On March 2, 2021, John Deere entered the optical spot spray (OSS) market with its first product, See & Spray Select™. This “Green on Brown” system identifies green material on a non-green background and is thus suited for pre-seed burnoff, chem fallow, or post harvest. It is competing for the same market space as Cropland’s WEEDit and Trimble’s WeedSeeker, but uses a slightly different approach.
At the heart of the See & Spray system is a relatively simple RGB camera that is mounted directly to the boom and looks about 1.5 m ahead. When this camera detects a spot of green colour, it assumes that this is a plant and activates a nozzle in line with that plant. John Deere says the weed size threshold is about ¼” (6 mm), and is evaluating its experimental data to identify exceptions to that rule of thumb.
See & Spray Select uses an RGB camera to detect weeds (Image courtesy John Deere)
In 2017, John Deere conducted a highly publicized acquisition of Blue River Technologies, a start up that pioneered artificial intelligence (AI) plant identification and coined the term “See & Spray”. However, the technology John Deere announced this time originated with the University of Southern Queensland near Toowoomba, Australia. The university’s Centre for Agricultural Engineering had received some initial seed financing from Sugar Research Australia, Cotton Research and Development Corporation, and Hort Innovation, and eventually partnered with John Deere. This is yet another example of the value of farmer investments in research.
Blue River contributed to this project but remains committed to its path of developing Green on Green OSS through machine learning. John Deere says this first product is part of an evolution of spraying with ever-increasing precision that will culminate in spot spraying weeds within a canopy.
The pixels in the See & Spray camera chip are mapped during its initial calibration, allowing the processor to know which nozzle to turn on. There are two user-selected modes. In “Single Nozzle” Mode, the system turns on as few nozzles as possible. If the weed is directly under a nozzle, just that nozzle is turned on. Should the weed be in between two nozzles, both will be turned on. In “Overlapping” Mode, a detection will turn on at least three, and up to six adjacent nozzles. This mode is intended for herbicides that contain specific nozzle recommendations on the label, such as dicamba. By fitting these tips on the spot spray location, the required overlap and subsequent coverage can be guaranteed to be compliant with that label, a unique feature of See & Spray.
The number of nozzles activated by a weed detection depends on the location of the weed relative to the nozzles, and the mode selected by the user (Image courtesy John Deere)
In all modes, the user can specify the distance before and after the detected plant that the nozzle will spray. This feature is useful when boom height varies or when travelling faster to provide extra assurance that the target will be covered by the spray. The boom height range for See & Spray is 26 to 47” (66 to 120 cm), and the maximum travel speed with nozzles pointed down is 12 mph.
Installation of a 40 degree angled adaptor allows sprays tom be emitted backwards, and increases the spray speeds to 16 mph due to the extra distance and time afforded the sensors andoin processors to make a decision.
See & Spray has a built in contingency for suboptimal conditions, for example when the boom falls outside its height range, or the nozzle speed (not tractor) exceeds the 12 or 16 mph maximum in a turn, or a light or sensor or processor fault occurs. Called “Fallback Mode”, the boom can be configured to shut off, or to go into broadcast mode (using the spot spray nozzles) at that time. These types of insurance are a necessary part of an OSS on the market today.
To prevent fallback mode from occuring unecessarily, operators often choose to reduce their tractor speed one or two mph to allow for yaw without triggering all the nozzles.
No OSS system is perfect. Tiny weeds, or those obscured from camera view by crop residue, may be missed. The contingency for WEEDit is “Combined Mode”, where the entire boom emits a broadcast spray at a user-determined fraction of the full dose, while still maintaining spot spray capability at the full dose when a detection occurs. The reduced dose is sufficient to control the smallest weeds, whereas the spot spray is emitted at the full label rate for the larger ones. This capability is made possible through Pulse-Width Modulation (PWM) control of each nozzle.
John Deere has developed a mode of its ExactApply system to create the same outcome. Called “A & B Mode”, the rear nozzle (B location on the ExactApply nozzle body) is being activated by See & Spray. The front nozzle (A location) can be asked to spray simultaneously over the entire boom width. By choosing a smaller nozzle, a fraction of the label rate can be applied as a broadcast while maintaining spot spray capability. The broadcast boom is pulse-width modulated and retains the swath control and turn compensation of ExactApply. This mode also makes it easier to ensure coverage of these smaller weeds by selecting a finer, wider (110 degree) angles spray on the broadcast boom, and retaining a coarser, narrower fan angle banding nozzle for the spot application. The spot spray does not use PWM, relying on conventional speed and pressure to ensure the correct rate.
If planning to use A & B Mode, a user would first need to decide if they will calculate the spot spray dosing on a single or a multiple activated nozzle system. If priorizing the single nozzle actiation, one would first determine the band width of that nozzle, and size the nozzle accordingly. The band width should be ar close to the nozzle spacing as possible to maximize savings. Say the sprayer has 15″ spacing, and the nozzle’s band width is 20″. Now, whenever multiple nozzles are activated, they would operate as a 15″ spacing and would over-apply 20/15 = 1.33, or 33%. Say you want to apply 15 gpa (you may need to boost the spot spray volume to allow you to cut that with the broadcast feature). You can do it with the band (and overdose when using multiple nozzles, or apply 15 gpa with the multiple nozzles, underdosing by 28% when a single nozzle is activated. Or split the difference.
The next step is to select the application rate of the broadcast. If you want to apply 30% of the spot spray rate using the broadcast nozzles, size these accrodingly to apply 5 gpa.
For band- and spot-sprays, the width of the spray pattern at the target height determines the dose, therefore careful selection is advised. A worksheet that shows boom heights at various fan angles and nozzle spacings is downloadable here. TeeJet and Hypro offer a selection of narrower flat fan tips, but none yet in a low-drift design. Other nozzles are in development. Agrotop has already developed a low-drift “Spot Fan”, and MagnoJet, a Brazilian ceramic nozzle supplier, has 30 and 40 degree low drift tips for sale. Wilger has develped the DX series ComboJet tips in 20, 40, and 60 degree fan angles, in a low drift (pre-orifice design that works with PWM.
The camera sensing threshold can be adjusted to optimize a specific target. For example, to specify a certain weed size, that weed can be held in view of the sensor and the user can adjust the sensitivity until the weed is properly detected. As with any higher sensitivity, this runs the risk of more false detections, resulting in over-application. But it gives the user some knowledge that an important weed stage is being targeted properly.
The See & Spray camera relies on ambient light conditions, and John Deere recommends it not be used within 30 minutes of dawn or dusk. Both WEEDit and WeedSeeker, in contrast, can operate under any light conditions.
One of the challenges of running a OSS boom is the unpredictable fluctuation in flow requirement, which can theoretically range from just a few nozzles spraying to the whole boom activated in less than one second. While this extreme example is rare, a sophisticated and fast-responding pressure-based flow capability is nonetheless required. WEEDit uses a Ramsay Valve into their units to handle this challenge, whereas John Deere is relying on its existing plumbing design.
As a factory install, the See & Spray is fully integrated into the Series 4 display and is tied into JD Link. As a result, it can generate a high resolution map that shows each spot spray activation, by nozzle. The agronomic utility of this capability is significant, as it provides a very high resolution plant density map. This capability is also inherent in WEEDit and most green on Green systems available..
See & Spray Select is a factory option and comes integrated into the 4600 series monitor (Image courtesy John Deere)
It’s no secret that I believe optical spot sprays represent the future of pesticide application (see here). And it’s great news to see John Deere enter the OSS area with a factory installed option. As an influential force in ag, it lends credence to the concept and will benefit all other companies vying for this space. As they say, a rising tide raises all ships.
Waste (noun): an act or instance of using or expending something to no purpose.
In agriculture, environment and economy are intertwined. Producers strive to obtain the maximum return on their inputs. They study incremental returns and avoid applying more inputs than necessary, especially if conditions don’t warrant it. The financial incentive is powerful, and waste is a four-letter word. This applies to seed, fertilizer, and pesticide. Pesticide labels identify the rate needed to obtain the desired result, and there is no incentive to over-apply. In fact, it’s illegal.
But there are plenty of other places where applications incur waste. As with time efficiency, it’s a good idea to identify where this waste occurs, and the only tool needed is a sharp pencil.
When might we incur waste in the spray application process?
Mixing more than we need because we don’t trust the flow meter or the tank gauge entirely, or don’t know the exact field size.
Priming the boom before the first swath.
Overlapping due to curvy terrain and coarse sectional control.
Spray drift away from the intended swath.
How big are the losses?
Let’s say we have a clean sprayer and need to spray 160 acres before moving to a new crop and product. We plan to apply 10 gallons per acre and have a 1,200 gallon tank with a 120 foot boom. That means we need 1,600 gallons of spray mix in total.
Once we’re at the field we prime the boom. Each sprayer is different, but depending on operator experience, 30 to 50 gallons are usually needed to push product from the tank to the last nozzle. Only part of that is lost to the ground, as boom sections can be shut off as soon as product has reached every nozzle of that section. We’re assuming 0.2 gallons per foot of boom is lost.
Spraying itself is relatively straightforward. Swath and sectional control handle the overlaps, but in less ideal terrain, double application is known to account for 4 to 5% of the area to reach non-square parts of the field. This is even more likely when the outer section is 10’ or more. Early turn-on of the boom prior to leaving the headland, to allow boom to reach operating pressure, adds to this.
Air-activated shutoff for individual nozzles reduces section size at a reasonable cost.
With an average nozzle, we can expect about 2% of the product to airborne drift. Most airborne won’t return to the ground within the field borders, so it’s a complete loss.
Most of the spray that travels more than 5 m after leaving boom stays airborne and should be considered a total loss from the field.
As we finish, the pump will draw air before the tank is empty due to sloshing or foaming, and a 50 to 60 gallon remainder may not be unusual. This simulation has assumed 5% of tank volume remains.
We also need to purge spray from the boom at cleanout, consuming approximately 0.4 gallons per foot of boom. This occurs after the field is completely sprayed and is therefore considered waste.
So how does this add up? The following table shows the approximate losses associated with five setups.
Table 1: Spray mix losses during a sprayer operation. Setup 1 = baseline, Setup 2 = low application volume, Setup 3 = baseline with recirculating boom, tank level monitor, and low-drift nozzles, Setup 4 = large area between cleaning, Setup 5 = large area with recirculating boom, tank level monitor, and low-drift nozzles.
In the first scenario, we spray just 160 acres at 10 gallons per acre. Priming the boom with 0.4 gallons per foot (allowing for all associated feed lines) consumes 48 gallons, but only wastes half of that, or 1.5% of the total volume needed for the field.
Four percent overlap consumes another 64 gallons.
If we have 5% of the tank volume left over, that’s 60 gallons. That amount is so small it doesn’t even register on the sight gauge but nonetheless it represents another 4% of the total sprayed amount.
Upon cleaning the boom, we need to push the spray mix out of all the plumbing after the pump, as it has nowhere else to go. At an assumed 0.4 gallons per foot, that’s another 48 gallons or 3%.
If we add to that a conservative 2% drift loss, it sums to a surprising 14% of the total spray volume. For those that use lower water volumes (the second scenario), the volumetric losses are slightly less, but their proportion is higher, now accounting for 23% (!) of the total spray mix.
In the third scenario, let’s assume we use a recirculating boom that returns the initial prime volume to the tank, eliminating any waste. We’ll also upgrade to individual nozzle sectional control, reducing overlap to 1%. And, since we want to know exactly what’s left in the tank, let’s invest in an AccuVolume system to precisely monitor tank volume. This allows us to make small rate adjustments up or down to be sure as much of the mixed product goes onto the sprayed swath as possible.
Recirculating booms allows the spray mix to pass through entire length of boom without being sprayed, saving waste during priming and allowing waste-free boom rinses.
When the sump begins to empty, we can introduce some water from the clean water tank to push the last of the mix to the boom (a continuous rinse system makes this easy).
An AccuVolume sensor shows the exact volume left in the tank at any slope position and with 1 gallon resolution, allowing greater accuracy when filling and emptying.
We’ll assume our sump waste is now reduced to 12 gallons. We still need to dispose of the content of the boom somehow, so the recirculating boom offers no saving there. But let’s also add better low-drift nozzles to reduce drift by 50% (now 1% total volume). Total loss is now just 6%.
Low-drift nozzles such as this AirMix (Agrotop) SoftDrop reduce airborne drift by 50 to 90%.
The last two rows in the table repeat the first and third scenarios for a larger sprayed area (1000 acres) before a tank cleaning is needed. This doesn’t change the magnitude of the volumetric loss, but reduces its proportion. Percent loss is down by a factor of two from the 160 acre interval, to 3 to 7%.
Experienced operators might cheat the system a bit by mixing the required pesticide with some extra water to make up for the plumbing waste. Doing so prevents extra pesticide from being consumed, but it doesn’t reduce the inherent inefficiency.
Lessons
This exercise suggests that waste from spraying is probably higher than we assumed. If we average the scenarios, there is 10 to 15% waste. At, say, $200,000 spent on pesticide for a single spraying season, that’s $20 – $30,000 worth of product and water hauled that ends up where it doesn’t belong. Beyond the time and money, there can also be environmental consequences depending on how that waste is treated.
Improve monitoring of tank content to allow lower remainders.
Consider individual nozzle shutoff to improve sectional control. These are part of Pulse Width Modulation (PWM) systems, but can also be achieved with less expensive valves.
Plan spray operations to minimize the amount of product changeovers.
Consider direct injection.
The return on investment for plumbing improvements can be high and result in considerable future savings over the life of the sprayer. It’s worth thinking about.
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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Bals, E. J. 1978. The reasons for C.D.A. (Controlled Drop Application). Proceedings of the 1978 British Crop Protection Conference – Weeds, pp 659-666.
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.
Edwards, C.J. and Ripper, W.E. 1953. Droplet size, rates of application and the avoidance of spray drift. Proceedings of the 1953 British Weed Control Conference, pp. 348-371.
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.
Hartley, G.S. and Brunskill, R.T. 1958. Reflection of water drops from surfaces. In: J. F. Danielli, K. G. A. Parkhurst, and A. C. Giddiford, eds., Surface Phenomena in Chemistry and Biology, Pergannon Press, London, pp. 214-223.
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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.
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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.
Some things have improved a lot. Others have lost ground.
Some years ago, a few of us weed scientists sat around a table and debated the most important developments in agriculture in our lifetimes. It was a great discussion, and we arrived at a few that included direct seeding (for its soil and moisture conservation as well as improved fertilizer placement), GMO crops (for slowing Group 1 and 2 herbicide resistance), and the abandonment of summer fallow in much of western Canada. Let’s apply this exercise to spray application to see what we come up with.
What follows are my version of the most important spray technology developments in the last 50 years.
Low-drift Nozzles. Spray drift is the biggest time management challenge and also perhaps the biggest public relations battle. These nozzles reduce drift, making more time available for spraying and doing it safely and effectively.
Rate Controllers. I both love and hate these things. On the one hand, a rate controller matches sprayer output to travel speed. On the other, it has allowed spray pressures to go wherever they need, even beyond the optimum, to match travel speed, and that can lead to nozzle performance issues.
Pulse Width Modulation. The pulsing nozzle fixes the rate controller problem mentioned above. Now, travel speed and pressure are independent. Plus, of course, a whole host of other flow management options, such as turn compensation and rate boosting, become available.
Optical Spot Spraying. Once you see these in action, you can’t go back. Why would you spray a whole field when weeds only cover 10% of it? Products like WEEDit and WeedSeeker are proven green-on-brown performers after years of field success around the world.
GPS Guidance. Some of us grew up with foam or disk markers, others learned to aim for brave family members perched on headlands. Achieving accuracy was stressful, overlap was insurance, and misses were common. The importance of this development is probably under-estimated.
Sectional Control. The ability to adjust the spray width in individual nozzle steps makes sense, and this can come with or without PWM. In fact, that alone can save 5% of an annual chemical bill compared to conventional sections measuring about 10 to 15 feet. And it’s definitely better than the left boom or right boom options from the 70 and 80s.
Operator Comfort and Safety. The refuge of the cab makes longer days bearable for all equipment, but for spraying it dramatically improves safety as well.
But we’re far from done. We still need work in these areas.
Cleaning and Waste Management. I can’t imagine another industry where managing potentially hazardous leftover materials are left to the discretion and circumstances of the applicator. Let’s make it easy and fast to thoroughly clean the sprayer and safely dispose of leftovers. Step 1 is smarter and simpler plumbing.
Boom Stability. Booms are too high, resulting in more drift and poorer nozzle performance, and adding to operator stress. The sole reason is unsatisfactory levelling. It’s possible to solve this, but it seems to not be a priority.
Weight. The road to productivity seems to be paved with larger, heavier machines. The side effects are fuel consumption, compaction, getting stuck. Let’s get smarter with frame design and logistics and talk acres/h rather than tank capacity and power.
Cost. All farm equipment has seen cost increases that far outstrip inflation or any reasonable accounting of productivity and features. Sprayers lead the way. Yes, it’s possible to spins this as a value proposition. But it shouldn’t be necessary.
Drift Management. Sprayer design continues to ignore drift management. We need sprayers that produce less drift by design, and this requires consideration of tractor unit, wheel, and boom aerodynamics. It’s more than a droplet size issue.
Direct Injection. Although very handy for single product application, the plethora of product formulations and mixes has limited the success of direct injection systems. The complexity of injecting at the nozzle, and the resulting lack of available systems, has stymied some very attractive options, such as site-specific rate or product use.
Ergonomics. If you need training, or to call someone before using your new sprayer for the first time, something’s wrong. Interfaces need to be intuitive and simple. The golden age of spray monitors was the 1980s. Those featured a main power toggle switch, a pump power switch, boom section switches, an agitation switch, and a simple way to enter the important information which was basically desired application volume. The screen can still be pretty, and you can still paint and monitor or tweak all the functions if you like that. But let’s at least have different tiers so beginners can also use the machine. Make interfaces using the philosophy Steve Jobs instilled in his trusted designer Jony Ive with the first iPod: no more than three clicks to achieve any desired outcome.
A few areas show promise and may suit certain niches.
In-Crop Weed Sensing. The green-on-green sensing that has been made possible by machine learning has shown some encouraging early success. Continuing improvements will eventually bring its reliability to within commercially acceptable standards. There is significant activity below the radar in this area, as all players recognize the enormous upside of a breakthrough.
Autonomy. While dispensing a pesticide adjacent to sensitive areas isn’t exactly the low-hanging fruit of autonomy, such field sprayers will have a fit in the temperate plains of North and South America, Australia, and Asia and may help solve the cost and weight problem.
Drone Application. The rapid pace of advancement in remotely piloted aerial systems, along with a seemingly low barrier to entry of new companies, will put pressure on the industry to make a decision on this alternate application method. If it can be done safely, it will have a dramatic impact.
If you want to improve your sprayer, don’t ignore the small things you can do in your operation. Although we’re conditioned to look for game-changing technology, the most sustained improvements don’t come from a single innovation, but from a period of persistent evolution. A lot of small improvements add up. Spray application is no different.
