURROUNDING CHARACTERISTICS ........................................................... 15
3.3.1 Crop type .............................................................................................. 15
3.3.2 Neighbouring structures at the field borders ......................................... 15
4. MEASUREMENTS OF DRIFT ............................................................................. 16
4.1 WIND TUNNEL MEASUREMENTS ............................................................... 17
4.2 FIELD MEASUREMENTS ............................................................................. 18
5. TOWARDS DRIFT REDUCTION ...? ................................................................... 19
6. DRIFT RISK ASSESSMENT ............................................................................... 23
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1. INTRODUCTION: AGRICULTURAL SPRAY APPLICATIONS
World-wide, most pesticides are applied through hydraulic sprayers of various size
and complexity. From the smallest knapsack to aerial equipment, the main parts
are a tank, a pump and a set of nozzles, interconnected by pipes/hoses and control
valves.
Aerial equipment (not further discussed here) is common in America. In Europe,
spraying equipment at ground level is used. We can distinguish two kinds of ground
spray applications: Spray application for orchards (e.g. fruit orchard, vineyard…)
and spray applications for fields [e.g. grasslands and field crops (potatoes, wheat,
sugar beets…)]. In orchards vertical spray equipments are used (figure 1), whereas
in fields we encounter horizontal spray booms (figure 2).
Figure 1. Examples of orchard spraying
Figure 2. Example of field spraying
Spray nozzles are mounted to the boom. Nozzles can be classified according to the
type of energy that is used to atomize the liquid into a spray: hydraulic, rotary or
air-blasted.
Hydraulic nozzles are the most important in agricultural spraying: Liquid is
atomized by being forced under pressure (hydraulic energy) through the nozzle tip
forming droplets. In this way a spray is created. See figure 3.
Rotary nozzle: The spray liquid is atomised into droplets by a spinning disc. The
higher the rotation speed, the finer the droplets.
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Air-blast nozzle: An air stream at high velocity is used to blast the spray liquid into
droplets.
Figure 3. Example of a spray nozzle (left) and a nozzle spray (right)
A very important characteristic of the spray is its droplet size. The finer the
droplets, the better the coverage of the canopies and the pests. Figure 4 illustrates
this. If the droplet diameter is reduced from 400 µm to 50 µm for a constant
amount of sprayed liquid, then 512 (=28 where 8 = 400 µm/50 µm) droplets instead
of one single droplet can be formed, and hence the coverage and biological efficiency
of the insecticide strongly increase!
Figure 4. Illustration of the relation between droplet size and coverage of an insect pest
On the other hand, spraying with fine droplets (< 150 µm) leads to drift (figure 5).
Drift is defined as the quantity of pesticide active ingredients (a.i.) that is deflected
out of the treated area by air currents at the moment of spray application. The
deposition of a.i. outside the target field can have negative effects on the health of
humans and the environment (especially surface waters are known to be driftsensitive), it also leads to economical losses.
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Figure 5. Illustration of drift
Note that the droplet size is an interaction between three factors: nozzle selection
(size and type), spray pressure and physicochemical properties of the spray liquid
(viscosity and surface tension). These parameters will be discussed later on.
The droplet size is not one single value, but it is a uniform distribution of sizes,
which shows a maximum. An example of a droplet size distribution spectrum is
given in figure 6.
Figure 6. Droplet size distribution spectrum of a spray with nozzle FF 110 03 at pressure 3 bar
The left spectrum of figure 6 is only presented here to give an idea of the shape and
the magnitude of a droplet size spectrum, but this kind of spectrum is not used in
reality! The spectrum is commonly expressed on a logarithmic scale, as illustrated
in the middle graph.
The most important parameter derived of the spectrum is the Volume Median
Diameter (VMD or Dv0.5), which is defined as the droplet size of which 50 % of the
spray volume has a higher diameter than this value, and 50 % has a lower diameter
than this value. Therefore droplet size distributions are often presented as
cumulative logarithmic spectrum (right graph in figure 6), because in this way of
presenting, the VMD value is easily derived out of the graph.
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The VMD values do not give an indication of how broad the range of the
distribution is. As illustrated in figure 7 (left), two spectra can have the same VMD
value, but the blue spectrum is broader than the orange one.
Figure 7. Illustration of Dv0.1, VMD and Dv0.9 values
For this reason, the Dv0.1 and the Dv0.9 values are defined respectively similarly as:
-
the diameter value for which 10 % of the spray volume has a lower diameter
value, and 90 % has a higher value
-
the diameter value for which 90 % of the spray volume has a lower diameter
value, and 10 % has a higher value
The Dv0.1 and Dv0.9 values give indications on how large the ranges of the spectra
are. Again it is better to present the cumulated spectrum (figure 7, right) because in
this way of presenting, the Dv0.1 and Dv0.9 values are easily derived out of the graph.
2. DEFINITION OF DRIFT
Spray drift is defined as the quantity of plant protection product that is deflected
out of the treated area by air currents at the moment of, or after, spray application.
The deposition of plant protection products outside the target field can have
negative effects on the health of humans and the environment (De Schampheleire et
al., 2006). Spray drift is also an undesired phenomenon because it causes great
economical losses.
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Figure 8. Particle spray drift (left) versus evaporation drift (right)
There are two forms of spray drift: particle spray drift and evaporation spray drift.
Figure 8 illustrates the differences: Particle spray drift occurs immediately the
moment of spraying, while evaporation spray drift happens afterwards spraying.
Even days or weeks after spray application, evaporation drift can occur.
Also see figure 5 for a realistic particle spray drift-scenario!
This course deals with particle spray drift. For shortness in the text that follows, we
will briefly talk about ‘drift’.
Drift is expressed as percentage (%) of the application dose (l/ha or kg/ha) in
function of the distance. For example if one says that there is 5 % percent drift for
an application dose of 180 l/ha of a herbicide at 5 m from the field border, then this


l 

means that at 5 m distance of the field border we will find 9 l/ha  5% * 180  of
ha
herbicide. Figure 9 gives an example of a typical drift curve in function of the
distance at 15 °C temperature, 60 % relative air humidity and 2.3 m.s-1 wind speed.
DRIFT IN GRASSLAND
(t° = 15 °C, R.H. = 60 %, v = 2.3 m/s)
6
Drift (%)
5
4
3
2
1
0
0
5
10
15
Distance from field border (m)
Figure 9. Example of a drift curve for grassland
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3. FACTORS THAT AFFECT DRIFT
The main factors that affect drift are droplet size distribution of the spray, weather
conditions and surrounding characteristics.
3.1 DROPLET SIZE DISTRIBUTION
As already mentioned, spray droplet size distribution is an interaction between
nozzle (type and size), spray pressure and physicochemical properties of the spray
liquid.
Good coverage is essential for insecticides and fungicides because of the small size
of the target organism (see figure 4). Therefore, small- to medium-size droplets are
desirable when applying insecticides and fungicides because they provide better
penetration into the canopy and better coverage. However, small droplets can drift
long distances. The smaller the droplets, the lighter they are and the more driftprone.
It is stated that droplets smaller than 150 µm in diameter pose the most serious
drift hazards, while some researcher take 100 µm as drift-prone limit.
3.1.1 Nozzle type and size
There are a lot of nozzle designs available on the market. For agricultural sprayings
the most important nozzles are the hydraulic nozzles. Some commonly used
hydraulic nozzles are explained below.
Flat Fan nozzle and Low-Drift nozzle
The Flat Fan (FF) nozzle was already illustrated in figure 3.
Figure 10 shows how a Low-Drift (LD) nozzle operates. An LD nozzle is an FF nozzle
with an extra orifice. The pre-orifice creates an extra compartment (turbulence
chamber) before the nozzle tip. When the spray liquid enters this compartment, the
pressure of the liquid drops, and thus the liquid looses some of its energy and has
less energy to break up in droplets. Coarser droplets will be formed at the nozzle
tip.
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Figure 10. Construction of an FF nozzle and an LD nozzle
Air Induction nozzle
The air induction (AI) nozzle (figure 11) consists of a narrow channel that ends into
the nozzle tip. At the beginning of the channel there is an air hole, through which
air is sucked in a natural way (Venturi principle) when liquid is being forced
through the channel under pressure. Inside the channel there is an intens mixing
between the liquid and the air phases due to the high shear forces. At the nozzle
tip, where the spray liquid is atomized into droplets, the air is incorporated inside
the droplets as small air bubbles. In this way – due to the incorporation of air –
coarser droplets are generated compared to the FF nozzle.
Air hole
Nozzle tip
Figure 11. Construction of an AI nozzle
Nozzle codes
Nozzles are indicated with a code that consist of 3 parts:
-
A part that gives information on the nozzle type (FF, LD or AI)
-
A part that gives information on the nozzle spray angle (80°, 90°, 110° or
120°)
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A part that gives information on the nozzle size (02, 03, 04 or 06)
For example the code ‘FF 110 03’ means a Flat Fan nozzle with spray angle 110 °
(see figure 12 for an illustration of the spray angle) and nozzle tip opening 03. The
code ‘LD 120 02’ means a Low-Drift nozzle with spray angle 120 ° and nozzle tip
opening 02.
110°
Figure 12. Spray angle of a hydraulic nozzle
The nozzle tip opening is given as 02, 03, 04 or 06. This number indicates the
numbers of gallons that are atomized through the nozzle tip when water is being
atomised through the nozzle at a constant spray pressure of 3 bar. For example an
FF 110 02 nozzle gives a volume rate of 2 gallons a minute at spray pressure 3 bar,
while the FF 110 04 nozzle has a volume rate of 4 gallons a minute at spray
pressure 3 bar.
3.1.2 Spray pressure
Of course spray pressure is also a determining factor for the droplet size. Table 1
illustrates the effect of spray pressure on the VMD for an FF 110 03 nozzle.
Table 1. VMD values for F 110 03 @ various spray pressures
Spray
VMD
pressure
2 bar
270 µm
3 bar
175 µm
4 bar
150 µm
3.1.3 Physicochemical properties of the spray liquid
The physical properties of the tank mix, atomized during a spray application, can
affect the droplet size spectrum, and therefore the drift potential.
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Several researchers examined the drift-reducing (or -inducing) effect of additives or
adjuvants, but they used a solution of the additive or the adjuvant in water, while it
is generally known that not the physicochemical properties of the additive or the
adjuvant itself, but the physicochemical characteristics of the complete spray
mixture is the determining factor for droplet size, and hence drift.
The major physicochemical properties affecting atomization are:
-
Extensional viscosity E
-
Surface tension γ
-
Density
Extensional Viscosity E
Fluids offer some degree of resistance to shearing motion. This resistance to
shearing motion is a form of internal friction called viscosity. Viscosity tends to
resist liquid stretching, thereby affecting atomization. An increase in extensional
viscosity can produce coarser sprays. Viscosity is calculated out of the time a fluid
needs to flow through a vertical, bulbed capillary between two marked points (upper
mark and lower mark) under the influence of gravity. A schedule of a viscosity
meter is given in figure 13.
Figure 13. Equipment for the measurement of viscosity of agricultural spray liquids (ASTM
E2408–04)
The absolute viscosity of a fluid is calculated as η = α.t where t is the time the fluid
needs to flow between the two marks and α is a constant that is characteristic for
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the viscosity meter (depending on diameter of the capillary, dimensions of the
bulb…). The value for α can be calculated by measuring t for distilled water, since
ηwater is a known value (ηwater = 1.10-3 N.s.m-2 at 20 °C).
For spray liquids, absolute viscosities are rarely used. One rather defines the
viscosity in a relative fashion, as the “extensional viscosity”.
Extensional viscosity E is measured (equation 1) as the ratio of tf and tw where tf is
the flow time of the spray liquid between the upper and the lower fixed mark of the
viscosity meter, and tw is the flow time for water (not distilled water, but tap-water,
which was used to formulate the spray liquid).
E
tf
(Eq. 1)
tw
Surface Tension γ
Molecules in liquid state experience strong intermolecular attractive forces, they are
referred to as cohesive forces. The cohesive forces between liquid molecules are
responsible for the phenomenon known as surface tension: The molecules in the
centre of the liquid experience net force zero, while the molecules at the surface
experience a net inwards force (figure 14).
Figure 14. illustration of the attractive forces between molecules of a liquid
To increase the surface of the liquid, energy is needed to overcome these inward
forces. The energy dW (Work) needed to expand the liquid surface area with one
unit dA (Area) is defined as the surface tension γ:

dW
dA
(Eq. 2)
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Surface tension tends to resist atomization, so a decrease in surface tension tends
to cause a decrease in droplet size.
Density
The density of a liquid may affect atomization, as with rotary atomizers, where
higher density causes the rotation rate to decrease, with a resultant increase in
droplet size.
However, with hydraulic nozzles in agricultural sprays, where the density is similar
for most spray liquids (equalling the density of water = 1 kg.l -1), density may not
have a significant effect on atomization.
3.2 WEATHER CONDITIONS
Wind is the main weather condition affecting drift. Temperature, air humidity and
atmospheric stability also affect drift, but their effect is less pronounced than the
effect of the wind.
3.2.1 Wind
The higher the wind speed, the more doplets will be blown away and cause drift.
According legislation, it is not allowed to apply field sprayings above a wind speed of
5 m.s-1. When sensitive areas are located in the neighbourhood of the sprayed field
(surface waters, recreation area, parcel with sensitive crop…), then the wind
direction also becomes an important parameter.
3.2.2 Temperature and Air Humidity
Most days there is a correlation between temperature and air humidity (R.H. =
Relative Humidity). High temperature correlates with low R.H., and vice versa.
Figure 15 illustrates the effect of temperature and R.H. on drift. In the right case,
the R.H. is low (20 %) and the temperature is high (30 °C). The water content of the
droplets evaporates, the droplets refine, and they become drift-prone. In the left
case, the R.H. is high (80 %) and the temperature is low (15 °C). The droplets
behold their water content, no drift occurs.
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t° = 15 °C
t° = 30 °C
Figure 15. Influence of R.H. and temperature on drift
3.2.3 Atmospheric Stability
Note that when one talks about wind, then the horizontal movement of air, caused
by the local differences in air pressure, is meant.
Atmospheric stability is defined as the vertical movements of air layers, caused by
Height
the differences in temperature between these layers.
UNSTABLE
CONDITION
Temperature
Height
Dominating
wind direction
NEUTRAL
CONDITION
Height
Temperature
Dominating
wind direction
STABLE
CONDITION
Temperature
Height
INVERSION
Dominating
wind direction
Temperature
Figure 16. Influence of atmospheric stability on the movement of a smoke plume in the
atmosphere
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Four regimes are distinguished. They are illustrated in figure 16 with the chimney –
smoke plume story.
Remember from your physics course that air expands when it is heated! Hot air
therefore has a lower density than cold air, and thus has the tendency to rise…
Depending on the temperature profile in function of the height above the
earth surface, the air will move into a certain direction…
-
Unstable condition
Temperature strongly decreases with height. Hot air is situated near the earth’s
surface, and cold air in the upper layers. The smoke plume spreads out in an
upwards direction. There is ‘vertical mixing’. The chance that drift damage occurs is
low, because the pesticides are strongly diluted into the atmosphere. But because a
main part of the sprayed pesticides is lost into the atmosphere, it is not advisable to
perform spray applications under these circumstances!
-
Neutral condition
Temperature slightly decreases with height. This condition occurs in the morning,
after the inversion layer (see further), that has been built up during the night,
breaks up. There is a slight, upwards spreading-out of the plume. This is a good
condition to perform a spray application.
-
Stable condition
Temperature increases with height. The heavy (cold) air is situated near to the earth
surface, while the hotter air layers are located at higher heights. The smoke plume
is ‘pushed’ towards the earth surface.
Stable conditions are often accompanied with low wind speeds (it is often difficult to
distinguish the effects of wind speed and stability in this condition). Sprayings are
allowed.
-
Inversion
We talk about an ‘inversion’ when a maximum in the temperature profile is found.
Then there is an isolated layer of cold air near the earth surface. This phenomenon
typically occurs when the night has been clear with little wind; the earth surface
then looses more heat than it receives. The inversion slowly disappears in the
morning.
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When a spraying is performed under these circumstances, the drift cloud will stay
located at its constant height, between the two cold layers. When there is wind, the
suspended pesticide particles are able to move horizontally towards distant places.
In this way highly concentrated drift clouds can be transported far away from the
field, and cause serious contamination problems elsewhere.
Sprayings are not allowed when there is an inversion!
3.3 SURROUNDING CHARACTERISTICS
Two types of surroundings considering drift are distinguished: The crop type that is
being sprayed, and intercepting structures neighbouring the field.
3.3.1 Crop type
In the Netherlands, drift experiments were performed in different crop types (bare
ground, potatoes, sugar beets and cereals) and in fruit orchards (leafless trees and
trees with leafs). Figure 17 illustrates the results for some of the measurements.
They were performed under ‘typical’ Dutch weather conditions.
50
aardappelen
Potatoes (conventioneel)
45
40
suikerbieten
Sugar beets
Drift (%)
35
30
granen
Cereals
25
blote
Baregrond
soil
20
15
10
5
0
0
2
4
6
8
10
12
14
Distance (m)
Figure 17. Drift results for field measurements in the Netherlands
3.3.2 Neighbouring structures at the field borders
Sometimes field borders (1 m – 5 m) are left unsprayed to intercept drifting pesticide
droplets. These ‘buffer zones’ can be bare, or a drift-intercepting vegetation can be
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grown on them, for example a grain border. In orchards, hedgerows are often used
for this purpose.
It is also possible to surround the field borders with artificial structures, like
screens and shrouds. Little research has been done on the effect of these structures
on drift.
4. MEASUREMENTS OF DRIFT
Two tools are used to perform drift experiments: the wind tunnel and field
experiments.
A wind tunnel is an artificially constructed corridor in which a wind can be
generated with the aid of a fan. Figure 18 gives a scheme of the wind tunnel of the
International Centre for Eremology (I.C.E.), at Ghent University, Belgium. This
closed-circuit blowing-type wind tunnel has a 12-m long, 1.2-m wide and 3.2-m
high test section.
Figure 18. Discription of the wind tunnel by Cornelis
The advantages of the wind tunnel are that the experiments take place under
conditioned circumstances: Wind speed can be controlled, so the experiments have
a high repeatability. Field experiments on the other hand take place under
unconditioned circumstances since we cannot control the weather conditions! Field
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experiments are also more laborious and time consuming than wind tunnel
measurements.
But the main disadvantage of wind tunnel experiments is that they are not
realistic…
 The wind tunnel is an ideal screening tool to evaluate drift-influencing factors,
e.g. a set of LD nozzles is compared to a set of FF nozzles. In a second phase,
based on the wind tunnel outcome, field experiments are conducted with the
nozzles that were found to be drift-reducing under wind tunnel circumstances.
In this way a lot of time, labour and money can be saved!
4.1 WIND TUNNEL MEASUREMENTS
The set – up for drift tests in the wind tunnel is given in figure 19.
Figure 19. Set – up for drift measurements in the wind tunnel
The spray nozzle is placed at a fixed height of 0.5 m at the beginning of the test
section. At this height the wind speed has a constant value of 4.0 – 4.1 m.s-1. The
relative humidity and the temperature are kept constant at values 42 – 44 % and 16
– 17 °C. Ground collectors (paper strips) are placed at distances 0.25 m, 0.35 m,
0.50 m, 0.75 m, 1 m, 1.25 m, 1.75 m, 2.25 m, 3.5 m, 4.65 m and 6 m in plastic
drains incorporated in the false bottom of the tunnel. Each spraying is performed
for 15 seconds at a constant spray pressure. Spray pressure can be chosen between
2 bar to 6 bar.
As spray liquid, various liquids, containing a tracer, can be used:
-
To evaluate the influence of spray application technique on drift (nozzle type,
nozzle size and spray pressure), a solution of 150mM NaCl is used. The
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paper strips are collected immediately after spraying and extracted into a
known amount of distilled water (100 ml – 500 ml). The concentrations of Na+
in the extraction water are determined with Flame Atomic Absorption
Spectroscopy (FAAS).
-
To evaluate the influence of the physicochemical properties of the spray
liquid on drift (various commercial pesticide formulations), the active
ingredients (a.i.) of the formulations are used as tracer. The paper strips are
collected immediately after spraying and extracted into a known amount of
hexane (100 ml – 500 ml). The concentrations of the a.i. in the hexane are
determined with GC-MS, GC-ECD or HPLC.
In the wind tunnel experiments, drift is expressed as a volume percentage of the
sprayed volume. This sprayed volume is mathematically calculated out of the
spraying time (15 sec), the spray pressure (2 – 4 bar) and the nozzle tip opening size
(02 – 04).
4.2 FIELD MEASUREMENTS
Field experiments are performed in a grassland (meadow) with a Hardi Commander
Twin Force field sprayer with boom length 27 m (see figure 20 - left) at the
agricultural research centre ILVO (http://www.ilvo.vlaanderen.be) at Merelbeke.
Figure 20. Spraying equipment (left) and set – up (right) for drift measurements in grassland
The test-site consists of a spraying and a sampling zone (figure 20 – right). The
spraying zone has dimensions 100 m x 27 m. The drift is collected on filter paper
ground collectors with dimensions 0.25 m x 0.25 m. Three parallel rows of these
ground collectors (fig. 20 – ■) are placed in the sampling zone. In each row the
collectors are placed at distances 0.5 m, 1 m, 2 m, 3 m, 5 m, 10 m, 15 m and 20 m.
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The sprayings are done perpendicular to the wind direction. The spray liquid
consists of a solution of the fluorescent tracer Brilliant Sulfo Flavine (BSF) in water.
The filter papers are collected immediately after spraying. The fluorescent dye BSF
is extracted from the filter papers in a known volume of water. The concentration of
BSF in the extraction water is determined with a fluorimeter. Out of these
concentrations, the amounts of drift collected by the filter papers are calculated.
Drift percentages are expressed as percentages of the applied dose.
Fluorescent dyes such as BSF are sensitive to UV – light. Therefore three filter
papers are also placed windward the test – site (fig. 20 - ●). A correction factor, to
bring into account for the photochemical breakdown of BSF, and also for the
extraction efficiency of the filter papers in water, is determined by grafting these 3
papers with a known amount of BSF. The correction factor is calculated as the
amount of BSF recovered from these papers to the amount grafted on it.
A weather station is used to monitor wind speed, wind direction, temperature and
relative humidity (RH).
5. TOWARDS DRIFT REDUCTION ...?
Based on what we have seen in this course, we can formulate following fist-rules to
avoid drift:
-
Use LD and AI nozzles instead of FF nozzles. Since LD and AI nozzles
generate coarser droplets, less drift will occur, this is especially the case with
AI nozzles (90 % or more drift reduction, compared to LD nozzles which give
medium reductions of 50 %).
On the other hand… do AI nozzles offer a solution? Recently it is reported in
literature that some AI nozzles have lower biological efficiencies since the
droplets they produces are too coarse (see p. 3: we need small droplets for
good coverage of the canopies and pests!).
-
Use the lower end of spray pressure range. Manufacturers of nozzles give a
pressure range in which their nozzles should be applied. Enquiries have
shown that farmers have the tendency to use the higher end of the pressure
range, and a lot of farmers even spray at higher pressures than the highest
end!
-
Increase nozzle size.
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Spray at favourable weather conditions, such as low wind speed, low
temperature and high RH. Avoid spraying during temperature inversions!
-
Do not spray when wind is directed towards sensitive areas and surface
waters.
-
Drive not to fast with the spray equipment. Typical driving speeds for
ground sprayers are 4 – 8 km.h-1. High driving speeds (> 8 km.h-1) will cause
air flows (turbulences) around the boom, so small droplets will be captured
by drift.
-
Take a low as possible boom height (in case of field spraying – see figure 2).
The higher the boom height, the longer the distance that droplets need to
travel towards the canopy/pest, so the more change there is that the droplets
are captured by drift.
Note: Driving speed of the tractor and boom height are not free to choose.
For example driving speed is determining for the application dose (l.ha-1).
Lowering driving speed increases the application dose.
Figure 21 gives an illustration of the importance of the boom height. The
profiles of the spray cones should be slightly overlapping at ground level. If
boom height is lowered too much, the spray cones will not be sufficiently
overlapping anymore.
Figure 21. Boom with spray nozzles
-
The spray equipment can be provided with shields and shrouds as driftintercepting structures, figure 22 gives an example for field spraying
(shielded boom) and orchard spraying (tunnel spraying)
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Figure 22. Shielding of a boom (left) and tunnel spraying in fruit orchards (right)
-
Drift-intercepting structures can also be introduced at the field borders. In
orchards natural hedgerows (this can be the last row of the fruit trees) are
commonly used. In field crops field margins of a certain width are left
unsprayed. These ‘buffer zones’ can be provided with a natural vegetation
(the crop itself or a grain border) or an artificial structure, such as a screen
(figure 23).
Figure 23. Artificial screen bordering the field
-
The spray equipment can be provided with air assistance along the boom
(figure 24). In this system an air flow is generated to ‘blow’ the droplets
straight towards the canopy.
Note: air assistance ≠ air-blast nozzles (p.3)!
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Figure 24. Spray tractors equipped with air-assistance (= grey bags at the boom)
-
Electrostatic spraying is a topic of the future. In this system droplets are
given an electrostatic charge, so an electric field is induced between the
droplets and the canopy. In this way an additional attractive force, directed
towards the canopy, is given to the droplets.
-
Drift-retardants can be added to the spray liquid. This practice is not yet in
use in Europe, but it is popular in America. Drift-retardants are polymers,
which increase viscosity of the spray liquid (p.10-11), so coarser droplets are
generated.

In the past polyacrylamide (PAM) was popular in America, but the last
few years, concerns on the use of PAM have risen. It is believed that
under certain conditions of sunlight (UV light) PAM unzips into its
monomer acrylamide. Acrylamide is carcinogenic. On the other hand
acrylamide is not very stable in nature, it is readily micro-biodegraded,
but at places where the concentration of PAM is high, acrylamide can
be formed more rapidly than it is broken down, so human exposure
problems could occur.

Polyacrylate [polyacryl acid (PAA)] is an alternative, but at pH = 5.514, PAA is negatively charged (carbonic acid!). In combination with
some spray formulations, this can give rise to problems (e.g.
interaction with other components of the spray liquid with the
formation of insoluble particles).

Polyethers, polyesters, polyvinyls… were found to be irreversibly
destroyed by the high shear forces, during the (re)circulation of the
spray mix in the pipes/hoses/nozzles… of the spraying equipment.

Hot topic drift-retardants are the natural polymers. They are natural
sugars, thus the toxicity is low and the biodegrability is high. Guar
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Dynamica en Residu’s van Pesticiden
Drift van pesticiden
gum is already in use in America. In the future, more polymeric
sugars will undoubtedly become important: xanthane gum, inuline…
6. DRIFT RISK ASSESSMENT
For information on drift risk assessment it is referred to De Schampheleire et al.
(2006):
De Schampheleire M, Spanoghe P, Brusselman E & Sonck S. 2006. Risk
assessment for pesticide spray drift damage in Belgium. Crop Protection – in press.
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