Category: General Concepts

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Six Spray Technology Skills for Agronomists
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Agronomists help farmers manage their crop with advice on everything from crop cultivars to fertilizer rates to marketing. It’s challenging to be an expert on everything, but a few core competencies can go a long way to improving the level of service.
Agronomists are also responsible for communicating environmental best practices. Along with fertilizer rates come messages of source, time, and place, the 4R principles. The same is true for spraying, with messages of spray drift, resistance management, and economic thresholds part of the consultation. Let’s remember that we should not be indifferent to the potential consequences of our recommendations.
Here are six skills that an agronomist should know about spray technology.
1. Recognizing major nozzle models and their spray quality and pressure requirements.
Application technologists are often asked to identify nozzles and recommend spray pressures for clients. It’s a skill that anyone can develop with just a bit of homework.
First, learn the colour-coding of nozzles – colours identify flow rates and follow an international standard that all manufacturers have adopted.
ISO Colour coding of major nozzle sizes, as well as application volumes at benchmark speeds.
Next, focus on the common nozzles on the major sprayers. John Deere sprayers will typically have three main air-induced nozzles, made for John Deere by Hypro, the Low-Drift Air (LDA), the Ultra Low-Drift (ULD), and the GuardianAIR Twin (GAT). Those with ExactApply, John Deere’s PWM system, will see the non air-induced 3D, the Guardian (LDX), and the Low-Drift Max (LDM). Recall that PWM flow control should not be used with air-induction tips.
Almost all Case sprayers have PWM, called AIM Command. Case uses Wilger ComboJet bodies and nozzles, with the ComboJet ER, SR, and MR most common, sometimes the DR or UR for dicamba.
New Holland/Miller with PWM (called IntelliSpray) are also likely to have these tips, but because these brands have TeeJet bodies on their booms, they require an adaptor for the proprietary ComboJet caps.
Otherwise, PWM units often use TeeJet’s TurboTeeJet (TT), Turbo TwinJet (TTJ60), and Air-Induced TurboTwinJet (AITTJ60), the only air-induced tip approved for PWM use by TeeJet.
Conventional spray systems (i.e., no PWM), will commonly have (in alphabetical order) the Air Bubble Jet (ABJ, actually labelled BFS for their manufacturer, Billericay Farm Systems), the Greenleaf AirMix (AM), the Hypro GuardianAIR (GA), and the TeeJet AIXR.
Many sprayers will have a twin fan for fungicides, primarily for fusarium headblight (FHB) management. The Greenleaf Turbo Asymmetric Dual Fan (TADF), the Hypro GuardianAIR Twin (GAT), and the TeeJet AI3070 dominate, as well as a number of custom configurations using splitters and twincaps.
Where dicamba is applied on Xtend trait soybeans, some special nozzles may be used to meet label requirements for coarseness. The TeeJet TTI is very common, but Greenleaf developed a special set of tips called the TurboDrop XL-D and the TADF-D. Wilger’s version, mentioned earlier, is the UR. John Deere has just announced their new ULDM.
That covers 95% of what you’ll encounter in the North American market. In Europe, add some Lechler nozzles (ID3, IDTA, IDK, IDKT) to the mix. In Australia, Arag is gaining ground.
Identifying the nozzles on sight is the prerequisite to finding out their average droplet size, called spray quality. Often, the inscriptions are worn off, so visual recognition is required to get there.
We’ve published a visual identification guide with pictures of the major nozzles here.
Knowing the relative spray qualities produced by these various nozzles will get you bonus points, but you’ll need to do some extra research to get there.
2. Using a spray calibration chart
This skill will make you popular on the farm and at the office. A very frequent question is “what size nozzle do I need for this new sprayer?”. The best way to approach the answer is to ask several questions.
- Does the sprayer have 20” nozzle spacing? (90% of sprayers do).
- What is the desired water volume?
- What is the expected average travel speed?
The first question guides you to the appropriate calibration chart, which can be downloaded here or can also be found in all sprayer catalogues. We explain how to use these charts here.
Calibration chart for 20: spacing, in US units.
If you don’t have a chart handy, use this shortcut: on a boom with 20” spacing, at 5 mph, every 0.1 US gpm capacity at 40 psi delivers 6 US gpa. So if you need to apply 12 gpa at 15 mph, an 06 size will get you there at 40 psi. That’s ballpark.
In metric, with 50 cm spacing, at 10 km/h every 400 mL/min (01 size) at 3 bar delivers about 50 L/ha. To deliver 200 L/ha at 20 km/h would require an 08 (white) tip.
Of course, if the tip is air-induced, make adjustments to speed or size to accommodate the higher pressure requirement of these types of nozzles.
Remember that spray pressure is key to performance, therefore the operator needs to drive at a speed, or use a volume, that results in the correct spray pressure.
3. Understanding Pulse Width Modulation
PWM technology has been on the North American and Australian market for two decades, but it remains poorly understood by those who do not use it. PWM will continue to gain popularity and has implications for nozzle selection and sizing.
Traditional rate control in the field involves the use of spray pressure to match liquid flow rates to travel speed. The rate controller knows the width of the boom (entered by the user), the travel speed (from gps), and the desired application volume (entered by the user). It does some math to identify the flow rate it needs, and compares that to the sprayer’s current flow meter reading. If the current flow is less than what’s needed, the sprayer increases pressure to increase flow. This happens continuously in the background.
When an operator speeds up, the pressure increases, and vice versa. As a result, the pressure (and therefore droplet size) will fluctuate with travel speed, and that can result in inconsistent spray patterns, coverage and drift.
PWM involves the installation of electronic solenoid valves at each nozzle body. These valves pulse on and off at 10, 15, 50, or 100 Hz, depending on the manufacturer. Each pulse contains a brief, complete shutoff of the flow. The proportion of the time the valve is open during a pulse is called the Duty Cycle (DC), and this is proportional to the flow through the nozzle.
Capstan PWM solenoid on Case AIM Command
When the system requires more flow, it no longer increases pressure. Instead, it increases the DC. The advantage of this approach is that nozzle pressure can now stay constant, ensuring consistent coverage and drift.
There are other advantages of these systems. Each nozzle can be controlled independently, offering high resolution sectional control and turn compensation.
Nozzle selection and sizing are both affected by this technology. Nozzles need to be sized larger, with about 30 to 40% more flow capacity ideal. The DC will therefore run at 60 to 70%, optimal for speed fluctuations and turn compensation. Air-Induced tips are not usually recommended because their pattern deteriorates with pulsing.
We’ve written about PWM here, here and here to get you started.
4. Validating coverage of the target
A very useful indicator of the success of a spray operation is an assessment of “coverage”. This term refers to a qualitative combination of droplet density and percent area covered, and can be quickly assessed using water sensitive paper. We’ve explained the use of WSP here and here.
It’s very useful to have some of this paper on hand (available from any retailer that sells TeeJet or Hypro products, or on-line from Sprayer Parts Warehouse in Winnipeg or Nozzle Ninja in Stettler, AB). The coverage can be assessed in four different ways:
Water-sensitive paper being used to assess spray coverage.
- using the “SnapCard” app (gives % coverage only);
- using the “DropScope” scanner (gives a comprehensive assessment of coverage, density, size, plus image editing tools);
- using a template of coverage examples;
- using experience built on years of doing this.
Water-sensitive paper is also useful as a record, for quality assurance. A spray application is conducted and part of the record is an image of the deposit. Should a performance issue arise, this will help settle it.
5. Understand basic sprayer plumbing
Often, a sprayer problem can be traced back to an issue with its plumbing. There could be mysterious sources of contamination. The pump might not be building pressure. The agitation isn’t running. Or you need to drain all the remaining liquid from the tank.
Sprayer plumbing seems intimidating for a number of reasons. It’s become complex on most modern sprayers. It’s hidden under the sprayer belly. All the lines are the same black colour, so they’re hard to tell apart.
But it’s not as bad as it seems. Basic plumbing is the same on all sprayers. The pump draws the spray mix from the bottom of the tank, the sump. It may also have options to draw clean water from an external supply, or from the clean water tank for wash-down.
The pressurized supply goes to three places:
- to the booms, via sectional valves;
- back to the tank, via a control valve that can be used to adjust the spray pressure;
- to the wash-down nozzles.
Typical sprayer plumbing for a centrifugal pump (Courtesy TeeJet).
When spraying, the less is returned to the tank, the higher the boom pressure. There may be several ways back to the tank, via agitation, via bypass (sparge), or via wash-down (used only when the pump draws water from the wash-down tank). Usually engineers can’t help themselves and introduce several what-if features that complicate the situation. But with a bit of know-how, and a flashlight, the plumbing system can be deciphered.
Pro tip: A centrifugal pump’s inlet (suction) is always the centre of the pump, its outlet (pressure) is at the periphery.
6. Matching a pesticide recommendation with application advice
It’s commonplace to recommend a specific crop protection product that matches the crop and pest situation. Recommending an ideal crop or pest stage improves the recommendation. But a truly successful outcome requires one additional step, advice on the application method. The customer may need to know if product performance depends on water volume and droplet size. Some products are more sensitive to this than others. Perhaps there is a specific nozzle type that may be helpful.
The classic example for application method is Fusarium headblight in wheat. The basics are straightforward. An agronomist recommends the fungicide, and guides the tight application window with a field visit to stage the crop, plus a look at the disease risk forecast map. But true application success requires an angled spray, with a coarser spray quality plus relatively low boom height to make it all worthwhile. That’s a full-featured recommendation.
Common herbicide applications also benefit from additional information. Some tank mixes and weed spectra allow for coarser sprays than others, and the ability to spray coarser means a wider application window and therefore more accurate timing. Other tank mixes may pose a significant risk to drift damage, requiring special measures to prevent a problem. Identifying those opportunities adds value.
Water volume and spray quality recommendations for major herbicide mode of action groups.
Newer labels for dicamba (Xtendimax, Engenia, Fexapan) and 2,4-D (Enlist Duo) have very specific instructions for drift prevention. This information must be shared with customers to ensure that their drift liability is covered.
Are there other skills that you feel agronomists should have? Please share them with us by contacting us at the bottom of this page.
February 3, 2019
How Airblast Spray Droplets Behave (or Misbehave)
Listen to article here.
Some pesticide labels require or prohibit certain droplet sizes to reduce the potential for drift. But, even when labels are silent about size restrictions, operators should be aware of the potential for droplet size to affect coverage. In the case of airblast, droplets should be:
- large enough to survive evaporation between nozzle and target.
- small enough to adhere without drifting off course.
- plentiful enough to provide uniform coverage without compromising productivity (e.g. affecting refills and travel speed).
Once spray leaves the nozzle, the operator has no more control over the application, so it’s important to plan for as many contributing factors as possible. Deciding which nozzles to use (and yes, you have alternatives beyond disc-core), requires an understanding spray quality symbols and basic droplet behaviour.
Spray Quality
Droplet diameter is measured in microns (µm). For a given pressure, a nozzle creates a range of droplet sizes which are described by the American Society of Agricultural and Biological Engineers (ASABE) standard S572.3 (Feb. 2020) In North America, these spray quality ratings range from “Extremely Fine – XF” to “Ultra Coarse – UC”. For interest, the scale is based on the British Crop Protection Council (BCPC) system, which is slightly different.
To make sense of the spray quality rating, we must first understand that not every droplet produced by a hydraulic nozzle is the same size. We noted that a single nozzle produces a range of droplet sizes. Spray quality captures that span using a few key metrics. The first is the Volume Median Diameter (VMD) or DV_0.5. Think of it this way: Let’s say you have a hollow cone nozzle that breaks a volume of liquid up into droplets. Let’s arrange them from finest to coarsest as in the following graph.
The DV_0.5 refers to the droplet size where half the spray volume is comprised droplets smaller than the DV_0.5, and the other half is comprised of larger droplets. But we need more to understand the variation in the population. In other words, are they all the same size, or do they vary a great deal?
That’s why we also assign a DV_0.1 which tells us the droplet size where 10% of the spray volume is comprised of smaller droplets, and a DV_0.9 which indicates that 10% of the spray volume is comprised of larger droplets. Let’s add them to the graph:
With all three numbers, we can calculate the Relative Span (RS) by subtracting the DV_0.1 from the DV_0.9 and dividing by the DV_0.5. The smaller the resulting number, the less variation there is in the spray quality. Two nozzles might produce a range of droplets with the same DV_0.5, but the one with the larger RS is more variable, and is more likely to drift. Since we don’t typically have access to the RS of each nozzle, we rely on the spray quality symbols in nozzle catalogues to alert us to potential drift issues.
Relative Droplet Size
Did you notice in the graph that there are a lot of Fine droplets compared to Coarse? Disc-core (or disc-whirl) nozzles do not have spray quality ratings, and moulded hollow cones may or may not. This is, in part, because the standard was developed for flat fan nozzles, but mostly it arises from the nature of airblast spraying. No matter the original droplet diameter, the air shear from the sprayer and the distance-to-target reduce the DV_0.5 considerably by the time spray reaches the target. It is safe to assume that the final spray quality will be much finer than the nozzle’s rating.
Incidentally, this is a big difference between boom sprayers and airblast: Where the boom sprayer operator should be aware of how pressure affects droplet size, it’s of little consequence to an airblast operator. On an airblast sprayer, pressure really only affects nozzle rate.
So, while shear and evaporation raise drift potential, shear also increases droplet count. Imagine the volume a nozzle emits as a cake. No matter how many slices you cut the cake into, you still have the same amount of cake. The finer the slices, the more people can have a slice, albeit not very much. Similarly, a single Coarse droplet can contain the same volume as many finer droplets. Mathematically, a droplet with diameter X represents the same volume as eight droplets with diameters of 1/2X. See the illustration below:
The eight to one rule: Every time the diameter of a droplet spray is doubled, there are eight times fewer droplets. Conversely, every time the diameter of a droplet is halved, there are eight times more.
Droplet Behaviour
The droplets that comprise the spray behave differently from one another. Finer droplets have a low settling velocity, which means they take a long time to fall out of the air. Conversely, coarser droplets fall out of the air more quickly. Think of how a ping pong ball (the finer droplet) has much less mass than a golf ball (the coarser droplet). When thrown into the wind, the golf ball follows a simple trajectory before falling. The ping-pong ball behaves erratically, like a soap bubble. Wind, thermals, humidity and many other factors will change where it goes because it is too light to resist them. It may even land behind the thrower, blown by the prevailing wind.
It is because of the behaviour of finer droplets, and the airblast sprayer’s inclination to create them, that we must be so diligent when we adjust the air settings.
We once explored this at a nursery workshop. The operator was spraying whips, which are young trees with very few lateral branches. He used a cannon sprayer to cover 30 rows (15 from each side) and felt he would incur less drift if he just used pressure, not air, to propel the spray. Water sensitive paper exposed the erratic coverage that resulted. Coverage uniformity was greatly improved when air was used, even when only spraying from one side of the 30 row block. Of course, this was only to demonstrate a principle; we don’t recommend alternate-row-middle-spraying.
Air-induction nozzles can be used to increase the median droplet size on an airblast sprayer. When used in the top nozzles positions, the coarser droplets that miss the top of tall targets will ultimately fall (reducing drift). They can also be used in positions that correspond to restricted airflow. In this case the operator relies on pressure to propel the coarser droplets where there is limited air to carry finer droplets.
Conclusion
The net result of all this is that the sprayer operator must choose a nozzle, pressure, and travel speed while considering the effect of distance-to-target and the weather. The resultant range of droplets should be fine enough to increase droplet count and be carried by sprayer air to deposit uniformly throughout the canopy. However, droplets should also be coarse enough to reduce drift if they miss.
Hey, if it was easy, anyone could do it!
Move ahead to 29:40 to watch a video describing how droplets behave an misbehave. Ahhhh Covid-hair. It was a thing.
January 21, 2019
Electrostatic Spraying in Agriculture
Dear reader: This article is intended to provide basic information on how electrostatic sprayers work in an agricultural setting. The author does not sell or manufacture sprayers. If your interest is related to spraying disinfectant in private or commercial settings, please contact retailers or manufacturers of electrostatic sprayers.
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Electrostatic nozzles have been tested in agriculture since the late 1970’s. Predominantly used in aerial applications, they are sometimes employed on airblast sprayers in orchard and berry operations, and on horizontal booms in vegetable crops. To a lesser extent, they are even mounted on wands for low acreage applications.
Claims
Independent research, manufacturer claims and user testimonials are intriguing:
- Improved coverage uniformity (i.e. underleaf coverage, panoramic stem coverage and canopy penetration).
- Improved retention (>50% better than conventional) and/or potential savings of 50% spray mix.
- Reduction in losses to soil.
- Improved efficacy with both insect and disease control.
So it begs the question: “Why doesn’t everyone have an electrostatic sprayer?” We performed a study in carrot in Ontario’s Holland Marsh to explore some of the claims and to get a first-hand experience with the technology. That article might help answer the question. But first, read this article which explores the basic principles behind how electrostatic applications work.
Charging the Droplet
Spray is charged by a high voltage supercharger. Commonly, the charge is induced by an electrode positioned close to the atomizing spray plume as droplets begin to form. This is referred to as coronal discharge. An intense electric field imparts a positive or negative charge depending on the polarity of the DC power used. Think of it as high-voltage static electricity.
Sometimes the spray is atomized by a hydraulic nozzle (e.g. a hollow cone) and sometimes using an air-shear nozzle. The latter has the added advantage of blowing droplets away from the electrode and projecting them into the canopy.
Let’s consider a negatively-charged droplet (see diagram below). The droplet becomes polarized when it passes through the electric field. The field attracts electrons to the droplet surface and repels positrons towards the centre. The droplet now has its own field that electrically motivates it to land on neutral objects. As they approach such an object, the negative charge on the droplet surface repels mobile electrons on the surface of the target, which redistribute, creating a relative positive charge on the surface and attract the droplet.
Another style of electrostatic technology employs a highly charged plate along the air outlet of the sprayer, generally attached just inside the duct. The clearance between the droplets and the plates is quite large in relation to that in a twin-fluid atomizer, coil-type charging system.
Droplet Size
Droplet size is a critical factor. Droplets must be large enough to resist evaporation and drift but small enough that the charge can change their trajectory when it comes close to a target (I.e., the Charge-to-Mass Ratio). Most electrostatic nozzles produce ~50 µm droplets, categorized in agriculture as Very Fine. For comparison, a human hair ranges from 20 to 180 µm. Fog is about 5 µm. Such a small droplet means that the distance between nozzle and canopy is a determining factor for the spray depositing, or drifting.
Droplet Behaviour
Many forces influence droplet behaviour (E.g., inertia, wind, gravity, etc.). Very Fine droplets have a low terminal velocity causing them to fall slowly (~40 seconds to fall 3 m). This makes them highly drift-prone. However, simulations have shown that a charged droplet released close to a grounded target would be “pulled” faster than an uncharged droplet. Further, their trajectories would be less affected by air movement and they have the potential to move upwards against gravity towards the underside of a leaf.
Of course the droplets must reach the canopy before any of these potential advantages can be realized. Even with air-assist to project the spray into the canopy, it has been shown that the droplet must be within two centimetres of the target before attraction improves deposition. There are many physical phenomena that influence this process:
The Faraday Cage Effect can occur when spraying dense canopies. The spray deposits on the first grounded object it encounters. This is the outer surfaces of the canopy and the spray can be prevented from moving deeper into dense canopies. Regarding arable crops, there is often a naturally occurring negative charge on the earth’s surface that repels negatively charged spray. This may be why studies often report reduced loss to soil.
The Corona Effect is a very complicated relationship between the shape, density and spacing of the crop and it’s influence on charged spray. Research has shown that deposition is better for rounded targets than pointed. The gaseous exchange of charges between leaf tips and spray can neutralize or even repel droplets. This may be why electrostatic demonstrations so often include fruit or spheres.
The Expansion Cloud Effect (or cooler, the “Space Cloud” Effect) describes how charged droplets are repelled by objects with a like charge. Coulomb’s Law describes how objects with an opposite charge attract, but it also says objects with a like charge repel. Since the droplets all have the same charge, they repel each other. While this causes the plume to expand into the canopy and helps to distribute the droplets to give uniform coverage, it also causes droplets to expand upwards away from the crop, making them susceptible to drift.
Observations
The opportunity for reduced pesticide use is appealing and it may entice consumers to consider the electrostatic sprayer as a more environmentally-conscious choice. However, we have found very few studies relating to drift, and opinions are mixed whether electrostatic applications are any more drift-prone than conventional applications.
Considered collectively, electrostatic applications seem to perform well in controlled conditions, but the complications arising from variability in a natural environment coupled with the cost of equipment has slowed adoption. The current rules for practical adoption are poorly defined. More fulsome drift studies are required and coverage uniformity and canopy penetration (particularly from ground rig systems) must be consistent in real world settings.
Nevertheless, electrostatic applications have a lot of “potential”.
October 10, 2018
Spraying in Dusty Conditions
Dusty conditions are common in spraying, and in dry springs they are often associated with a further challenge, drought-stressed plants. There is no magic cure for these problems, but here are a few guidelines:
1. Most products are not strongly affected by dust. But two important products are very dust-sensitive, glyphosate and Reglone. The active ingredients in both products are very “charged”, therefore they bind readily and strongly to soil particles, which includes not only dust on plant surfaces, but also suspended soil in spray water that gives the “turbid” appearance.
2. Dust can be viewed as similar to hard water cations, as a game of relative concentration. We try to get the herbicide concentration to be higher, essentially over-powering the antagonist. For glyphosate, two approaches are common: (a) reduce water volume; (b) increase herbicide rate. Reduced volume is tricky if the glyphosate spray contains a tank mix partner such as a Group 6, 14, or 15 to combat resistance. Those products require more water. For Reglone, low water is a bad idea for the same reason.
3. Some specialists recommend the use of higher water volumes to reduce the effects of dust. Although spray volumes are usually too low to actually wash dust off surfaces, the higher water volumes permit the use of larger droplets which may have better absorption characteristics in the presence of dust.
4. Another remedy is to increase the application rate in the spray swath where dust is most severe, usually behind the wheel tracks. Slightly larger nozzles in those regions are widely used by sprayer operators.
5. Even when dust is not a problem, roadside field edges may contain dust from traffic. Higher rates may be justified on the outside rounds for that reason.
6. A report in No-Till Farmer makes the following useful statements: “Greenhouse research conducted by researchers at North Dakota State University in 2006 found that control of nightshade species with glyphosate was reduced when dust was deposited on the leaf surfaces before, or within 15 minutes after, glyphosate application. If the dust was deposited later than 15 minutes after application, phytotoxicity was not reduced. Dust generated from silty clay soil tended to reduce glyphosate phytotoxicity more than dust generated from loamy sand soil.”
7. Several additional management opportunities exist for dusty conditions. Slowing down tends to reduce turbulence and dust generation. Although front-mounted booms apply the spray before the dust is generated, it will deposit before the spray is dry, limiting the benefit, as indicated by the NDSU study.
8. Don’t mistake aerodynamic turbulence for dust. Weed control may be lower behind the tractor unit or near the wheels because the spray is displaced by air currents. The use of water-sensitive paper can help identify if this is part of the problem.
One of the better references on dust and wheel tracks was produced by the GRDC in Australia, and can be found here.
May 15, 2018
Deciding on the Right Way to Spray
“What is the right way to apply this pesticide?” It’s one of the classic questions. Applicators know that spray method determines the efficacy of the application as well as its environmental impact. And it has to use time and water resources efficiently to make sense.
To answer the question properly, we need to take things one step at a time.
1. Canopy: To start, we need to look at the canopy that our application will go into. If it’s an early season spray into a seedling crop, then the canopy won’t be much of a barrier. Lower water volumes can be possible. Droplet size will only depend on the target type and the pesticide mode of action.
Small weeds require more smaller droplets to secure effective targetting
If it’s a later application into the bottom of a maturing canopy, the foliage may intercept the spray before it reaches the target area. More water will likely be needed, and droplet size may become more critical for getting the spray to its destination. Dense canopies are a real challenge and lower-canopy deposition usually benefits from finer sprays because the small droplets can turn corners better.
Dense canopies are very difficult for a spray to penetrate. Higher water volumes and smaller droplets are the key tools that help.
2. Water Volume: Regardless of canopy, the range of application possibilities will depend on the water volume and spray quality combination. It’s math: assuming some constant amount of coverage on each leaf, more layers of foliage will require more water. Using less water volume will make it necessary to use finer sprays to keep droplet numbers constant. More water will allow coarser sprays. This decision has implications for drift, and by extension, affects the number of hours we can spray in a day. More drift tolerance means better application timing and overall productivity.
The tradeoffs between water volumes and droplet sizes are seen in this figure. Once a certain threshold of coverage has been reached, a further increase in coverage may not provide any additional control.
3. Target Type and Droplet Behaviour: Whatever spray we use, the target plant or insect needs to intercept, collect, and retain the spray droplet. This is where the fun begins. Target leaves may be vertical or horizontal, large or small. Their waxy surface may be easy-to-wet or difficult-to-wet. The general rules of thumb are that larger, more horizontal and easy-to-wet surfaces are better suited for coarser sprays – these are intercepted more efficiently and stick readily. That is a reason why most broadleaf weeds and crops are very compatible with low-drift sprays.
Large targets (left) are most efficient at intercepting larger droplets (provided droplet bounce is not a problem) because smaller droplets may evade capture. Smaller targets are usually missed by larger droplets but are very capable of capturing smaller droplets.
On the other hand, smaller, vertically oriented and difficult-to-wet plants require finer sprays for effective targetting. Larger drops tend to miss these targets or bounce off them. Most grassy, and some broadleaf weeds (especially at early growth stages) fall into this category.
4. Mode of Action: There are nearly 30 modes of action on the herbicide world, and another ten modes for insecticides and fifteen for fungicides. The effect of droplet size and water volume on their uptake and translocation varies, and it’s probably not correct to generalize too much. There is one notable product, glyphosate. For this product, research has consistently shown that large droplets and more concentrated mixtures provide better uptake. But we’ve also seen problems when this is over-done, causing localized toxicity and limiting translocation.
With many products, we’ve sometimes seen better performance with finer sprays due to improved coverage, yet at other times less performance due to rapid evaporation. On the whole, it’s probably still fair to say that contact modes of action require finer sprays and higher water volumes, even if there is the occasional exception. And systemic products can typically handle coarser sprays. We’ve always been surprised just how coarse we seem to be able to push the system before any loss of efficacy.
What does it all mean? In spraying, we need to accommodate a lot of diversity. The average application is broad-spectrum, targeting large and small broadleaf and grassy plants. Many sprays are tank mixes of several modes of action. It’s impossible to prescribe a specific spray for each situation. We need a little bit of everything. And the spray should not be drift-prone. It’s easy to see that we need to aim for the middle to accommodate everything.
The traditional flat fan nozzle, either in its conventional or low-drift form, generates a wide range of droplet sizes that can range from 5 µm to about 2000 µm. If we need fine droplets, they’re there. If we need larger droplets, they’re also there. The proportion of the total spray volume in each specific size fraction depends on the nozzle choice and size, the spray pressure, and the adjuvant mix in the tank. Overall, the system is very robust, and although it requires some tweaking, a well chosen average spray can achieve most tasks well enough.
A typical spray quality chart shows the expected spray quality for a range of nozzle sizes and pressures. Spray quality measurements follow standards set by the ISO and ASABE, these change from time to time and therefore charts tend to become outdated.
Our research has repeatedly shown that a Coarse spray is a good starting point that does most things well. It is acceptable to move into a Very Coarse or coarser category provided water volumes are also raised, and provided the target types and modes of action are suited for this change.
It is rarely necessary to spray finer than Coarse, and when this is done, we recommend against spraying finer than a Medium spray. There is simply no advantage from product performance, and drift risk becomes unacceptable.
Tweaking the System. In order to maximize the performance of your spray, and the efficiency of your overall spray program, here is some advice:
1. Know the spray quality of your nozzles, and their response to spray pressure. Manufacturers publish this information in their catalogues and on-line. Make this your homework assignment.
2. Use the coarsest spray that you can afford to. This will make the application safer, it will widen the weather window, and it will simply let you get more done in a day or a season. Coarse sprays work.
3. Use spray pressure and water volume to fine tune the application for a specific purpose. If using a contact product, you can keep the same nozzle you used for a systemic product. Apply more water or use more spray pressure to generate more droplets.
4. Do not skimp on water. Higher water volumes tend to make an application more uniform, robust, and crop-safe. Spray coverage improves. Canopy penetration improves. Coarser sprays are possible. The only exception to this rule is glyphosate, which works better in lower water volumes. But with higher glyphosate rates and more tank mixing, even that exception is disappearing.
5. Learn as much as you can about how your pesticides work and where they need to be in your canopy. Apply your knowledge to select optimal water volumes and spray qualities.
6. Be wary of people who advise very low water volumes in conjunction with fine sprays. They want to appeal to your need for efficiency, but do so at the cost of consistency and environmental stewardship. Plus these types of applications are illegal for many of our products.
June 7, 2017