In This Section |
Six Elements of Effective Spraying
in Orchards and Vineyards
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| Agdex#: | 605 |
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| Publication Date: | July 2009 |
| Order#: | 09-039 |
| Last Reviewed: | July 2009 |
| History: | |
| Written by: | Dr. Jason Deveau - Application Technology Specialist/OMAFRA |
The objective of spraying is to deliver an effective, uniform dose of
product to a target area in a safe and timely manner. Any product not
deposited on the target is called "wastage". Wastage includes
drift (vapour and droplet), run-off and any off-target deposition. In
high volume airblast applications studies show that 80 per cent of the
product can be lost to drift and ground deposition. Wastage not only costs
time and money but may reduce the effectiveness of the application and
increase the risk of environmental contamination.
There are six elements that affect spray accuracy and wastage to consider
before every application. See Figure 1.
Figure 1. Six elements of effective
spraying
The elements overlap, illustrating how changes to one element often means
reconsidering another. Only the operator affects all elements. Currently
no technology or technique can compensate for an inattentive operator;
an operator's skill and attitude can greatly improve the accuracy of an
application.
Each element is made up of a group of related factors (see Figure 2).
This Factsheet provides an overview of how each element affects spraying.
Use this information to modify spraying practices to increase accuracy
and reduce wastage. It is this balance between benefit and compromise
that makes spraying both an art and a science.
There are many varieties of directed sprayers commercially available
for orchards and vineyards. There seems to be no naming convention between
manufacturers and equipment is often customized, so it can be difficult
to classify sprayers.
Figure 2. Expanded elements of effective spraying
Any design that maintains a minimal effective distance between each nozzle
and their respective target improves deposit uniformity and reduces drift.
For example, consider the distance-to-target at the top nozzles of a
conventional airblast sprayer. A lot of product is lost to intervening
environmental exposure and spray deceleration, reducing penetration and
deposition. The ability to control air speed and redirect flow to match
the target canopy is of great benefit when adapting the sprayer to different
crops, different zones within a crop (such as the grape zone) or during
different stages of growth.
Ducted conveyance (e.g. towers) and vertical booms are growing in popularity
as operators pursue better work rates, more accurate deposition, smaller
buffer zone requirements and minimal wastage. They are the best choice.
Air assistance carries droplets to the canopy and stirs leaves and branches
to aid in penetration. The converging air flows create air turbulence
in the canopy and expose all leaf surfaces to the spray. It has been shown
that penetration and deposition in full canopies is improved when airflow
is angled slightly forward or backward from the direction of travel. If
using a tangential/cross-flow system for example, the British Crop Protection
Council (BCPC) recommends orientating flow 45° backward and 10°
upward. Air can also be used as a deflector, preventing spray-laden air
from blowing over the target.
There are differing opinions about the benefits of changing air speed
and air volume. Air speed is measured in metres per second and air volume
(or airflow rate) is measured in cubic meters per second. In orchards,
high volume/low speed air improves overall coverage when the speed is
just enough to form openings in the canopy. This does not work under high
wind conditions. In vineyards, however, higher air volumes reduce spray-side
deposition by blowing spray too far through the target. Some vineyard
studies show that using airflow may not be necessary during pre-bloom.
Therefore the appropriate air setting is situation-specific, but in general
"more air" does not imply better coverage. Too much airflow
tends to carry droplets over or through the canopy, increasing drift potential
and reducing coverage. Spray-laden air can actually completely miss a
leaf in a process known as slipstreaming. Given the current practice of
using too much air, reducing the average fan speed by 25 per cent can
potentially eliminate 75 per cent of drift without compromising coverage.
Try to match air speed and air volume to canopy density by:
Strategically place water-sensitive cards throughout the canopy, or use
kaolin clay (a non-toxic compound that leaves a white, powdery film) to
receive feedback for further adjustments. Regardless of air-assist settings,
as canopies begin to fill, it is imperative to spray both sides for proper
coverage. Even if spray appears in the next alley, it is not providing
consistent coverage on the far side of the row being sprayed.
Air deflection research at Cornell University found that fitting adjustable
deflectors to the top and bottom of airblast sprayers will direct all
exit air into the canopy. The commercially-available deflectors are often
too short to effectively redirect air. The variety developed at Cornell
is as much as eight times longer and features a metal plate on the front
to prevent spray moving forward. Alternately, and preferably, retrofit
commercially-available towers to airblast sprayers to redirect air into
the target canopy. The Cornell group also developed a low-cost ducted
retrofit for Kinkelder vineyard sprayers that reduced wastage and increased
deposition by 25 per cent.
Hydraulic nozzles are classified by their spray patterns, flow rates
and the range of droplet sizes they produce. This is called "spray
quality". More pesticide labels now specify droplet size as well
as flow rate. Review this carefully when selecting nozzles.
Note that the droplet size classification system (very fine to extremely coarse) was originally developed for flat fan nozzles spraying into still air under specific conditions. While nozzle manufacturers sometimes provide droplet size classifications for cone-pattern directed-spray nozzles such as disc-core, these ratings do not account for the effects of shear from air-assistance or spraying over larger distances. Tip selection and orientation requires a particularly good understanding of the behaviour of droplet.
Droplet size decreases by increasing:
With cone-pattern nozzles, often used in directed spraying, a wide-angle
spray pattern is subject to more shear and creates finer droplets and
vice versa. Droplet size affects droplet behaviour:
The exception is air-induction (AI, or sometimes referred to as Venturi)
nozzles, which produce coarser droplets that contain air bubbles. Manufacturers
and independent studies show that AI nozzles are a good choice for certain
directed applications. The droplets arrive at the target intact and shatter
upon impact, increasing coverage while greatly minimizing drift.
Nozzle Orientation
During calibration make sure to aim nozzles at the target. Recent work
in Poland with a prototype dual-fan orchard sprayer demonstrated that
nozzle orientation plays a larger role in mitigating drift than fan location.
This means:
For fan-driven airblast sprayers, the counter-clockwise turn of the fan
carries spray up and over the canopy on the right hand side and downward
on the left. Work at Cornell determined that the best spray pattern for
the grape zone occurs when nozzles are angled to counteract the "spin":
the right-hand nozzles are set to horizontal with the top two set 20°
downward and the left-hand nozzles aimed 45° upward. The angles are
specific to the sprayer and the intended target. Spin may not be as pronounced
on dual-fan and turbine-driven sprayers.
While re-orienting nozzles is very effective, two-fold variations in
leaf and fruit spray deposits are typical between the inner and outer
portions of vine and orchard canopies, even after spray plumes are optimized
for the target. The lesson: spray crops from both sides.
The pesticide label is a legal document; do not exceed dose rates. Unless
specifically prohibited, lower rates and higher concentrations are allowed.
However, there may be consequences, such as reduced protection and increased
potential for pest resistance and phytotoxicity. The registrant is only
liable if the operator follows the label exactly.
There are many techniques to apply pesticides in vineyards and orchards,
some more effective than others.
For all techniques, turn off outward-pointing nozzles at row ends and
outer rows.
Timing is very important. It relates to pest growth stage, pest pressure,
weather conditions and work rate. Given the limited timeframe, it is tempting
to speed through an application; but as the canopy fills, it is increasingly
important to give spray time to penetrate. As a generality, perform directed
spraying between four and six kilometres per hour.
As forward speed increases, spray can be diverted backwards into upward
wind currents and vortices behind the sprayer. This increases variability
in spray deposit, which is generally undesirable and it adds to drift.
This effect is amplified when driving into the wind because the shearing
effect increases the number of driftable fines, even when using coarser
droplets. One study on boom spraying showed that reducing speed from eight
to six kilometres per hour has the potential to reduce drift by ~50 per
cent.
When performing airblast applications, canopy penetration and uniformity
is greatly improved at slower speeds. Air and droplet velocity has a high
rate of drop off, and this loss of momentum means it takes time for spray
to get to the target. Work in apple orchards demonstrated that high volume/low
speed air used at low forward speed results in a large increase of air
volume penetrating trees
Studies in grapes demonstrated that increasing air volume does not compensate
for higher forward speeds; it reduced deposition on the spray side of
a fully developed canopy, while it did not affect deposition on the far
side. Moreover, the backward angles increased variability and ground deposition
beneath vine rows. Adding more carrier liquid will not permit a higher
forward speed, either. This will only increase the material deposited
on the area already sprayed.
Ways to save time without increasing forward speed include:
The purpose of the carrier (usually water) is to convey product to the
target and distribute it in the desired pattern and concentration. The
volume of liquid to use during directed spraying is seldom indicated on
a label except in general terms (i.e. maximum and minimum). Therefore,
carrier volume is often determined by the equipment used, crop morphology,
pest location, the mode of action and personal considerations including
past performance, work-rate and environmental impact.
The volume of liquid retained on a surface is limited, so once wetted
the surplus drips down to the lower leaves and so on to the soil. Once
run-off begins, the amount of product deposited is proportional to concentration
but independent of carrier volume. Properly calibrated, the same amount
of product will be deposited on all portions of the target without incurring
run-off.
High Volume Spraying
High volume spraying (>2,000 L/ha in a high-density orchard, 500-1,000
L/ha in vineyard) uses a wide range of droplet sizes and is the simplest
and arguably more dependable form of spraying. The objective is to thoroughly
wet all portions of the branches, blossoms, fruit and leaves just to the
point where excess spray begins to run off. This can be difficult to achieve
given that outer leaves reach the point of run off before inner leaves
are suitably covered. Pruning, training and hedging combined with a slower
forward speed can help by allowing spray materials to travel throughout
the canopy.
Although at first glance the simplest technique, spraying large volumes:
Low Volume Spraying
Low volume spraying employs less liquid than needed to thoroughly wet
the target. While benefits include reduced soil contamination and fewer
refills, it requires very accurate spraying to compensate for the lower
volume. Coverage can be improved by using smaller droplets in greater
numbers, but this incurs an increased potential for drift. In addition,
the dose rate for most agricultural chemicals is determined through high
volume spraying, so the concentration of the product is often increased
proportionally as the carrier volume decreases. Higher concentrations
are intended to maintain effective dosage, but this increases the risk
of phytotoxicity.
Historically, dose rates for orchard spraying in Ontario were determined
through efficacy trials where standard trees (4.5-5.5 metres high) were
sprayed to runoff with a handgun at 3,740 L/ha (400 US gal/ac), and the
effective rate was expressed per 378.5 L (100 US gal.). Label rates are
still determined through trial and error, but are tested using today's
denser, size-controlled plantings.
These modern crop protection products often express dose rate as the
lowest effective amount of product applied per hectare of crop. While
area-based rates are ideal for boom or aerial spraying over workable crops,
orchards and vineyards are three-dimensional targets planted in rows and
are typically sprayed from within the canopy. Depending how efficacy was
originally determined, it is possible to over-apply for small plants with
large spacing in early season, or to under-apply for large plants in dense
plantings late in the season.
When direct spraying do not rely solely on product per hectare. Instead,
adjust spray volume to match the density and volume of the target canopy
by:
Crop-Adapted Spraying matches the carrier volume and/or dosage to the
size, shape and density of the target crop.
Many models have been developed to guide operators in their decision
to modify application rates, each relying on certain assumptions with
varying degrees of success.
For orchards, the most popular CAS models include the relatively new
Pesticide dose Adjustment to the Crop Environment (PACE+) and the more
established Tree-Row Volume (TRV).
PACE+
The PACE+ system relies on a pictographic key and a simple formula for
growers to calculate dose adjustments for certain pesticides. However,
it assumes sprayer efficiency, has no link to growth stage, does not perform
well in highly variable orchards and has only been demonstrated on a few
varieties in the UK.
TRV
The TRV system translates ground-area rates to volumetric rates using
a formula based on tree height, width, row length and distance between
rows. It does not account for sprayer efficiency or the area-density of
the canopy, which accounts for 80 per cent of the deposit variability
between canopies. Therefore, TRV is generally considered too complex and
is only a partial solution.
UCR
For vineyards, the Australian Vine Row Volume (VRV) was replaced by the
Unit Canopy Row (UCR) concept which also adjusts the application rate
requirements on the basis of crop row-length rather than ground-area.
However, UCR does not consider the canopy type, area-density or sprayer
efficiency.
Ontario options
Many of these models are complicated, and have proven inaccurate or inappropriate
for Ontario growers. As a result, growers have adopted their own methods
using a trial and error approach, which may or may not provide adequate
coverage.
Until a simple and consistently effective CAS system is developed, only
mechanical changes to nozzle orientation, airflow and droplet size are
recommended.
Recalibrate sprayers as the season progresses, not only to confirm good
working order, but to reorient nozzles and air assistance to match canopy
changes as they grow and fill.
Dense Foliage
As noted previously, increasing foliar density requires pruning, training
and hedging, changes to air assist settings and slower forward speeds
to ensure adequate penetration and uniform coverage.
The goal of spraying is to deliver an effective, uniform dose of product
to the target area. Depending on the nature of the pest, the target area
may or may not include all portions of the canopy.
Tips to help reach the target:
It is important to understand when and how to apply any pesticide or
growth regulator. Timing considerations include:
It has been suggested that droplet number per square centimetre is more
important than droplet size for determining pesticide efficacy. A product
can work by immediate physical contact with a pest or in a secondary manner,
such as a systemic product.
For example, a product may require an insect to move through the deposit
when dry, or impact the pest at a susceptible stage of development during
application. Alternately, the product may be taken up by the plant, or
require the pest to ingest it.
It is for this reason that labels increasingly specify droplet size.
But if this information is not provided, use finer droplets for contact
insecticides and fungicides, and coarser for systemics. There will always
be exceptions, such as using localized treatments or discrete droplets
to allow greater survival of natural enemies. Never use directed applications
(e.g. airblast) to apply herbicides.
Nozzle rates in catalogues are based on spraying water, which weighs
1 kg/L. When spraying solutions are heavier or lighter, use a conversion
factor (found in nozzle catalogues) to determine the correct nozzle size.
Density conversion formula :
Desired application rate (L/ha) x
Conversion Factor based on weight =
Actual application rate (L/ha)
In some cases, the difference may be enough to require the next lowest
or next highest flow rate. Depending on the volatility of the product,
consider shorter distances to target and larger droplets to counteract
evaporation.
Adjuvants
Adjuvants are sometimes added to a tank mix, separate from the pesticide
formulation, to improve performance. There is a wide variety of registered
adjuvants; caution is advised. Some pesticide formulations are not compatible
with certain adjuvants and the label specifies what to use.
Many effective application technologies and pest control products are
available - but they must be used correctly. Effective application begins
with observing the Integrated Pest Management (IPM) process:
The first two steps mean identifying the pest, understanding the nature
of the pest (such as the life cycle) determining a threshold of tolerance.
If spraying is required to control the problem, the operator needs to
know the basics of spray technology. This includes the equipment and the
effects of changing spraying parameters (such as pressure or carrier volume),
the impact of weather conditions (such as wind and relative humidity)
and the product being applied (such as correct timing and safety requirements).
Knowing how to properly maintain, calibrate and orient the sprayer according
to the nature of the target is also important.
Finally, monitoring the results requires responding to changes in the
environment and target during application and to consider these factors
when evaluating the outcome.
Ultimately, the effectiveness of an application is determined by the
operator's understanding of the elements that influence spray application
and the decisions they make when balancing benefit and compromise.
This factsheet was authored by Dr. Jason S.T. Deveau, Application Technology Specialist, OMAFRA, Simcoe. It was reviewed by Helmut Spieser, Engineer - Field Crop Conditioning and Environment, OMAFRA, Stratford.
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