Spray that is not directed at the target is wasted spray. Many pesticide labels specifically require the operator to restrict spray to the target canopy. Spray that escapes above the canopy is a significant source of off-target drift. Foliar applications that extend below the canopy are not efficacious and represent waste and lost productivity.
A spring application or oil and chloropyfiros. Estimate of 50% waste (in red).
Air carries spray droplets, so the first step in any adjustment should be to perform a ribbon test to ensure the air outlets are oriented correctly. This is achieved by adjusting deflectors (e.g. low profile axial), the air outlets on a tower, or the entire head on a wrap-around design with individual fan/nozzle combinations.
Spray height should always exceed the canopy height by a small degree. This compensates for the increase in wind speed with elevation, the potential loss of spray height with faster travel speeds, and uneven alleys that cause the sprayer to rock, which changes the spray angle.
Spray angles change as a sprayer rocks on uneven alleys. It is more important that spray is directed at the top of a canopy than at the bottom.
It is less critical that spray align with the lower portion of the canopy. As air energy wanes, or as droplets begin to lose momentum, finer droplets will slowly fall, depositing on random surfaces. Coarser droplets will quickly fall towards the bottom of the canopy, settling primarily on upward-facing surfaces. This secondary deposition can also occur from the cumulative impact of blow-through from upwind rows.
Once the air is aligned, park the sprayer in an alley. Stand behind the sprayer and extrapolate a direct line from each nozzle to target canopy. Nozzles that point at the canopy should be left on. Nozzles that point above or below can be blocked, or turned off via valves or rotating roll-overs. Some roll-over nozzle bodies can be swiveled up or down 15 degrees to fine tune the spray angle. An alternative would be to permanently rotate the nozzle body fitting in the boom line. When aiming nozzles using a roll-over nozzle body, be careful not to swivel them too far or the valve will partially close and compromise the spray pattern.
Use a ladder when adjusting nozzles on a tower sprayer. Some sprayer chassis and tanks are designed to accept a climber, but even so they can be slippery. Please be careful.
When extrapolating, remember that the centre of a nozzle only indicates the centre of the spray pattern. Cone and fan angles can span 60 to 110 degrees, depending on the influence of air. Therefore, even though the centre of the lower-most nozzle intersects the bottom of the target canopy, you may still be able to turn it off because the nozzle above has that portion covered.
Adjust spray distribution across the boom at the beginning and roughly mid-way through the spray season to ensure the sprayer will uniformly cover the target with the optimal volume. These adjustments should account for both canopy growth and fruit set.
For example, as the season progresses in an orchard, fruit may cause limbs to hang lower and warrant a new spray distribution. Turning on the bottom nozzle position will help, but it doesn’t account any increase in density throughout the canopy. You may need more volume distributed across the entire boom. Another example: as grape bunches begin to close, sprayer operators may direct fungicides exclusively at the fruit zone and not the entire canopy.
Remember to always check coverage using water sensitive paper. It’s not worth saving a bit of spray if you’re missing a bit of your target.
Funny how some issues never go away. For as long as I’ve been in the sprayer business, the question of ideal droplet size for pesticide application has remained a hot topic. At its root are the basic facts that small droplets provide better coverage, making better use of water, but large droplets drift less. So why are we still debating this? Because we need both of these properties to be efficient, effective, and environmentally responsible. Ultimately, the droplet size question is reduced into one of values, where everyone’s individual priorities play a role.
First, let’s talk about basic principles. To be effective, an active ingredient must make its way from the nozzle to the site of action in the target organism. On the way, it encounters several obstacles as summarized by Brian Young in 1986.
Figure 1: The dose transfer process of pesticides (after Young, 1986)
After atomization and before impaction, the spray encounters two main losses, evaporation and drift. Both of these are more severe for smaller droplets. Smaller droplets have a greater ratio of surface area to volume for any given spray volume, and can evaporate to a much smaller size, even to dryness depending on the formulation, in seconds. For water-soluble formulations, one consequence is lower uptake. Oily formulations may maintain efficacy, but neither type can escape the second effect, spray drift.
Figure 2: Time to evaporate all water from droplets of various sizes, based on the “two-fluid” model developed by Wanner (1980). Based on 0.8% v/v non-volatile, non-soluble addition, 20 ºC, and 50% RH. This model suggests that final droplet diameter is 20% of initial diameter. Reproduced from Microclimate and Spray Dispersion by Bache and Johnstone (1992, Ellis Horwood Ltd).
Small droplets are more susceptible to displacement by wind currents due to their small mass. There is no magical size above which drift is no longer possible, but we’ve generally used diameters of 100, 150, or 200 µm as a theoretical cutoff. The proportion of the spray’s volume in droplets smaller than these diameters can be called “drift potential”, and this value is useful to measure the impact of nozzle type, pressure, or formulation on that phenomenon.
But it’s not quite that simple. Even a small droplet may resist drift if its exposure to wind is limited, perhaps through a protective shield shroud, or lower boom height. Or by increasing its speed through air assist. Higher energy droplets resist displacement.
These mitigating strategies aren’t lost on sprayer manufacturers who have used them for decades to build lower drift sprayers.
The next phase of the dose transfer process is interception. The droplet has to encounter its target, but the process is mostly coincidence. Simply put, the target has to be in the way of the droplet’s flight path for the two to meet. Denser canopies are therefore more effectively targeted. A larger number of droplets (smaller droplets or more carrier) also improve the odds. But it’s not that simple. Flight paths can change. That’s where small droplets are more inventive. Because they respond to small air currents, and because such small currents surround most objects, the smaller droplets can weave around objects, following the small eddies generated by air flows. As a result, we’re more likely to find smaller droplets further down in denser, more complex canopies where the eye can’t follow. They simply cascade through.
Larger droplets, on the other hand, resist displacement by air and travel in straighter lines. They tend to hit the objects they encounter. For that reason, larger droplets are intercepted by the first object they reach and only make their way deeper into a canopy if the path is clear. In other words, vertical, sparser objects allow larger droplets to pass by.
These properties are related to the droplet’s inertia, and are best described by a parameter known as “stop distance”. Assuming an initial velocity, stop distance is the distance required by a droplet to slow to its terminal velocity.
Figure 3: Stop distance as a function of droplet size. Assuming a 20 m/s initial velocity (similar to exit velocity of a hydraulic nozzle) and gravity assistance. Note that smaller droplets without the benefit of air assist lose their initial velocity within a few cm of the nozzle exit. Reproduced from Microclimate and Spray Dispersion by Bache and Johnstone (1992, Ellis Horwood Ltd).
These characteristics, combined with the aerodynamic properties of objects such as tiny insects, cotyledons, leaves, stems, etc. govern the collection efficiency of sprays. Small, slow moving droplets are thus best captured by small objects that don’t create strong enough deflections of airflow to steer the droplets past. Large objects that redirect air around them very effectively are better collectors of the larger or faster droplets whose kinetic energy can guide them through this turbulence. It’s also a matter of probability, as the smaller objects tend to have a lower likelihood of encountering the relatively scarce large droplets of any given spray.
But once again, that’s not the end of the story. Interception is followed by a critical stage, retention. Objects must be able to hold onto the droplets they intercept. Slow motion video has shown that droplets flatten out on contact with an object as the liquid converts impaction velocity into lateral spread. Once at full extension, the flattened droplets will collapse even beyond their original round shape, pushing them away from the surface and possibly causing rebound. A rebounding droplet may eventually land on target, but that would be a matter of fortune. It’s better if the leaf can offer enough adhesion, diminishing the power of the rebound oscillation, allowing droplet to stick the first time.
Figure 4: Droplet deformation during impact (C. Hao, et al. 2015. Superhydrophobic-like tunable droplet bouncing on slippery liquid interfaces. Nature Communications. August 2015).
Small droplets have less mass, and tend to be retained more easily. But more than size is at play here. The morphology and chemistry of the leaf surface is also important, with crystalline or more oily surfaces offering less adhesion for droplets. The physico-chemical properties of the spray mixture becomes important, as characteristics such as dynamic surface tension and visco-elasticity affect spray retention. These properties are optimized through the product formulation effort, and possibly via adjuvants added to the tank.
We sometimes classify targets as “easy to wet” or “difficult to wet” to summarize these properties. Most grassy plants (foxtails, cereals) are difficult to wet (there are exceptions, such as the sedges) and broadleaf plants vary from the easy to wet pigweeds to the difficult to wet lambsquarters and brassicas. Easy to wet species can retain larger droplets than difficult to wet species, and that’s one reason why finer sprays are preferred for grassy weed control (leaf orientation and size are another).
Figure 5: Droplet deformation, and surfactant molecule alignment, during impaction. The inability of surfactants to reach optimal alignment quickly, and for the target surface to absorb these forces, leads to rebound.
A few words about surface tension. Although surfactants reduce surface tension and facilitate spreading, this may not be enough to improve spray retention. To be effective, surfactant molecules need to align themselves with the surface of the droplet so they can be a “bridge” at the interface where the droplet meets the target surface. This takes time. The oscillations that occur during impaction continuously create new surfaces, and if surfactant molecules don’t follow suit immediately, the droplet will behave as if no surfactant is present. Specialists measure “dynamic” surface tension, i.e., the surface tension at young surface ages – a few milliseconds – to better predict spray retention. Very young surface ages have surface tensions of plain water, even with a surfactant present. Only certain surfactants, or higher concentrations of surfactants, can actually improve spray retention.
When air-induced nozzles were introduced in the mid 1990s, one of their claims was the improved spray retention due to air inclusions (bubbles) in the individual droplets. These bubbles made the droplets lighter, and also reduced their internal integrity, promoting breakup on impaction. As a result, the coarser sprays they produced actually had some of the same efficacy performance as the finer sprays they replaced. And indeed, research showed that coarser, air-induced sprays did in fact maintain good performance. Interestingly, performance of non-air-induced coarse sprays used with pulse-width modulation also showed similar robustness of performance. Research comparing air-induced to conventional sprays of similar droplet size rarely showed differences, and when they occurred, they were small in magnitude and could be corrected through improved pattern overlap.
Figure 6: Air Bubbles in spray droplets (Source: EI Operator. Believed to originate with Silsoe Research Institute, UK)
One reason larger droplets still work well is due to the pre-orifice designs of modern low-drift nozzles. This design reduces the internal pressure of the nozzle itself, with the effect being a slower moving large droplet. This reduced velocity takes away some of the force at impaction, reducing rebound.
Figure 7: Droplet velocity of larger droplets is reduced by lower pressures from pre-orifice and air-induced design nozzles. Lower velocities reduce droplet rebound.
Another neat effect of coarser sprays is their ability to entrain air. All sprays move air (simply spray into a bucket to see this), and larger droplets do this better and for longer distances. The entrained air is a form of air assist for the smaller droplets, increasing their average velocity and thus reducing their drift potential while they move in the spray pattern.
The final stage of the dose transfer process is deposit formation and biological effect, and that’s where we once again see differences attributable to droplet size.
Once established on a target surface, the active ingredient usually needs to move to its site of action. In some cases, resting on the surface is sufficient, it depends on the specific product. But for the majority of herbicides, the active ingredient must move across the cuticle into the cytoplasm where it eventually migrates to the enzymes involved in photosynthesis or biosynthesis of fatty- or amino acids. The cuticle is waxy, with only a few water-loving pathways and the uptake process is basically driven by diffusion and concentration gradients. As such, it is more effective when the product is in solution and the longer the droplet can stay wet, the better. That’s one reason why spraying during hot, dry days may reduce performance. Again, it depends on the formulation and the mode of action. Too high a concentration can damage membranes, physiologically isolating the active ingredient and reducing its subsequent translocation. It’s always a balancing act.
If you’ve been keeping track of the score, it’s more or less a tie between large and small droplets. One deposits better and makes more efficient use of lower water volumes, while the other has lower losses from drift and evaporation, helps smaller droplets resist drift, and may improve uptake of some products.
And this draw is why the venerable hydraulic nozzle has been so successful for so many decades. Hydraulic atomization, by its nature, creates a wide diversity of droplet sizes, ranging from 5 to 2000 µm or greater. As Dr. Ralph Brown of the University of Guelph used to say, this nozzle provides a drop for all seasons. Some small ones for coverage and retention in hard to reach places, and some large ones for uptake and drift-reduction. The result is a robust delivery system that provides reliable results on many different targets under many conditions. In recognition of the heterogeneity of sprays, we don’t refer to specific droplet sizes, but rather their composite, grouped into international categories of Spray Quality such as Medium, Coarse, and Very Coarse.
Our challenge is to find the spray quality sweet spot, the ideal blend of these contradictory and yet complementary features of our agricultural sprays. And I believe that task is very achievable. Simply put, broadcast agricultural sprays in field crops work reliably when applied as Coarse and Very Coarse sprays in volumes between 7 and 12 US gpa. There is no need to spray any finer than Coarse for good efficacy, as coverage is already sufficient and any additional coverage has small marginal returns. There is, however, value in adding more water when canopies are denser or when leaf area index grows as the crop matures. To gain coverage, adding water is preferred to reducing droplet size because of the value of environmental protection. It so happens that Coarse to Very Coarse sprays provide or ecxeeed the drift protection required by most agricultural labels.
There is occasional reason for spraying even coarser than what I’ve suggested. It’s certainly required by law for dicamba products on Xtend traited soybeans and cotton, but even then, only in conjunction with higher water volumes to offset losses in droplet numbers. In practice, moving to Extremely Coarse or Ultra Coarse sprays may allow an application to proceed in higher than average wind without adding drift risk. The use of some additional water is a relatively small price to pay for that additional capability.
There will always be opportunities for efficacy improvement in specific cases for those willing to spend the extra time to optimize that situation. That’s one of the reasons I’m excited to see the widespread adoption of pulse width modulation (PWM) in the industry, allowing users to change spray pressure and therefore spray quality with no impact on application rate or travel speed. Or the introduction of nozzle switching from the cab, employing the optimal atomizer for a specific situation. Although it remains difficult to define the ideal spray, selecting a spray quality has never been so easy.
Originally published in: Wolf, T.M. and Downer, R.A. ILASS Americas, 11th Annual Conference on Liquid Atomization and Spray Systems, Sacramento, CA, May, 1998
Note to reader:It’s been nearly 23 years since we wrote this paper at the invitation of organizers of the Institute for Liquid Atomization and Spray Systems Conference. At the time, custom operators, not farmers, bought self-propelled sprayers. Air-induction tips had just been introduced. Pulse-Width Modulation was only beginning to be available. GMO crops were available but not widely adopted. Buffer Zones were more rumour than policy. How badly out of date are the thoughts we mulled over?
Abstract
The goals of an agricultural spray application are to provide effective control of the pest at low cost without adverse environmental impact. A spray must transport effectively from the atomizer to the target, be intercepted and retained by the target, and form a biologically active deposit. Improvements in efficiency are elusive because of interactions between successive stages in dose transfer. Progress will depend on atomizers providing increased control over droplet size and velocity spectra without sacrificing mechanical simplicity: (a) elimination of the interdependence of flow rate and spray quality, (b) control over size span at any given nominal diameter, (c) reversal of present relationship between droplet size and velocity. Such an atomizer would drive a new research thrust to improve spray efficiency
Introduction
Polydisperse sprays provide consistent results yet suffer from inherent inefficiencies in dose transfer. Drift potential and poor spray retention at the extreme ends of their spectra are classic examples of this inefficiency, and environmental aspects of spray application have been criticized as a result (Pimmentel and Levitan, 1986). Yet, despite ongoing research, efficiency breakthroughs remain elusive (Hislop, 1993). Due to the interdependency of the factors governing dose transfer, progress in one area (i.e., greater retention with finer sprays) has often been at the expense of spray drift, and vice versa (Young, 1986). Theoretical improvements in efficiency with monodisperse sprays (Controlled Droplet Application, CDA) have not translated into widespread adoption due to drawbacks in consistency and robustness of the results. After 50 years of research, the same compromises which have been discussed since the early days of spray application are apparently still unresolved.
Nozzle designs have certainly improved – wider pressure ranges, improved spray patterns, more options for achieving various spray qualities, better quality, longer wearing materials, and lower costs are all important for the end-user. But the basic atomizer – the hydraulic flat fan nozzle generating a polydisperse spray – has hardly changed over the years.
A New Start?
The questions posed in this paper are: If a biologist could design the ideal spray, what would it be? What are the criteria for achieving the best result in the most efficient manner? Such a discussion represents a unique opportunity to think about what we know about sprays and their biological impact, providing atomizer design information to meet our future needs.
Unfortunately, biologists still know relatively little about the impact of kind of spray quality on efficacy. General statements can be made relating spray quality to herbicide, insecticide, or fungicide effectiveness, but for the most part, the ideal spray or subsequent deposit has still not been defined for most situations (Hislop, 1987). The situation reflects the lack of choice in spray atomization, creating a catch-22: not being able to easily produce customized sprays has made it difficult for biologists to identify (without confounding factors) the ideal spray for any particular situation. Further, the need by the industry for a simple, reliable, and standard application system has inherently hindered efforts to optimize the system. All stakeholders will need to be flexible to present a fertile environment for improvements to take hold.
“Integrated Spray Management”
In this era of integrated pest management, cropping systems are optimized to provide the most effective pest management strategy on a case-by-case basis with minimal crop protection agent (CPA) use. This underlying philosophy can be extended to spray application. When CPAs are used in such systems, they, too, must rely on diverse strategies to make them more efficient. Within this philosophy, a single standard application technique for all pests will not be acceptable. Two developments are needed to put such a development into action: (a) an application system capable of delivering a wide variety of spray qualities (droplet sizes, spans, velocities) at a range of carrier volumes; and (b) the knowledge to utilize specific spray qualities under identifiable conditions.
Application Objectives
During the development of such a new application philosophy, the objectives for spray application must remain clear. They are to deliver a CPA in its most effective form to the pest, with no off-target effects, at the lowest possible cost, i.e., effective, economical, and environmental.
Figure 1: Typical droplet number and volume spectra for an agricultural hydraulic flat fan nozzle
The status quo for most post-emergent CPA applications is the hydraulic flat fan nozzle. As we know, such a nozzle produces a heterogeneous mix of fine and coarse droplets (Figure 1) with a droplet speed and size relationship (Figure 2). This nozzle has frequently been criticized for inefficiency because only a small portion of spray is optimally targeted (Adams et al., 1990). At the same time, it has been applauded for consistency because a portion of its size and velocity spectrum (although not necessarily the same one) is usually appropriate for the pest complex at hand.
Figure 2: Typical velocity spectrum for an agricultural hydraulic flat fan nozzle.
The most frequently documented drawbacks of the hydraulic nozzle are driftability of fine, and poor retention of coarse components. An additional drawback is interdependence of flow rate and droplet size for any given nozzle, i.e., at the same work rate, lower carrier volumes are applied with finer sprays. Research into droplet size effects has been difficult because no variable can be held constant while another changes. Keeping dose constant, studies of carrier volume have to accept a simultaneous change in travel speed or droplet size, droplet size studies have to contend with changes in droplet density, and droplet density studies must alter active ingredient concentration. Given the complexity of the problem, few researchers commit themselves to solving these dilemmas.
Maximizing Effectiveness – No Easy Answers
For any given spray mixture, an atomizer controls spray pattern, droplet size, and droplet velocity. Spray patterns determine spatial uniformity. Droplet size and velocity in turn affect spray fate by controlling canopy penetration spray interception, spray retention, spray coverage, evaporation rate, etc. Considering the variety of active ingredients, formulations, concentrations, environments, pests, and plant canopies present, it is not surprising that the scientific body of evidence is often contradictory (Knoche, 1994). It should also come as no surprise that there is no single “best” droplet size to optimize these factors.
Basic principles: In order to better understand why a single ideal spray cannot exist, a brief review of the principles of spray drift, interception, and retention are appropriate. Larger droplets are driven mostly by inertial and gravitational forces (Spillman, 1984). As such, they tend to have vertical trajectories from which they cannot easily be displaced. This makes them a good choice for drift reduction, and also for canopy penetration into vertically oriented canopies, such as cereal grains. Collection efficiency by a target is a function of target size and orientation – horizontally oriented, larger objects will be favoured by larger droplets. Spray retention is a function of leaf surface wettability and microstructure, as more difficult to wet species will be more likely to reflect larger droplets (Hartley and Brunskill, 1958).
Small droplets, on the other hand, are more subject to viscous drag, have shorter stop distances, and can therefore move with local air turbulence to reach shadow regions (Nordbo, 1992). Thus the finer sprays have a propensity for displacement from their flight path by air turbulence, but they also are better able to penetrate dense broadleaf canopies because they can move around larger objects. Small, vertically oriented objects such as stems and petioles have the best collection efficiency for small droplets.
Upon depositing successfully on a target, the deposit must be in a form which exerts the desired biological effect. Given the same spread factor, deposits with greater volumes remain wet longer, providing more opportunities for uptake into the leaf. Small droplets provide more efficient coverage per unit volume, but dry rapidly, which may limit their uptake.
Uptake and translocation of active ingredients by biological targets are physical processes driven by concentration gradients. Concentrations of active ingredients and surfactants per unit leaf area are a function of carrier volume, droplet size, and spread factor. Less than optimum concentrations can result in reduced uptake and translocation (Wolf et al., 1992).
Further complications arise due to the heterogeneous nature of weeds. Individual regions of weed plants have unique anatomical and physiological features that can affect retention, uptake, and translocation processes on a spatial level. For example, Merritt (1982) showed that for wild oats (Avena fatua), younger leaves and the basal region of leaves absorbed more difenzoquat than older leaves.
All these factors conspire to complicate the quest for optimization in a field setting.
The Ideal Spray
Based on the previous discussion, it may be obvious that a single droplet size cannot meet all demands within such a complex system. Therefore our focus must shift from a theoretical optimum solution, as was the basis for controlled droplet applicators (Bals, 1980) to one which emphasizes flexibility.
One advantage of speaking on behalf of biologists is that one can feign complete ignorance about atomization, and propose seemingly ludicrous ideas. Perhaps a prerequisite to a fresh approach is such ignorance.
Biologists need a spray to not only implement optimum application, but as a means with which to learn how to optimize the process in the first place. Further, since there is no single optimum spray quality to meet all application scenarios, the most important feature in a spray is flexibility. The following features will be important:
Spray quality independence: The first criteria is the ability to adjust spray quality easily, without affecting carrier volume or droplet velocity, and vice versa (Figure 3). A shift towards a coarser or finer spray can then be achieved without introducing other confounding effects. Some progress has already been made in this area (Giles and Comino, 1990).
Figure 3: Shift in droplet size spectra from medium to fine or coarse qualities, achievable without a change in carrier volume.
Relative span factor flexibility: The relative span factor of the spray should be adjustable (Figure 4). It will be important to narrow the broad spectrum sprays produced by flat fan nozzles to determine the importance of specific droplet sizes. While such research was conducted during the 1970s and 1980s with controlled droplet applicators (CDAs), the unique droplet velocity associated with such atomizers would question results if they were to be applied using hydraulic atomizers.
Figure 4: Narrowing the span of the droplet size spectra, while preserving its polydisperse nature, will be useful to strike a balance between specific droplet sizes and spray heterogeneity.
Velocity control: The third criteria is for improved droplet velocity control. The droplet velocity dependency on size has meant that in the absence of air assist, smaller droplets are always moving slower. This factor has reduced the efficiency of their collection and made them more drift prone. Additionally, the larger droplets, being faster moving, were more likely to rebound from targets. Acceleration of small droplets is a strategy for reducing spray drift and enhancing collection efficiency, but greater velocity for larger droplets may reduce the efficiency of their retention by the target. If the droplet size – velocity relationship were reversed, then smaller droplets would be less drift prone and larger droplets would be less likely to rebound (Figure 5).
Figure 5: Droplet velocity spectrum for a typical agricultural hydraulic spray, accelerated with air assist to reduce drift potential of smaller droplets, and with smaller droplets travelling faster than larger droplets to maximize transfer efficiency.
Spray heterogeneity: Spray heterogeneity will remain important in an optimized system, especially in the absence of specific knowledge on droplet function by size class. In this sense, a polydisperse spray does more than provide insurance for changing conditions, it adds diversity to static conditions which strengthens the overall effect. While a quantitative dose-based approach to CPA delivery is often appropriate, it under-emphasizes the role of deposit structure and spray redistribution, where quality is more important than quantity (Wolf, 1996). For example, let us assume that canopy penetration is maximized with a spray of 400 µm VMD, with a relative span factor of 0.7. In such a spray, fine droplets contribute relatively little to overall dose. However, their ability to redistribute in the canopy, targeting areas left untouched by the larger droplets may be more important than their total dose contribution would suggest. In this way, they provide benefits which are total dose independent. A heterogeneous spray would ensure that these benefits remain.
Deposit uniformity: Efforts at optimizing dose transfer are compromised if spatial dose uniformity cannot be maintained within the treated area. High deposit variability has been associated with reduced control of insects (Uk and Courshee, 1982; Cooke et al., 1986). As such, uniformity remains a fundamental requirement for spray application and should not be compromised with new atomizer designs.
Environment as a Priority
Spray must land on the intended target, be it a plant, insect or ground, and in some cases on the optimal pest part, i.e., specific leaves, leaf sides, stems, etc. Off-target placement not only represents inefficiency, but also undesired environmental input. With any application system, an important criteria is the ability to manage off-target impacts.
Past solutions to spray drift or droplet rebound has been two-fold: (a) eliminate those droplets which do not impact on the target efficiently, or (b) protect them from displacement. For spray drift, the elimination of small drops through production of coarse sprays has been successful (Edwards and Ripper, 1953). The challenge is to provide drift protection without compromising the advances made in the previous exercise of maximizing effectiveness. The protection of fines with barrier (shrouds) is an effective strategy for reducing drift, and provides the advantage of maintaining a spray quality established to meet separate criteria (Wolf et al., 1993). Another successful strategy has been to assist transport of fines with an external energy source (air or electrostatics). This also allows the preservation of an optimized spray quality, with the added advantage of modifying the droplet velocity spectrum in favour of canopy penetration.
Nozzle design may offer some opportunities for the reduction of rebound. Novel atomization systems such as venturi or twin-fluid nozzles, which offer air-inclusion in droplets, may reduce rebound of larger droplets. If larger droplets are required, but retention is of concern, such approaches may be useful. Spray adjuvants can also play important roles in this area (Downer et al., 1995)
Economical Considerations
Underlying any attempt to provide effective pest management is an economical consideration. The producer must see a benefit in making a technical investment. Any atomizer solution must therefore not only meet the technical requirements for optimizing dose transfer, it must also be a cost-effective and practical system. A system which is complicated to use is not likely to be widely adopted. Without strategies for implementation by the end user, innovations in delivery are merely theoretical exercises.
Putting it into Practice
During a typical work day, an applicator may be called to treat crops for a range of pests with broad-spectrum products. These pests will likely be present in a range of densities, some above and others below an economic threshold. There may also be a range of canopies present, some broadleaves, others grasses, some dense, others sparse. Depending on the area, there may have been a range of environments under which pests became established, or during which application is made. Each field will also have a range of bordering ecosystems with unique trespass sensitivities.
There will obviously be a limit to the degree of customization that is possible. But some efforts will be rewarded. The applicator uses GPS technology to collect or recall relevant data – sensitive areas, high and low infestation levels, or changes in canopy structure. With the new atomizer, the applicator can emit the most effective droplet size, velocity, span, and dose appropriate for the pest or canopy on a site-specific basis. The use of spray quality classification systems such as those developed by the BCPC and ASAE will guide optimization efforts, but in the end, these classification systems will be too broad to fine-tune the system. A higher resolution, multi-parameter scheme which is sensitive enough to represent the criteria laid out in this paper will be necessary.
Possible difficulties emerge when the system resists optimization. For example, it would be comparatively easy for the applicator to control a broadleaf weed in a grassy canopy, as the size spectrum which optimizes grass canopy penetration is also likely to target the broadleaf weed effectively. If a tank mix is used to control both grassy and broadleaf weeds in this canopy, the applicator now needs a more heterogeneous spray, where a finer component targets the grassy weed, and the coarser component still effectively transfers dose to the broadleaf weed. As the situation increases in complexity, the simultaneous optimization of several criteria will be increasingly difficult.
Conclusions
Only an integrated approach involving all stakeholders (engineers, chemists, biologists, etc.) can result in improved application of CPAs. Individual goals and concerns must be communicated and reconciled in new design efforts. This paper represents a wish list from biologists’ perspectives. While greater flexibility and control are important objectives in our opinion, consideration must also be given to mechanical complexity and cost, possible interactions with formulations exhibiting a range of physico-chemical properties, biocontrol agents, and practical strategies for adoption. A continued willingness to establish and maintain lines of communication and cooperation between these disciplines will be pivotal to success.
Acknowledgments
The invitation by the ILASS Program Organizing Committee to make this presentation is gratefully acknowledged.
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Spillman, J.J. 1984. Spray impaction, retention and adhesion: an introduction to basic characteristics. Pestic. Sci. 15:97-106.
Wolf, T.M. 1996. Spray application into standing stubble – an exploration of physical and physiological components. Ph.D. Dissertation, Department of Agronomy, The Ohio State University, 192 pp.
Wolf, T.M., Grover, R., Wallace, K., Shewchuk, S.R., and Maybank, J. 1993. Effect of protective shields on drift and deposition characteristics of field sprayers. Can. J. Plant Sci. 73:1261-1273.
Wolf, T.M., Caldwell, B.C. McIntyre, G.I., and Hsiao, A.I. 1992. Effect of droplet size and herbicide concentration on absorption and translocation of 14C‑2,4‑D in oriental mustard (Sysimbrium orientale). Weed Sci. 40:568-575.
Picking the correct nozzle for a spray job can be a daunting task. There is a lot of product selection, and a lot of different features. We try to break the process down into four steps.
1. Identify Your Needs
Before making any assumptions about the right nozzle for you, review your needs and objectives. Are you trying to reduce drift? Do you want better coverage? Are you moving towards more fungicide application? Do you need a wide pressure range?
It’s always a good idea to review your experience with your previous nozzle. What, if anything, would you like to change?
2. Identify Flow Rates
Most spray operations fall into one of three categories, (a) pre-seed burnoff (3 to 7 US gpa); (b) in-crop early post-emergence (7 to 10 US gpa); (c) late season application to mature canopies (10 – 20 US gpa).
To find the right nozzle size, you need to know the application volume, the travel speed, and the nozzle spacing. Most sprayers have 20” nozzle spacing, but some have 15” spacing. Use these metric or US units charts to find the right flow rate for common nozzle spacings. Various on-line calculators from Hypro, Greenleaf / Agrotop, Lechler, or Wilger or their apps, are also helpful.
If you use our chart, the top row lists water volumes. The columns contain travel speeds. Travel speed is somewhat flexible and can change throughout the field.
Let’s assume the water volume is 7 gpa, and the desired application speed is 13 mph. Move down the “7 gpa” column, searching for 13 mph. You will encounter 13 mph about 5 times: 02 nozzle @ >90 psi, 025 nozzle @ 60 psi, 03 nozzle @ 40 psi, and 035 nozzle @ 30 psi (the 035 size is only offered by some manufacturers) and the 04 nozzle at about 25 psi.
Nozzle chart, in US units, solving for 7 gpa at 13 mph. Five nozzles can produce the required flow, each at different pressures.
Note that for the smaller nozzle sizes, the spray pressure is perhaps too high, and for the larger sizes, it is too low. Select a size that allows optimum nozzle performance and travel speed flexibility. In this example, the 025 size is optimal, producing an expected pressure of about 60 psi. The column for the 025 nozzle can now be used to predict the travel speed range from 30 psi to 90 psi, about 9 to 16 mph. For the 03 nozzle, the minimum speed would be 11 mph, too fast for some.
For Pulse Width Modulation (PWM), slightly different rules apply. See here for instructions.
3. Select the Nozzle Model
For general spraying, we recommend intermediate spray qualities ranging from Medium to Very Coarse.
These intermediate spray qualities offer good coverage at reasonable water volumes and good drift control. Their spray quality can be tailored with pressure adjustments to suit specific needs. For images, see here. In alphabetical order:
Air Induced:
There is plenty of selection in this popular category, all manufacturers offering similar specs and performance.
All nozzles should be operated near the middle of their pressure range, for air-induction this is 50 to 60 psi or higher, a bit less for non air-induced types. This allows maximum flexibility when travel speeds change or when spray quality is adjusted with pressure.
For fusarium headblight, consider a twin fan nozzle.
Keep your booms no more than 15” to 25” above the heads for best results.
Air Induced:
There is an excellent selection of twin fans from most manufacturers.
For finer sprays (lower water volumes), simply increase spray pressure or consider a non-air-induced design.
There has always been a large selection of finer sprays on the market, remnants from a time when drift was less important. Very few offer flow rates above 06 or 08, decreasing utility for PWM systems.
Notice that conventional flat fan tips and most pre-orifice tips are absent from these lists. These nozzles are not recommended for herbicides because they produce sprays that are too fine for acceptable environmental protection (ASABE Fine and Medium). The added coverage afforded by such sprays only has value with low water volumes, and in those instances is more than offset by their higher drift and evaporation. An exception is the use of insecticides with contact mode of action targetting small insects such as flea beetles or aphids. In thes cases, finer sprays (ASABE Fine or Medium) may be required to provide effective tragetting.
Very high flows are sometimes needed (11010 and above, usually for PWM). When this occurs, conventional flat fans have merit because the higher flow rates of any nozzle usually create coarser sprays, and even conventional tips will create sufficient coarseness to prevent drift.
For the best drift protection, consider these tips.
The advent of the dicamba-resistant trait in soybeans has spawned interest in very low drift tips that comply with the label requirements for these products. Although superior for drift control, they are not well suited for low volume or low-pressure spraying, nor for contact herbicides or grassy weeds, as spray retention and coverage may be poor. But they are very valuable when drift control is paramount and when higher volumes can be used to maintain adequate coverage.
The following advice is based on the rules at the time it was written. These may be suitable for 2,4-D application in Australia under the newest APVMA guidelines (check spray quality to be sure it is VC or coarser). Many are also suited for Dicamba in Canada (must be XC or coarser), or dicamba in the US (must be on approved lists such as this one for Xtendimax or this one for Engenia, but caution is advised, some low pressure limits make them impractical. Always check that spray quality can be achieved at pressures that offer travel speed flexibility.
Air Induced:
Excellent selection. This market has received much attention in recent years.
Before making a selection, check the nozzle’s recommended pressure range and the spray qualities within that range from the manufacturer info. The target pressure for these tips may differ from your expectations.
4. Tweak and Confirm
Under field conditions, the spray pressures which produce the desired water volumes can vary from the charts. Make sure you trust your pressure gauge reading and know the pressure drop from the gauge signal to the nozzles, particularly with PWM, where the solenoid adds additional drop. Add the pressure drop to your target pressure reading. If using a rate controller, use the pressure gauge as your speedometer to ensure optimal nozzle performance. Adjust travel speed until the nozzle pressure meets with your spray quality and pattern goals. If that speed is too slow or fast…you have the wrong size nozzle and/or water volume.
Spray pressure is more important than travel speed – make your pressure gauge your speedometer.
Airblast operators should know how to read a nozzle table. They are found on dealer and manufacturer websites as well as in their catalogs. Table layout varies with brand, but they all relate a nozzle’s flow rate to operating pressure. The better tables also provide the spray angle and the median droplet size (i.e. spray quality).
Operators need this information to complete calibration calculations (aka sprayer math) and when deciding how to distribute nozzle rates, angles and spray quality along a boom relative to the target canopy.
This article focusses on hollow and full cone nozzles, which are commonly found on airblast sprayers. For more information on flat fan nozzle tables (e.g. for banded under-canopy or, vertical booms or broadcast applications from horizontal booms), refer to this article.
Reading the table
Let’s use the table below to determine a nozzle’s flow rate for a given pressure. First, find the nozzle colour in the top row. Second, find the operating pressure in the left-most column. Finally, the flow rate is indicated in the cell at the intersection between the row and column. For example, a red ATR hollow cone nozzle operated at 9 bar will emit a flow rate of 1.83 L/min.
Perhaps you want to determine which nozzle will give a specific flow rate. Find the rate in the body of the table and trace the column and row to determine which nozzle/pressure combination will achieve it. For example, if we want a flow rate of ~1.00 L/min, we can use a Yellow at 10 bar or an Orange at 5 bar. Yellow is the better choice since the Orange would have to be operated at the bottom of its pressure range (more on that later).
This Albuz nozzle table for 60 and 80 degree molded hollow cones gives flow rates in litres per minute.
Note: Do not to confuse TeeJet’s ISO-standardized TXA or TXB nozzles with TXVK or ConeJet nozzles. They may be the same colour, but their outputs are very different.
Higher flow rates or full cone patterns can be achieved using combination disc and core (or disc and whirl) nozzles. Depending on the manufacturer, the disc plate is defined by it’s diameter in 64th’s of an inch. The core or whirl plate might be described by the number of holes (e.g. 2-hole, 3-hole, etc.), or some other manufacturer-specific nomenclature (e.g. 45’s, 25’s etc.).
Using the table below, we see that a D2 disc and a DC35 core will emit 0.34 gpm at 80 psi. By continuing along the row, we see that the spray angle for this combination will be 47 degrees at that pressure.
This TeeJet nozzle table gives the flow rate for a disc (D#) and core (DC#) full cone combination nozzles in US gallons per minute.
Pressure problems
Do not choose a nozzle at the extreme of their flow or pressure range. A trailed PTO sprayer will experience pressure changes from driving on hills, or rate controllers will create pressure changes in response to changes in travel speed. In either situation, coverage will be compromised if the nozzle is pushed outside its optimal range.
Note: Use pressure to achieve small changes in flow, but for more extreme changes, switch nozzles. Remember, it takes 4x the pressure to get 2x the flow. Stated differently, it takes 1/4 the pressure to get 1/2 the flow.
You may not find a nozzle/pressure combination that emits the rate you are looking for. When your desired rate or pressure falls between the figures listed in the table, you can take the average. When nozzling an entire boom with different nozzle rates, get each position as close as you can to achieve the overall boom rate for a given pressure. It’s always a compromise – don’t stress over it.
Looking up nozzle rates during a spring calibration. The operator was running at 190 psi, but the catalogue only listed 180 psi and 200 psi. When the increment is only 20 psi, it’s reasonable to approximate the output. When the span is 50 psi increments, it is more difficult to determine the rate without testing the output (it’s not a linear relationship). This issue usually occurs at pressures above 200 psi, and that’s far too high for cane, bush, vine and high-density orchards. In these situations, consider using a lower operating pressure.
Different nozzles, same rate
Different disc core combinations, or molded nozzles at different pressures, can produce similar flow rates. However, their spray quality and spray cone angles can be very different (see last three columns in the TeeJet table above).
The angle of the spray cone can have a big impact on spray coverage. When the target is far away from the corresponding nozzle (e.g. the tops of nut trees), or the canopy is very, very dense (e.g. citrus canopies), consider tight-angled full cones under high pressure. This is inefficient and can give variable coverage, but it is sometimes the only option in extreme situations.
Oops! Two hollow cone nozzles on top and five full cone nozzles below is the exact opposite of how things should be. Note the lack of spray overlap with the full cones for the first few meters. Spray from the top two positions will likely not reach the intended target.
When the target is very close to the sprayer, full cones do not overlap and create undesirable striping or banded coverage. Creating a full, overlapping spray swath that spans the entire canopy is a function of nozzle spacing, distance-to-target, and sprayer air-settings. It can also be affected by humidity, wind speed and wind direction at the time of spraying.
Confirm your settings by parking the sprayer in the alley between crops. With the air on, spray clean water while a partner stands a safe distance behind the sprayer to look for gaps in the swath. The partner will see things the operator’s shoulder check will not reveal.
Here’s what the operator sees. But, shoulder checks may not show you what’s really happening. Have someone stand a safe distance behind the sprayer while spraying clean water to see the nozzle spray overlaps sufficiently to span the entire canopy.Here’s what the partner standing behind the sprayer sees. Take a picture with a smartphone to show the operator.
Nozzle tables can be wrong
Sometimes nozzles do not perform per the nozzle table. We have discovered errors in published tables, worldwide. Here are the big three:
Conversion errors. Manufacturers publish catalogs in Metric and in US Imperial, but we have found many errors in the conversions.
Spray angle errors. When nozzles are operated at the extremes of their pressure ranges, spray angles deviate from those listed in the tables.
Flow rate errors. When tables are not updated to reflect changes in nozzle design, or the manufacturing process, actual flow rates deviate from those listed in the tables.
Perhaps it’s not the table, but the nozzle itself. Most nozzle manufacturers accept a flow variability up to +/- 2.5% for new nozzles, but we have seen higher. It depends how they are made (machined, stamped, printed) and the material they are made of.
Validate flow rate and pattern
When errors are discovered and reported, the manufacturers can be slow to issue corrections and the errors will persist in old tables. Yes, even apps (which are often based on tables) can be wrong. So, predicted flow rates can prove unreliable. This is why it is important to double check by observing nozzle overlap and validating flow rate when you replace nozzles – even when they are brand new.
Thanks to Dr. David Manktelow (Applied Research and Technologies, Ltd., NZ) for input into this article.
