Choosing the right time to spray can be tricky. Our gut tells us that spraying when it’s windy is wrong. The experts tell us that spraying when it’s calm is wrong. So when can you actually spray?
I’ve always advised my clients to spray in some wind, because it has a few advantages. The main one is that wind helps disperse the spray upward and downward, diluting the spray cloud fairly rapidly. Another advantage is that winds tend to be reasonably steady in their direction and velocity (or at least that can be forecast), so downwind areas can be identified and potential impacts are known or predictable. It helps if it’s sunny, because that improves the dispersion of the cloud even more.
First, let’s define “windy”. The classic wind scale is the Beaufort Scale, intended for the sea, but also used on land. The upper limit for spraying is probably Force 3 or Force 4, with upper limits of 20 – 25 km/h or so. The Beaufort Scale calls these “Gentle or Moderate Breezes” (they had to save the alarming words for hurricanes), and the scale provides good visual clues such as what wind does to flags, leaves, or dust.
Spraying under breezy conditions can be done fairly safely if you follow specific steps. The idea is to understand what the risks are and to manage them.
The cornerstone is to use a low-drift spray and match it to a pesticide that will work well with larger droplets. But there are other important aspects to consider. Below are the top ten to think about:
Choose a herbicide that can handle large droplets. Glyphosate products are well suited to coarse droplets. But glyphosate commonly has contact actives in the mix, members of Group 6, 14, and 15, and these are less likely to perform well with big droplets than those that contain Group 2 and 4 mixes. Actives with soil activity also have more tolerance for larger droplets.
Use a low-drift nozzle and operate it so it produces a Coarse (C) to Very Coarse (VC) spray quality, as described by the manufacturer. Dicamba labels call for Extremely Coarse (XC) to Ultra-Coarse (UC) sprays, and Enlist requires at least Coarse. To achieve these you may need to purchase new nozzles. Low-pressure air-induced nozzles operated at about 50 – 60 psi will generally be very low-drift, but lower drift models are available. If you need a finer spray, produce it either by increasing the pressure or moving to a finer tip. Do this when the weather improves, for contact modes of action.
The name, symbol and range of droplet sizes used to describe the median droplet diameter produced by nozzles according to ASABE S572.3
Keep your boom low. Lowering the boom ranks as the second-most effective way to reduce drift, after coarser sprays. But there’s a limit. For low-drift sprays, you need at least 100% overlap (more for PWM), which is for the edge of one nozzle pattern to spray into the centre of the adjacent pattern. In other words, the spray pattern should be twice as wide as your nozzle spacing at target height. For most nozzles, a boom height of close to 20 inches is enough to achieve this overlap. That’s pretty low by current standards from suspended booms on self-propelled sprayers, so being too low for a good pattern will only happen due to boom sway.
Maintain reasonably slow travel speeds. These reduce the amount of fine droplets that hang behind the spray boom, reduce turbulence from sprayer wheels, and they also make low booms more practical. An added bonus is less dust generation.
Know what’s downwind and what harms it. Survey the fields on all sides of the parcel you’re treating. When you have a choice, avoid spraying fields that have sensitive areas downwind such as water, shelterbelts, pastures, people, etc. If you can’t avoid being upwind of these areas, make sure you check and obey the buffer zone restrictions on the label. These will also give you an idea if the product can cause harm in water or on land, or both.
Let the weather help you.
Take the wind from the side if you can. Going straight into the wind creates a lot of extra drift.
Consider a dicamba tip for special situations, even if you don’t use dicamba. If you’re in a situation where quitting and waiting is a poor option, these tips allow you to finish the job with minimal drift risk and with only slight reductions in product performance due to poor coverage.
Use a low-drift adjuvant. Specific products such as Interlock or Valid have been shown to reduce driftable fines (<150 microns) by between 40 – 60%, without adding significant volume in coarser droplets. The response will depend on the nozzle and the tank mix, but can be very noticeable.
Study drift and how it forms and moves. It’s about more than wind speed and droplet size. Knowledge in this area can help you work out the best strategies.
Invest in productivity. You may not need it every day, but on occasions when you have a small window to avoid bad weather, it pays dividends.
If you feel that drift is unavoidable and someone might be impacted by it, talk to those people first. It’s one of the most important things you can do.
Keeping pesticide sprays on target continues to be one of our top responsibilities.
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.
Bridgette Readel (@BMReadel) is back! This time singing that holiday classic “Larry the Low-Drift Nozzle”! Read the lyrics and then head to the bottom of the article to enjoy Bridgette’s rendition.
You know Flat Fan and Flood Jet and Pulse Width and Wilger, Hypro and Greenleaf and TeeJet and Lechler. But do you recall The most famous nozzle of all?
Larry the low-drift nozzle
Had a very “big-drop” spray
And if you ever saw it
You’d never see it drift away.
All of the other nozzles
Used to laugh and call him names
They never let poor Larry
Join them on their spray boom frames.
Then one windy summer’s eve
Farmer came to say
“Larry with your spray so coarse
Won’t you fight this gusty force?”
Then how the neighbours loved him As they shouted out with glee “Larry the low-drift nozzle Stewardship for chemistry!”
Variable rate spray application is receiving a lot or attention with our increased ability to farm according to prescription maps. For dry products such as seed or fertilizer, metering is relatively straight-forward and variable rate application has been possible for many years. However, liquid product application has been more complex and requires special approaches
Hydraulic Pressure and Flow Rate
In conventional liquid metering, the liquid is forced through a metering orifice that is placed in-line. This could be an orifice plate for liquid fertilizer, or a flat fan nozzle for pesticides. Rate control is achieved by altering the spray pressure. It is usually impractical to change the nozzle or metering orifice during an application.
The main drawback to this approach is that spray pressure is not very effective at changing flow rates due to the square root relationship between spray pressure and flow rate.
For example, with reference to the table below, one can see that doubling the spray pressure (say, from 30 to 60 psi) only increases the flow rate by 40%. Tripling the pressure (from 30 to 90 psi) increases the application volume by 73% (we can call that a factor of 1.73). As a result, the use of pressure alone doesn’t offer a large range of application rates, and we accept a factor of 2 to be the limit for fertilizer streamer and broadcast nozzles (meaning a four-fold pressure range) and a factor of 1.73 to be practical for broadcast pesticide sprays over a 3-fold pressure range. Any wider application volume range would require adjustment to travel speed.
With these inherent limitations in flow rate capacities from hydraulic pressure alone, applicators are often forced to use wide pressure fluctuations to achieve reasonable rate responses. In some cases, this means that pressure needs can be too low for uniform distribution, or too high for pump or plumbing capacities.
For Variable Rate application, we are less interested in travel speed range, and are more interested in flow rate range. The above chart can be used for both purposes. In the above example, rows under each application volume identify the travel speed range. These headings can be flipped, so the 10 gpa column (with mph values in it) can also be a 10 mph column (with gpa in it). the numbers don’t change. same is true for metric units, except the convenience of being in the same magnitude that makes the flip easy in US units is absent.
There are a few options available that expand the flow rate range of liquid products. A brief overview of the main options follows:
Greenleaf / Agrotop
TurboDrop Variable Rate (TDVR): This nozzle appears like the traditional TurboDrop family, but has an innovative dual orifice in its venturi. The first stage is always open, but the second orifice is held closed under spring pressure until a certain threshold is reached. This design achieves a 3-fold flow rate range between 40 and 140 psi. Below the 40 psi threshold, the spray pattern fan angle deteriorates quickly.
TurboDrop VR tip provides about 3-fold flow rate range at any given speed, but requires higher pressures.
TurboDrop Variable Rate Fertilizer (TDVFR): Because fertilizer streams do not need to atomize the spray or form a fan, the minimum pressure can be reduced, in this case to 10 psi. From 10 to 140 psi, this design offers a four- to five-fold range of flow rates. Three exits are offered, a streamer, a hose barb, and a quick connect.
Three variants of the variable rate fertilizer orifice are offered by Greenleaf.
VariTarget Nozzle
This nozzle design uses a spring-loaded plunger to exert force on a flexible nozzle cap, deflecting it slightly. The deflection changes the orifice size, allowing for a change in flow. As a result, the flow rate response to a pressure change is increased dramatically. A single VariTarget nozzle equipped with a blue or green nozzle cap can deliver flows ranging from 0.2 US gpm at 20 psi to 1.2 gpm at 65 psi, for a stunning 6-fold change in application rate (link).
The VariTarget nozzle body
The main drawback of this nozzle is the poor metering accuracy of the system. In calibration tests, flows from various new VariTarget nozzles operated at the same pressures varied by more than 10%. While this amount of variability may be acceptable in liquid fertilizer application, it is not considered acceptable for pesticide application. Tightening or loosening the threaded spring cap even a little changes the flow.
TeeJet Variable Rate Fertilizer Assemblies
These metering assemblies, introduced in 2016, offer an elastomer (EPDM) metering plate whose orifice diameter expands with pressure, offering a wider range of flows. There are no moving parts in the assembly. Four models are available (link).
PTC-VR: Using a push-to-connect design for planters and toolbars, it offers versions that accomodate 1/4″, 5/16”, and 3/8” OD tubing diameters
QJ-VR Hose Barb: This unit offers hose barb diameters for 1/4″ and 3/8” ID hose.
Both units feature a pressure range of 10 psi to 100 psi, within which a flow rate range of approximately 8-fold is possible.
SJ3-VR: This unit generates three streams and operates over a pressure range of 20 to 100 psi, offering a flow rate range of about 3-fold.
GPA ranges for specific travel speeds for TeeJet SJ3 VR
SJ7-VR: Generating seven streams and operating over a pressure range of 30 to 80 psi, this unit allows a flow rate range of about 2.9.
In all cases, the realized flow rate range is significantly greater than would have been achieved with pressure change alone. TeeJet has tested the flow rate variance among units operating at the same pressure and has found them to be acceptable, according to company representatives.
Fertilizer banding has greater tolerances for application because pattern width is less important, and also because stream stability is less affected by pressure than spray pattern droplet size.
Pulse Width Modulation (PWM)
PWM utilizes conventional plumbing: a single boom line and a single nozzle at each location. Liquid flow rate through each nozzle is managed via an intermittent, brief shutoff of the nozzle flow activated by an electric solenoid that replaces the spring-loaded check valve. Typical systems pulse at 10 or 15 Hz (the solenoid shuts off the nozzle 10 or 15 times per second), and the duration of the nozzle in the “on” position is called the duty cycle (DC) or pulse width.
100% DC means the nozzle is fully on, and 20% DC means the solenoid is open only 20% of the time, resulting in the nozzle flowing at approximately 20% of its capacity. This is illustrated in the figure below. The ability to control the duty cycle is referred to as pulse width modulation.
The system has a theoretical flow rate range of about four- to five-fold. Within this range, spray pressure, and the corresponding spray pattern and droplet size, stay roughly constant. This makes it ideal for variable rate pesticide application, where spray patterns and spray quality are critical for performance.
The main disadvantage of this system, compared to the variable orifice designs, is cost. Although highly accurate and dependable, commercial sprayer units are priced between $15,000 and $65,000 per sprayer, depending on features and boom widths. The available systems are Capstan PinPoint II and EVO (as a retrofit to any sprayer), Raven Hawkeye (retrofit to any sprayer, available as factory option on Case (AIM Command), New Holland (IntelliSpray) and most other brands, John Deere ExactApply, WEEDit Quadro, Agrifac StrictSprayPlus and TeeJet DynaJet (available as retrofit). See our in-depth article on PWM for more information on these systems.
For ammonia and liquid fertilizer planters or toolbars, Capstan offers three different PWM products, N-Ject NH3, N-Ject LF or EVO LF. These systems offer more control over PWM pulse frequency and duty cycle and can achieve 8-fold rate ranges.
Flow rate ranges for Capstan N-Ject LF, on 30″ spacing
At low frequencies and duty cycles, the mobiliy of the fertilizer in soil needs to be considered, as significant gaps in a stream can be generated.
A variable rate for liquid fertilizer system for seeders, together with sectional control and turn compensation, is offered by Capstan EVO-LF. This system can generate 10 to 60 gpa at 4.5 mph on 12″ spacing.
Dual Boom Systems
A second boom fitted with different flow nozzles is installed, and is activated when the flow rate requirements can no longer be met with a single set of nozzles. Once the second boom is activated, the spray pressure drops significantly and additional flow capacity can be realized.
Dual boom system
Dual or Quadruple Nozzle Bodies
A similar approach to the dual boom is available as selectable nozzles in the same body from Arag (Seletron), Hypro (Duo React), John Deere (ExactApply) Amazone (AmaSelect), and others. These systems utilize a single boom and direct the flow through one of any two (Duo React, ExactApply, Seletron) or four (Seletron, others) nozzles, or several nozzles at the same time.
AmaSelect utilizes a unique switching system that allows the user to select only Nozzle 4, Nozzle 3, Nozzles 3 & 4, and Nozzles 2 & 4, making the placement of certain sized nozzles critical.
Amazone AmaSelect nozzle switching system
Similar pressure fluctuations as with a dual boom would be experienced, requiring careful selection of nozzle flow rates to avoid large pressure jumps. The system can also be used to manually change from one nozzle to another as needed. In the figure below, the pressure changes associated with the sequential use of 015, 02, and 035 flows are shown.
Direct Injection
Direct injection is an option for variable application of pesticides. In this system, undiluted pesticide is placed into canisters on the sprayer, and plain water (or water plus adjuvant) is in the sprayer tank. The chemical is metered and introduced into the water on the pressure side at some distance upstream from the boom sections. The pesticide rate can be varied with the speed of the direct injection pump, offering a very high dynamic range of possible rates. For example, Raven’s Sidekick Pro (available as factory option on Case and John Deere sprayers, or as a retrofit to any sprayer) offers a 40-fold range of flow rates.
After injection, an in-line mixer ensures that products are evenly distributed in the carrier. The amount of lag in the systems will depend on the amount of spray mixture in the plumbing upstream of the nozzles, the total boom flow rate, as well as the boom section configuration. With a variable rate map this lag can can be anticipated and accommodated.
Pump technology has improved the metering accuracy over a range of viscosities. However, dry formulations remain a challenge as slurries can settle and create problems for the pump and screen components.
Summary
High dynamic flow rate ranges for agricultural sprays are challenging to achieve, but will become more important as interest in site-specific management increases. Relatively inexpensive solutions are available for liquid fertilizer, whereas pesticide sprays require greater investments in technology to preserve spray pattern integrity. As mapping sophistication continues to grow, these application technologies will be integral to variable input prescriptions.
Some years ago, a friend recommended that I read The Tipping Point by Malcolm Gladwell. In this book, Gladwell tries to understand why some things catch on, and others don’t. It’s a compelling read full of Gladwell’s trademark stories and his knack to deftly interpret scientific studies. He talks of connectors, mavens, and salesmen, as well as the “stickiness factor”, a measure of how memorable something is, as keys to success of products and ideas. I think of the book often as I ponder the many good ideas in agriculture, many of which never see widespread adoption.
One of these good ideas is spot spraying. Green-on-brown detection was first introduced in the early 1990s. Anyone remember the Concord DetectSpray? It was great but had bad timing, as resistance wasn’t a big issue and glyphosate prices were about to slide. Green-on-brown grew to the NTech (later Trimble) WeedSeeker a few years later. Rometron’s WEEDit built on Trimble’s success and found widespread adoption in Australia in the past ten years. Spot spraying did not gain any traction in North America during this time.
Australia is unique in many ways, not the least of which is their summer spraying practice. Summer is the hot, dry season where land is typically fallow and weeds are kept in check with herbicide sprays (aaaah, the serenity). Making several passes over a field, combined with the need to control some larger and hardy plants, is expensive, and a spot spray saves much of the cost. The savings can be put to use with more effective herbicide tank mixes that delay the onset of herbicide resistance. Spot sprays pay for themselves in short order Down Under.
It’s more of a challenge in the northern plains of North America, where the fallow season involves snow cover and burnoff occurs in a short window before seeding and sometimes after harvest. But nonetheless, spot sprays have a fit for many of the same reasons.
WEEDit is the first system to make serious inroads in North America, with several dozen systems having been retrofitted to high-clearance sprayers. High detection accuracy and hardware reliability is proven in three seasons.
On March 2, 2021, John Deere entered the Green-on-brown spot spray area with See & Spray Select. This not to be mistaken as competition. Instead, the entry of a major brand provides validation of the concept like only a large manufacturer can. Yes, we’ve reached a tipping point.
While the first Green-on-brown units are becoming established, Green-on-green, the ability to detect weeds within a crop, continues to be developed around the world. French startup Bilberry has made enough gains in Australia to bring its product to market with Agrifac, where it’s called AIC Plus. In farmer field trials, they have achieved 90 per cent detection accuracy of wild radish in Western Australia, and claim that they are ready for broadleaf weed identification in wheat, barley and oats. Bilberry’s technology will also be seen on Australia’s Goldacres and France’s Berthoud. Other startups, notably Israel’s Greeneye Technology, plan to introduce a Green-on-green system in the U.S. in the near future. Amazone, the German farm equipment giant, partnering with Xarvio and Bosch, announced plans at Agritechnica to have a commercial unit for sale by 2021.
This technology will have significant impact on sprayer design philosophy. At present, productivity is synonymous with capacity, and large tanks with commensurate heavy and powerful tractor units dominate. Spot spraying savings will depend on weed density and hardware resolution, but 50 per cent to 90 per cent reductions in spray volume can be expected. A 1,600-gallon tank would no longer be necessary. The savings in frame weight and horsepower would be significant, as would the time savings from less intense tendering demands. These savings would offset the lower driving speeds that accompany sensing technologies, and, overall, provide a lower bar for autonomous operation. We may see lighter specialty spot sprayers.
The savings in brute size will be countered by increased sophistication. Better boom height management is essential for spot spraying, not just for the sensor to properly see the target and estimate the time needed for the boom to reach that spot, but also for the spot spray itself to deliver the right dose. In any fan spray, band width at ground level changes with height, and that, of course, is related to dose. Trailed booms can address this issue easily.
But not everyone wants a specialty spot sprayer that would require an extra pass over the field. With growing utility of soil residual herbicides, dual tank sprayers—small tank for the spot spray, large tank for the broadcast residual—make sense. Large sprayer frames can accommodate an additional smaller tank, second pump, and plumbed boom easily.
Plant detection and identification bring other opportunities. Adjusting dose for plant size is one of the first, or for harder to control weed species.
Spot sprays rely on fast, precise response of the nozzle, and this provided by fast-reacting solenoids that are part of pulse-width modulation (PWM) systems. On a broadcast sprayer, these solenoids can change the emitted dose instantly, within a certain envelope, by altering the duty cycle of the pulse. This, however, works best in the context of a boom with overlapping spray patterns. A single band spray would not change dose with duty cycle as easily.
Higher dosing would be an opportunity for multiple nozzle bodies that are able to spray one, two or more nozzles in the same spot simultaneously. These are already widely available and popular in Europe.
This also brings direct injection into play. Current systems introduce the active ingredient into the boom upstream of the nozzles, affording it time to mix into the water. For true spot spray utility, though, direct injection ought to be at the nozzle. Only then can custom mixes and rates be applied on a spot basis. It’s been done before, if only to show how difficult it would be to deliver uniform doses to a spot spray machine.
Spot spray sensors have agronomic benefits. By recording the location sprayed, weed patches can be mapped. As plant identification becomes possible, it’s conceivable to obtain plant species and stage distribution maps from the spray pass That would turn the sprayer into a high-resolution crop scouting tool. As machine learning and sensor sophistication grows, other plant and soil parameters can be mapped. The agronomic value of such maps, especially if created over the course of the growing season, is immense. Of course, data density, handling, storage, and analysis will constrain this.
If the past has taught us anything, it’s that there seems to be a appetite for investment in farm equipment. Sprayers have been the most-used implement on the farm for some time, and their popularity continues despite sharp price increases. These new capabilities will only add value to these implements. Prepare for sticker shock, followed by acceptance and adoption.
What will a future spot sprayer look like? Although it will have tanks and booms, the level of electronic sophistication will make it so much more versatile we can’t yet imagine all the ways in which it might be used. But it seems to me the situation has tipped and we’re already accelerating toward that future.
