The year 1989 marked my first spray drift trial under the watchful eye of Dr. Raj Grover and John Maybank. We evaluated the performance of several spray shrouds, Flexi-Coil, AgShield, Brandt, and Rogers, and wanted to measure just how effective they were. But in my heart I wasn’t interested in drift. I wanted to study herbicide efficacy. Anyway, I thought, we’ll do this trial and I’m pretty sure we won’t have to revisit the topic.
It’s now thirty-two years later and spray drift has interwoven itself into all my projects, remains one of the most powerful drivers of regulatory activity, is likely the most visible consequence of poor stewardship, and will stay as one of the dominant creators of public opinion around modern agricultural practice.
Drift has not gone away. And yet our understanding of it is far from complete.
Spray drift is defined as the wind-induced movement of the spray cloud away from the treated swath. Droplet drift can occur for all sprays, and it happens within minutes of the spray pass. Its cousin, vapour drift, is limited to active ingredients that are volatile, that is, they can evaporate from dry deposits after application. Vapour drift happens after the spray application is complete and can last several days.
Droplet Drift
Droplet drift can be divided into two phases that are separated by about 1 second and that are measured differently. “Initial drift” happens first and refers to the product that leaves the treated area immediately after atomization. It is airborne and can be measured by placing air-samplers (any device that can capture droplets in air) close to the downwind edge of the spray swath.
Figure 1: Initial vs Secondary drift. Once the drift cloud leaves the treated swath, the relative strengths of turbulence and sedimentation determine the amount that remains airborne and the amount that lands downwind.
Secondary drift describes the airborne spray cloud that continues to move downwind from the swath edge, where it either remains aloft or deposits on the surface below it. It is typically measured using samplers placed on the ground that capture sedimenting spray droplets. The difference in method is important because it goes to the heart of the problem of understanding spray drift.
Figure 2: Droplet drift occurs when displacement energy exceeds droplet energy. The droplet’s combination of mass and velocity cannot withstand the energy presented by moving air.
Initial drift is actually quite easy to understand because its creation is intuitive. The displacement of droplets from the spray plume is a function of balancing two types of energy. The first, droplet energy, is the product of droplet diameter and velocity. The more energy in the droplets, the more difficult they are to displace, and that’s why larger, heavier droplets or fast-moving air assist are useful drift reducing tools. The second, displacement energy, comes from relative air movement, either from forward travel speed or wind and the associated turbulence. More wind or turbulence means more power to displace.
Figure 3: Initial drift follows an expected response to greater wind speeds and coarser sprays. Data from a pull-type sprayer travelling 13 km/h with 60 cm boom height.
Because initial drift is easier to understand, our most common advice for reducing drift is based on maximizing droplet energy and minimizing displacement energy. Lower booms, larger droplets, slower travel speeds, shrouds, or properly implemented air assist all help reduce initial drift. It makes sense that creating less initial drift will also reduce downwind deposition arising from secondary drift.
Figure 4: Management of initial drift is intuitive. We reduce drift by adding energy to the droplet and by protecting the droplet from exposure to moving air.
Downwind Deposition
After leaving the spray swath, the moving secondary drift cloud has two main options. It can deposit or it can remain airborne. Basic physics suggest that all objects eventually fall to the ground, and since smaller objects need more time, they drift further. But when atmospheric turbulence and topography are considered, it’s not quite that simple. These two complicating factors control what proportion of the drift cloud remains airborne, and what proportion deposits.
Drift trials show that about 20% of the initial drift amount returns to the surface within the first 100 m or so of the sprayer. The rest remains and rises in the atmosphere where it evaporates and gets mixed further.
Figure 5: The majority of secondary drift remains airborne. Data are for Medium spray quality from a pull-type sprayer with 60 cm boom height and 13 km/h travel speed
It happens quickly. Just 5 m downwind of the spray swath, the cloud is already 4 m tall. At 100 m downwind, we’ve measured its height to be 30 m.
The proportion of the spray that remains airborne depends on the spray quality and the nature of the atmosphere. If it’s windy and sunny, or if the spray is finer, turbulence sends more into the air. If it’s cloudy and the wind is low, we have little atmospheric mixing. As a result, a smaller proportion will remain airborne and more will sediment, and overall, we may actually have more potential to damage downwind areas.
When we graph spray drift deposit data from a windy day, the deposit amount decreases exponentially with downwind distance. Usually, drift damage follows the same pattern. The larger droplets that contain the majority of the dose deposit first. The smaller droplets go further and are more likely to mix in the atmosphere and rise with thermals.
Figure 6: Deposited drift decreases logarithmically with distance. Top, linear axes. Bottom, log axes.
Under temperature inversion conditions that are common on calm summer evenings, overnight, and early mornings, the damage from the drift cloud does not decrease the same way. The cloud containing the buoyant mist lingers over a large area. Without atmospheric mixing and its resulting dilution with time and distance, large areas can be damaged.
The Effect of Turbulence on Deposition
We’ve established that the more atmospheric mixing we have, the less spray will deposit on the ground, at least in the short term. How does this affect our thinking on the role of wind?
When we evaluated drift data from a number of trials, we always saw more initial drift with higher wind speeds, as expected. However, the downwind deposit did not usually increase significantly. We attributed this observation to turbulence generated by wind which lifted more of the initial drift higher into the atmosphere. To be clear, deposited drift did not go down with higher wind. It just didn’t rise as fast as initial drift.
Figure 7: The effect of wind speed on airborne drift (top line) vs deposited drift (bottom line) from a high clearance sprayer travelling 23 km/h and emitting a Very Coarse spray.
The effect of turbulence can be viewed as a good thing because it protects downwind objects. Rapid dilution reduces immediate drift damage. We can use turbulence to protect objects on the ground. It’s certainly better than the alternative, emitting sprays when the atmosphere can’t dilute them, such as in an inversion. In that case, downwind areas remain at risk for a long distance, and for a long time.
But we have to also consider what happens to airborne spray droplets. Some pesticides degrade in sunlight and stop being a problem. But others are more stable and may persist in the atmosphere for days or longer. During that time, they may move significant distances, ultimately returning to the earth’s surface in precipitation or in dust. Even though the atmosphere has diluted them, these deposits are measurable, and will show up in environmental monitoring of air, soil, and water. We may not be able to find out where they originate, but the public knows who to blame. Agriculture.
Vapour Drift
Vapour drift is another issue altogether. It occurs hours and days after application, as long as the volatile product remains on a surface and conditions that allow formation of vapours persist. Vapour pressure is related to surface temperature, and losses increase with warmer surfaces. Some products enter the vapour phase when in contact with water, and release vapour after a rainfall.
In situations where vapour is released for several days after application, it becomes impossible to control its subsequent movement. For droplet drift, if we know the wind direction at the time of spraying, we know where the impact is likely to be. But vapour movement depends on conditions that may occur between now and three days from now, and these could include high temperatures, various wind directions, and even inversions in which vapours accumulate. Ultimately, the best way to avoid off-target vapour movement is to avoid using volatile products.
The Public Good
Spray drift is one of agriculture’s most important stewardship challenges, and our industry needs to continue to improve its track record. Sprayers have a difficult task of converting a relatively small volume of liquid into a spray that offers good target coverage yet doesn’t move off the treated area. Favourable weather combined with droplet size management are at the heart of making this system work, but there isn’t a lot of wiggle room. Once again, an emphasis on sprayer productivity is one of the most fruitful areas to invest in, as this makes the best of the sometimes rare conditions in which spraying conditions are optimal.
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.
Citations
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Downer, R.A., Wolf, T.M., Chapple, A.C., Hall, F.R., and Hazen, J.L. 1995. Characterizing the impact of drift management adjuvants on the dose transfer process. In: R.E. Gaskin (ed.) Fourth International Symposium on Adjuvants for Agrochemicals. New Zealand Forest Research Institute, Rotorua, NZ, pp. 138-143.
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Giles, D.K., and Comino, J.A. 1990. Droplet size and spray pattern characteristics of an electronic flow controller for spray nozzles. J. Agric. Engng. Res. 47:249-267.
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Hislop, E.C. 1993. Application technology for crop protection: an introduction. Pages 3-12 In: G.A. Matthews and E. C. Hislop (eds.) Application Technology for Crop Protection. CAB International, Wallingford, UK.
Hislop, E. C. 1987. Can we define and achieve optimum pesticide deposits? Aspects Appl. Biol. 14:153-172.
Knoche, M. 1994. Effect of droplet size and carrier volume on performance of foliage-applied herbicides. Crop Prot. 13:163-178.
Merritt, C.R. 1982. The influence of form of deposit on the phytotoxicity of difenzoquat applied as individual drops to Avena fatua. Ann. Appl. Biol. 101:517-525.
Nordbo, E. 1992. Effects of nozzle size, travel speed and air assistance on deposition on artificial vertical and horizontal targets in laboratory experiments. Crop Prot. 11:272-278.
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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.
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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.
Drift symptoms can take a few weeks to be discovered, and to figure out the cause, people need to reconstruct the conditions during the application in question. Wind direction is the easiest. But when we consider factors like inversions, volatility, calm conditions, and others used to explain the movement of pesticides, it can quickly become quite confusing.
Let’s review how and why pesticides move.
There are about six main ways that pesticides can move off-target.
Droplet drift at the time of application;
Vapour drift at or after the time of application;
Pesticide movement in water (precipitation or runoff) after application;
Dislodgeable residues from plant surfaces after application;
Pesticide-containing soil movement after application;
Pesticide residue in sprayers applied to another site.
Whenever we find pesticides in a place where they do not belong, usually first indicated by plant symptoms specific to that herbicide, we need to find out the possible reasons and take steps to prevent that from happening again. We’ll focus on the first two items from the above list because those two are the most common.
Droplet Drift: Sprayer nozzles produce droplet sizes ranging from 5 to 1000 µm, some up to 2500 µm. All nozzles, even the venerable low-drift tips recommended for dicamba application, will have a fraction of their volume in driftable droplets, say, less than 150 to 200 µm. For the low-drift sprays, that fraction is indeed very low, only a few percent of the total spray volume. For conventional nozzles, the driftable fraction may be 10 to 20% or more if high pressures are used.
Tiny droplets have no energy of their own and move with the air mass they’re released into. If it’s windy, they move downwind. If the air is turbulent, they move up and down. If the atmosphere is stable, the buoyant fraction stays aloft and concentrated. So in order to understand their movement, we need to understand the atmosphere.
Vapour Drift: Some chemicals are inherently volatile. This means they convert from the liquid or solid phase to a vapour phase on their own in accordance with temperature. Water is a great example, it is highly volatile. It is also able to sublimate, which means it can convert from a solid directly to a vapour without going through the liquid phase. An example of that is freezer burn, in which ice cubes shrink due to water escaping as a vapour.
Volatile pesticides can also sublimate. On landing on a leaf or soil, a significant portion of a droplet is absorbed or adsorbed. Some fraction may dry on the leaf surface. This remaining solid can volatilize (form a vapour) for hours or days after application. The rate of evaporation is driven by two factors, (a) the background vapour pressure of the substance in the atmosphere, and (b) the surface temperature of the object the chemical is resting on. For water, the atmospheric vapour pressure can be expressed as relative humidity. Droplets evaporate slower when the atmosphere is already full of water.
Pesticide evaporation is driven primarily by surface temperature. The background concentration of pesticide in the air is much lower than saturation, and has no effect. Pesticide evaporation is not directly affected by relative humidity because vapour pressures are independent of each other. In other words, most active ingredients will evaporate at the same rate whether the RH is 30% or 100% (it’s actually a bit more complicated than that. See the Note on Evaporation at the bottom of this article). This will be on the test, kids.
Vapour losses can be minimized by choosing low-volatile pesticides and also by making the application on cooler days. We also need to watch the forecast and avoid spraying when tomorrow or the day after is forecast to be hot.
Sometimes a rainfall can affect vapour losses, prompting a release of pesticide into the atmosphere. This behaviour can be predicted by the Henry’s Law Constant of a chemical.
Inversions: There are two types of turbulence, mechanical and thermal. Mechanical turbulence results from air encountering friction as it moves across a landscape. Taller objects and stronger winds result in greater mechanical turbulence. This turbulence creates small eddies that allow different layers of the atmosphere to communicate with each other and transfer momentum and contents up and down. More mechanical turbulence means more mixing and more sedimentation and dilution of a contaminant. In other words, the downwind impact of drift particles is reduced with greater mechanical turbulence. Mechanical turbulence happens whenever it’s windy, day or night, and it tends to counteract thermal turbulence.
Thermal turbulence is more powerful than mechanical turbulence for dispersion of pollutants. Driven by the solar heating of the earth’s surface, that causes the lower atmosphere to be much warmer than the air higher up. The atmosphere normally cools the higher you go (at about 1°C/100 m, called the dry adiabatic lapse rate), but when it’s sunny, the gradient is greater. In other words, it cools faster because the air at the ground is warmer.
Thermal effects move large parcels of air up and down, and we call this an unstable or a turbulent atmosphere. When parcels of air rise and fall great distances, we get a powerful diluting effect which is usually associated with a breeze but can also happen under calm conditions. An unstable atmosphere is great at dispersing drift, minimizing its downwind impact. This can only happen during the daytime, is most powerful when it’s sunny, and almost never happens at night.
By the way, a neutral atmosphere occurs when the rate of air cooling with height equals the adiabatic lapse rate described above. A neutral atmosphere can occur on cloudy days just before a rain, or on windy nights. There are no thermal effects in a neutral atmosphere, and the only dispersion occurs due to mechanical turbulence (windy conditions).
A stable atmosphere (inversion) happens when there is no solar heating of the soil. In other words, it can only happen when the sun is low in the sky or at night. In this case, soil cools off and the cold soil cools air near it. As a result, the air temperature rises with elevation. Since it’s normal for air to cool with elevation (at the dry adiabatic laps rate mentioned earlier), the temperature profile is now…inverted. Hence the name “inversion”. To be clear (write this down kids, it’s on the test), an inversion describes an atmospheric condition in which (potential) temperature rises with elevation. That’s it. It rarely happens during the day, but is common on clear calm nights. (btw, “potential temperature is the temperature adjusted by its normal rate of cooling with height. To have thermal effects, the rate of cooling needs to be different from this rate.)
The atmosphere is called stable because there is no thermal mixing. Air parcels stay put. Suspended particles such as tiny droplets stay put. Drift clouds stay concentrated, potent. If you make a fire, smoke hangs around. Cool, dense air is near the ground, and moves laterally very slowly, and might run downhill, like water, in a sloped setting. This situation is dangerous because it can move pesticide particles or vapours great distances without them becoming diluted or dispersed. An additional danger is that relative humidity is higher at night, delaying evaporation of water from the droplets. They stay potent.
In Summary: Pesticides move in the atmosphere and are rapidly diluted by mechanical, and especially thermal, turbulence. That is why we like to see spray applications on sunny days with a nice breeze, which moves the product in a predictable direction and dilutes any drift rapidly along the way. We minimize particle drift through the usual measures such as the use of low booms, protective shields, slow travel speeds, and coarser sprays. We avoid spray application of volatile pesticides on or preceding hot days to minimize the risk of vapour drift. We do not apply pesticides when the atmosphere is stable (inversion), which usually means from just before sunset to just after dawn on a clear night.
OK, that’s the basics. Now let’s explore some common questions.
Can all pesticides move as particles and vapours? All pesticides that are atomized through a nozzle can move as particles. Only pesticides that are considered “volatile” can form significant amounts of vapour and move in that form. Dicamba is volatile. New dicamba formulations such as Xtendimax, FeXapan, and Engenia are much less volatile than older formulations, but they’re still capable of moving as vapours. Glyphosate and many other pesticides are not considered volatile and are not known to cause vapour drift.
Are inversions only a problem for dicamba? Inversions affect droplet drift from all pesticides equally. The key difference is the amount of harm that any given droplet or vapour cloud can impart. Dicamba can harm conventional soybeans, many vegetable crops, and many trees and shrubs at extremely low doses. That means that even a weak inversion or a small amount of drift can cause great harm for long distances. In comparison, most other products are not as harmful to most of our crops in such small doses (I’m generalizing, forgive me). Tiny amounts may never be noticed, but they are there. Dicamba lets us notice these tiny amounts.
Does vapour drift move only by inversions? No, although its movement is more damaging under inversions. Vapour drift clouds form above a recently sprayed canopy on hot days when leaf or soil surfaces contain a volatile product. On a sunny day (no inversion), this vapour will likely disperse rapidly downwind, causing diminishing damage with increased distance in relation to the sensitivity of the non-target plant. But towards evening, the dispersion (caused by thermal turbulence) ends as the sun sets and the atmosphere becomes stable. Now, the residual vapour cloud above the crop is no longer diluted, and may move in an unpredictable direction based on the slope of the land or a very gentle evening breeze. This movement may be significant, extending for miles in some cases, and potentially causing harm along the way.
How long after application can vapour drift occur? Under most conditions, vapour losses diminish rapidly and will likely be gone within a few days as the pesticide is taken up by plants, metabolized, converted to a non-volatile form, etc. For some products, a light rainfall event can release a new wave of vapour drift because these products would rather be vapours than be dissolved in water, in accordance with their Henry’s Law Constant.
Do some products drift further than others? Yes and no, but mostly no. Spray drift is a physical process governed by the behaviour of droplets in the atmosphere. Droplet diameter determines its mass and this mass controls the time it takes the droplet to sediment to the ground. The substance dissolved or suspended in that droplet has no bearing on this behaviour. But there are two key exceptions to consider. First, we know that some formulations generate more fine droplets than others even when atomized through the same nozzles. The greater abundance of small droplets will create more drift damage at any given distance, and also extend further downwind. And secondly, some formulations change the rate of water evaporation from the droplets. As a result, droplets moving downwind may shrink faster, in effect making them more drift prone and causing them to move further downwind. The same droplet size drifts the same distance, but droplet size changes. Question for the final: If you spray dicamba and glyphosate on the same day using the same nozzle, and the formulation has no impact on droplet size or evaporation, which one drifts further over a soybean crop? The answer is at the bottom of this article.
Do calm conditions indicate an inversion? Inversions are defined as a temperature profile, not a wind condition. But the two are associated. An inversion is most pronounced and persists the longest under calm conditions, and because it suppresses atmospheric mixing, an inversion does prevent a windier upper atmosphere from reaching the ground. But it can be calm in the middle of the day with an unstable atmosphere. The calm condition eliminates mechanical turbulence, and therefore reduces the dispersion of the spray cloud. Calm conditions are also undesirable because the winds that follow a calm period are often unpredictable in direction, force, or duration. So it’s not a great idea to spray when it’s completely calm, even on a sunny day.
Can inversions occur during the day? Yes, but it’s rare. Sometimes a large cold air mass moves into an area, say from a cool body of water, pushing warm air above it. So technically the air at the ground is cooler than the air above it, suppressing dispersion through that cap. Another situation is an inversion layer that forms at the top of a transpiring plant canopy. The air at ground level is warm, and cools suddenly where the crop evaporates water from its leaves. Air temperature rises with elevation above this transpiring layer, then cools again in accordance with an expected profile. So we have a thin layer in which vertical mixing is suppressed. This is most common in dense, thick canopies with adequate soil moisture on hot days.
Is there an inversion every night? No. Cloud cover suppresses the rapid cooling of the soil, and the air at soil level stays warmer longer. Wind mixes the cold air layer into a warmer air layer, returning a more neutral condition. Inversions are most likely on clear nights with little wind. Recent data in inversion frequency from Missouri and North Dakota shows that inversions occur on the majority of nights, but the frequency depends on the location.
Can drift be eliminated? Yes, we can eliminate spray drift by atomizing the spray in droplets (or, for dry soil-active products on carrier particles) large enough to resist movement in wind. We would need to be sure that absolutely no fine droplets or particles are produced, and that they don’t dislodge after application. That will require different atomizers and significantly more water volume and possibly new adjuvants. Some will argue that drift can also be eliminated by protecting the fine droplets with shields or air assist, but again, the protection would need to be 100%. Drift control has not been a high enough priority for these technologies to be developed and made available to applicators. Vapour drift can be eliminated by not applying volatile products.
Pesticide movement in the atmosphere is complicated. But pesticides don’t just move as a result of vapour or droplet drift. Consider all the options when investigating an affected field. And let’s all work together to better understand pesticide movement and to prevent it.
Answer: Both drift equally. But assuming the beans are susceptible to both herbicides, the dicamba damage will appear further downwind due to the greater sensitivity of the beans to this herbicide. This does not mean it drifted further.
Note on Evaporation: There is some discussion about the role of relative humidity on vapour loss. Although we stated that RH plays no direct role in pesticide volatility, we need to qualify that.
(a) Many pesticides dissolve in water. More water moves to plant or soil surfaces during periods of low RH, and this can carry dissolved pesticide with it. The supply of pesticide that can evaporate is thereby replenished.
(b) Evaporation is driven by temperature and the concentration gradient between the source and the atmosphere. In still air, the air layers closest to the evaporating surface are most concentrated with evaporated pesticide, slowing further evaporation. Air movement will remove these layers, increasing the rate of evaporation.
(c) co-distillation may occur for some pesticides. This means that the pesticide dissolved in water may evaporate with water, liberating it into the atmosphere. When co-distillation occurs, low RH would increase pesticide losses as well.
We still have much to learn about these phenomena, especially as it affects new formulations.
There’s a call that I’ve been getting for 20 years now. It came again this week. Someone has a twincap with two small air-induced tips, and they’re applying herbicides and fungicides with low water volumes, often 5 gpa, sometimes less. They call because they want to know how much wind they can spray in. Is 30 km/h OK? They want my blessing.
I don’t need to hear much more. Some nozzles are sold entirely on the premise that they provide superior coverage – more droplets per square inch – and that this improved coverage permits the reduction of water volumes. Furthermore, the claim goes, when water is reduced, the spray concentration increases and the whole darn package just works a lot faster and better.
This line of thinking is as old as spraying itself. Applicators seek pesticide performance as well as productivity, and this approach gives them both. The proponents are well aware of their customers’ desires, and sell into it. “Use these tips and cut back on water. Any more than this just runs off anyways. You’ll get better coverage and better performance, get more spraying done.” It’s a convincing argument. Get an edge on your neighbour, the person who’s not in on the secret and is wasting time and water.
Why don’t I embrace it? There are a few reasons.
First, it doesn’t tell the whole story. Invariably it involves a twin nozzle setup. Use two nozzles, get more droplets, right? If that were true, believe me, I’d be advocating for quintuples.
Fact is that the only factors that change droplet numbers are droplet size (spray quality) and water volume. Want more droplets at the same water volume? Make the spray finer. Want to keep spray quality and add droplets? Add water (not nozzles).
The easiest way to improve coverage at the same volume is to use a finer nozzle, or to increase spray pressure. Depending on how far you go, you could make the spray finer and cut water, and still have more droplets per square inch.
The hardest way to improve coverage is to purchase a twincap and buy two nozzles, each of them half the size. True, within any given nozzle type, smaller sized tips usually generate finer sprays. But why bother with two tips? They’re more expensive and plug more.
If someone asks me how to improve coverage without changing water volume, I usually tell them to speed up a few mph. The rate controller will increase pressure and the spray gets finer. If speeding up is not possible, get one size smaller nozzle and run at higher pressure, same speed. Or keep nozzle and speed, and add some gpa, pressure will go up. It’s that easy. No twins necessary.
Second, the twin nozzle/low volume approach exaggerates the value of the twin nozzle for herbicides. With small plants and relatively open canopies in the early season, plus our high booms and travel speeds, the twin tips are not adding a lot, if anything at all, to coverage. It remains a sum of droplet size and water volume, the angle is not important at this stage. Deposit is by turbulence and wind, most of the time.
Third, low volume believers ignore a few potential problems. Drift is a big one. Low volume, fine spray operators are surrounded by nervous neighbours. They have fewer hours per day during which drift is acceptably low. And they definitely should not be on the field when wind is at 30 km/h. Basically, they’re a bit uncomfortable (at least they should be) and get less done per day.
Another potential problem is evaporation. Most sprays, even when applied at lower volumes, are still 90% or more water. The same volume of water evaporates much quicker when atomized into smaller droplets. This has two main downsides: On their way to the canopy, small droplets evaporate and become even more drift prone, and may not impact at all. Those that impact evaporate shortly thereafter. Research has shown that pesticide uptake is better from wet than dry deposits.
When Delta T (dry bulb minus wet bulb temperature) is high, evaporation can be so strong that it reduces pesticide performance or causes solvent burn. Fine sprays make it worse.
I also hear about the use of oily adjuvants to control evaporation from small droplets. This could be even more dangerous. Small droplets drift, and evaporation to dryness is actually helpful in reducing the impact of that drift. How? It makes the small droplets disappear, with their remnants dispersing into the turbulent atmosphere. With oily adjuvants, the small droplets stick around and stay potent and their drift damage is much worse.
Lastly, the practice is possibly off label. Water volume and spray quality label statements are designed to offer good performance and acceptable drift risk. While that part of the label is often a bit dated, it does provide better support from the manufacturer should something go wrong.
If you’re spraying under hot, dry and windy conditions, the low volume, fine spray approach is irresponsible. Use sufficient water (7 to 12 gpa) to allow low-drift sprays, at least Coarse to Very Coarse, in some case, even coarser.
Agronomists provide the best possible information for their clients, based on scientific evidence and experience and in accordance with their professional code of ethics. Sometimes the news we deliver aren’t what the customer wants to hear. But we have to represent the interests of all of us, collectively. I find that pretty important.
