Category: Boom Sprayers

Main category for sprayers with horizontal booms

  • Optical Spot Spraying and AI Scouting

    Optical Spot Spraying and AI Scouting

    Listen to the Audio article here

    Site-specific treatments have long been a goal in agriculture. It makes sense to provide inputs or treatments at rates that reflect the local situation. And to a large degree, those capabilities have been available for fertility and seed inputs for some time, with input zones reflecting soil types or topography.

    Typical prescription map for nutrients (Source: Field Crop News)

    But the sprayer world has not seen as much site-specific treatment. One reason is that pest maps are time-consuming to generate and their usefulness may be short-lived. Or perhaps weeds are fairly ubiquitous, and it usually makes sense to treat an entire field. Another reason could be that sprays are relatively inexpensive compared to fertilizer or seed.

    For spraying, we need to re-define site-specific.

    While traditional zone maps (corresponding to, say soil type and/or elevation or slope position) allow unique treatments on a scale of acres, new sensors have allowed sprayers to basically leapfrog this approach and treat each square foot uniquely. These sensors identify plants directly and create an immediate treatment response.

    Optical Spot Spray(OSS) principle (adapted from WEEDit)

    The idea, and technology, has and been around agriculture since the early 1990s, with the Concord DetectSpray and later the Trimble WeedSeeker. For various reasons, these two never became widespread in North America, although a significant market formed in Australia and New Zealand.  New cutting edge technologies are about to change this.

    Green on Brown

    Two main manufacturers have occupied the traditional Green on Brown Optical Spot Spraying (OSS) space, the Trimble WeedSeeker and WEEDit. Both have been available for over 10 years and are well established and proven reliable. WeedSeeker uses the Normalized Difference Vegetation Index (NDVI) principle to detect green on a non-green background. It employs one sensor per nozzle and the nozzle is either on-or off based on what the sensor detects. The WEEDit system is manufactured in the Netherlands by Rometron (https://www.weed-it.com/), and is widely adopted for use in Australia and South America. It is now making inroads into North America. The most recent version is named Quadro.

    WEEDit spray booms contain sensors placed at 1 m intervals. These scan the ground ahead of the boom, identify the presence of plants, and trigger the nozzle in line with the plant. The newest Quadro sensor contains four channels so that its resolution is actually 25 cm (10″) wide. The boom therefore contains a nozzle every 25 cm, and this nozzle has a correspondingly narrow fan angle that treats just this space.

    Hypro even spray (banding) nozzle with 30 degree fan angle. 30 and 40 degree nozzles are currently installed on WEEDit on 10″ spacing.
    30 degree fan achieves approximately 8″ to 10″ band at target height. Boom stability is important

    The detection principle is based on the quality of light that is reflected from living plant tissue compared to everything else. A red (older generation) or blue (newest generation, Quadro) light is emitted, and chlorophyll-containing plants reflect a unique wavelength that differentiates them from ground or dead plant material.

    Older generation WEEDit sensors were placed at 1 m intervals and had five channels, each covering a 20 cm band. There were 180 nozzles on a 36 m (120′) boom.

    The response time of the system is very fast. Triggered by small solenoids, a sprayer travel speed of up to 15 mph is possible when the sensor looks 1 m ahead. Furthermore, the software allows the user two important controls: first, the sprayed distance before and after a detected plant can be buffered between 5 and 20 cm, resulting in a sprayed patch between 10 and 40 cm long. This could be useful when boom heights fluctuate and placement of the sprayed patch shifts accordingly. Second, the user can select from among four sensitivity settings. Higher sensitivity can detect smaller weeds but will also result in more false results.

    WEEDit Quadro sensor

    One reason the system has been successful in the southern hemisphere is the long growing season that may require multiple spray passes outside of the crop each year, and in which the weeds are relatively large at treatment time and therefore easier to detect.

    Water sensitive paper can be used to show whether a target has been detected (and therefore sprayed).

    In North America, the pre-seed spray window is relatively narrow and weeds may be very small or just be emerging. The risk of a miss due to non-detection is therefore greater. Fortunately, the WEEDit system has a feature that addresses this risk.

    PWM valve for WEEDit, capable of instantaneous response at 10 to 50 Hz

    The solenoids that trigger an individual nozzle are pulse-width modulated (PWM). This means that the application rate is adjusted according to travel speed via a duty cycle. And it offers an innovative capability: The entire boom can be programmed to spray a defined fraction of the full dose, to a maximum of 50%, as a background broadcast rate (called “Dual Mode” or “Bias”). The smallest weeds that escape detection are likely to be susceptible to this lower dose. Larger weeds are then detected and sprayed with an individual spot spray at the full dose. Dual Mode is typically set to about 25%; overall savings are less, but control is improved for those very early season situations.

    A WEEDit Quadro boom can also be operated in “Cover Mode” for broadcast spraying where it functions as a full PWM system with turn compensation.

    Currently, several hundred WEEDit sprayers are operating in Australia, and they’ve been available in Canada and the US since 2017. in 2019, Croplands, an Australian sprayer manufacturer owned by Nufarm, started representing WEEDit in Canada. It is available as a retrofit on existing booms, and can be ordered with a WEEDit Millennium aluminum boom that contains mounting brackets and wiring harness channels. Savings compared to broadcast spraying range from 65 to 85%.

    In early 2021, John Deere announced its entry into the Green on Brown space with See & Spray Select™. This system is built around the ExactApply nozzle body and uses RGB cameras to differentiate green plants from non-green background colours. It will be in fields in 2022 according to John Deere. Similar RGB-based systems are in development by other manufacturers. Although their performance has not been compared side by side with WEEDit or WeedSeeker, initial specs suggest that the RGB systems are slower and are less able to detect small plants. Nonetheless, the future looks very promising.

    In 2021, Hardi Australia announced a new product, called GeoSelect. This system does not have boom-mounted sensors, and instead sprays according to a prescription map developed by a drone. The advantage of this system is that the amount of herbicide needed is known in advance of spraying, and the knowledge of weed distribution in the field can allow for a more efficient coverage plan to be used. This system allows for spraying under any light condition, and adjusts for boom sway to ensure accurate placement. Drone map development is the responsibility of the applicator.

    Green on Green

    Green on Green spraying, which detects weeds within a crop and differentiates them from that crop, is advancing and the earliest commercial releases are now available in Australia, offered by a partnership between Bilberry and Agrifac (WeedSmart podcast here), as well as Bilberry and Goldacres with Swarmfarm. Others, notably the SmartSprayer from Amazone in partnership with Xarvio and Bosch and Greeneye Technology are entering field testing with commercial sized units in 2021 and 2022, respectively.

    Opportunities for Optical Spot Spraying

    Taken as a whole, optical spot spraying offers a number of opportunities for weed management.

    Cost Savings: OSS has an appealing rate of return on investment. On a 5000 acre farm, a pre-seed treatment of glyphosate plus tank mix for resistance management may cost $10/acre, or $50,000 per year. At an average savings of 75%, that represents $37,500 per year. Add other non-crop uses, such as post-harvest, and savings increase. With eventual weed recognition in-crop, virtually all herbicide treatments are candidates for such savings.

    Herbicide Resistance Management: Delaying the onset of herbicide resistance requires the use of multiple effective modes of action in a tank mix. Cost is a deterrent to this practice. With OSS, these tank mixes become affordable.

    Efficiency: With 75% product savings, a tank of product will last longer. The time lost to hauling water and product, as well as filling the sprayer, will decrease. For example, WEEDit users are spraying a full day on a single load. Or they may choose to use a much smaller load, decreasing equipment weight.

    Pre- and Post-Harvest: Whether for desiccation or weed control, site-specificity of late season sprays can also be based on living tissue. Only regions in the field requiring the desiccant are treated. Perennial or late-season weeds are selectively controlled pre-harvest. Since herbicide rates in these applications are typically higher, savings are significant.

    High value crops: Row crops requiring multiple fungicide applications per season, such as potatoes, can benefit from OSS. Sprays applied prior to canopy closure can thus avoid gaps between plants, saving product.

    Producer Innovation: One user of the WEEDit system in Saskatchewan developed an innovative use. Having missed a pre-seed spray, the applicator was faced with large weeds in a 1-leaf RoundupReady canola crop. By turning down the sensitivity of the system so the canola crop did not trigger the sensors and turning on Dual Mode, he was able to broadcast spray the field at a low glyphosate dose (sufficient to control the small weeds) and then apply a full dose to the larger weeds, triggered by the sensor.

    Equipment Innovation: Since individual zones or weeds require unique doses or products, technologies like direct injection, remote nozzle switching, multiple smaller tanks and booms, and PWM will make more sense and grow. But the whole concept of detection and treatment can be moved away from pesticides to mechanical control or other techniques such as lasers, as does Carbon Robotics.

    License to Farm: OSS makes intuitive sense not only to applicators, but also to the public at large. Showing and using these technologies demonstrates stewardship practices that are easy to communicate and understand.

    Artificial Intelligence Scouting

    Another approach is pioneered by several companies, for example Dronewerkers in the Netherlands (https://www.dronewerkers.nl/english/) Taranis (http://www.taranis.ag/), and Xarvio (https://www.xarvio.com). These companies have developed plant recognition algorithms that are currently able to identify over 100 different species. Each species can be divided into several growth stages. Taranis has launched a business in North America that scouts fields by high-resolution drone imagery, and then provides customers with maps that highlight potential agronomic issues such as weeds, disease, or insect damage.

    Example of information available from artificial intelligence scouting. In this case, plant and foreign material information by species, relative abundance, and growth stage.

    Resolution of the output can be species-specific (lambsquarters vs redroot pigweed), or by coarser resolution (broadleaf vs grass). The resulting output then shows the plant density at each location.

    Weeds in a soybean crop (courtesy of Taranis)

    Xarvio Scouting is a product in their Field Manager line (https://www.xarvio.com/en-CA/Scouting). App-based, the agronomist or producer takes pictures of their crops and the app is able to recognize weeds, diseases, insect feeding damage, as well as nitrogen status. The app is aware of other users in the area and basically crowd-sources emerging agronomic issues as they arise, communicating them back to the user.

    The Xarvio Scouting app can identify certain weeds, diseases, and insect feeding damage from pictures taken while scouting (Screenshot from Xarvio.com).

    The agronomic value of this information is clearly very high. Imagine knowing the distribution of weeds by species before and after treatment. Although we can already assess this when we walk fields, by conducting the task via drone we are measuring on a wide scale, permitting an accurate quantification of the treatment effect so its value can be assessed. This level of measurement intensity was not possible before. Yield loss models for time of removal of certain weeds at certain growth stages can be applied across the entire field, and economic analyses allows follow-up treatments to be tailored to specific portions of the field.

    Green-Eye Technology artificial intelligence can differentiate these ragweed plants from the pea crop. (Courtesy Green Eye Technology).

    Or imagine following specific patches of weeds over time, to monitor the effectiveness of a certain cultural practice, or be alerted to the establishment of a resistant population while it’s still feasible to contain it.

    Heat maps can be generated to document weed patches, and perhaps monitor their size over time. (Courtesy Green Eye Technology).

    When this information is converted to a prescription map, rate and tank mix composition (or cultural controls) could be varied as necessary by zone, or weeds could, in the future, be sprayed individually. Perhaps future autonomous robots could be deployed more efficiently.

    Identification of plant symptoms in canola (Courtesy of Taranis)

    Development and improvement of these technologies is ongoing rapidly. Finally, we may have all the pieces that can bring site specific weed, disease, and insect management to market.

  • Ten Tips for Spraying in the Wind

    Ten Tips for Spraying in the Wind

    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.

    Beaufort Scale-1

    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.
    • 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.

  • Spraying Asparagus in Fern

    Spraying Asparagus in Fern

    This research was performed in 2012 and since then there have been considerable advances in application technology for asparagus in fern that should be considered. Be sure to read the epilogue at the end of this article.

    Introduction

    Diseases such as purple spot can have major economic impacts for asparagus growers, and the best line of defence is spraying the appropriate control products. The good news is that asparagus growers know this. The bad news is that there are few things harder to spray than asparagus in fern.

    Asparagus infected with purple spot.
    Asparagus infected with purple spot.

    Asparagus in fern can stand 1.5 m (5 ft) high by 1.0 m (3 ft) diameter and is typically planted on 1.2 m (4 ft) centres. Asparagus in fern has a very dense canopy full of needle-shaped leaves. This dense canopy slows air movement, making conditions still, humid and very difficult for a spray droplet to penetrate.

    Spraying asparagus in fern.
    Spraying asparagus in fern.

    Spray coverage is a combination of two factors: the area of the target contacted by spray droplets, and the distribution of spray droplets over that target. For most insecticide and fungicide applications, reasonable coverage is reflected by 10-15% surface area covered paired with an even distribution of approximately 85 medium sized droplets per square centimeter. This is not a rule, but a guideline.

    In order to determine the best way to spray, we have to be able to compare the coverage achieved. To do this, we used water sensitive paper, which is yellow until contact with spray turns it blue. Three sets of three targets were placed in approximately the same location for each pass.

    Water-sensitive paper arranged on stands, ready to be placed in the fern.
    Water sensitive paper arranged on stands, ready to be placed in the fern.
    Diagram defining where water-sensitive papers were located relative to the fern and the sprayer.
    Water sensitive paper orientation and location in asparagus canopy relative to sprayer direction.

    We tested five popular nozzle types, at two ground speeds using three carrier volumes to answer three questions:

    1. Does spray volume impact spray coverage?
    2. Which nozzle style gives the best coverage?
    3. Does travel speed impact spray coverage?

    Does spray volume impact spray coverage?

    Five different nozzle types were used to spray three volumes onto the targets at 16 kmh (10 mph). This was repeated three times and target coverage was determined both as droplet deposits per cm2 (see Figure 1) and total % covered (see Figure 2).

    Figure 1. Average deposits per cm^2 for five different nozzle types at 187 L/ha (20 US gpa), 234 L/ha (25 US gpa) and 280 L/ha (30 US gpa) at a ground speed of 16 kmh (10 mph).
    Figure 2. Combined average percent coverage for five different nozzle types at 187 L/ha (20 US gpa), 234 L/ha (25 US gpa) and 280 L/ha (30 US gpa) at a ground speed of 16 kmh (10 mph).

    Cards in each position consistently received a significantly higher average deposit per cm2 and significantly higher average percent coverage at higher spray volumes. The relatively low coverage in the middle position was anticipated given the orientation of the targets to the sprayer.

    Therefore, it would appear higher volumes result in better coverage, at least up to 280 L/ha (30 gpa). Generally, there is a threshold where exceeding a given carrier volume results in a diminishing return.

    Which nozzle gives the best coverage?

    Coverage from five different nozzles was compared: the Hollow cone, Flat fan, Dual flat fan, Guardian Air and Air-induced hollow cone. Given that 280 L/ha (30 gpa) resulted in the best coverage, the following figures illustrate droplet deposits per cm2 (see Figure 3) and total % covered (see Figure 4) at 280 L/ha (30 gpa).

    Figure 3. Average deposits per cm^2 for five different nozzle types at 280 L/ha (30 US gpa) and 16 kmh (10 mph).
    Figure 4. Average percent coverage for five different nozzle types at 280 L/ha (30 US gpa) and 16 kmh (10 mph).

    The graphs show that each nozzle followed a similar trend, with more droplets at the top of the canopy, less or par at the bottom of the canopy, and considerably less in the middle of the canopy (which is not surprising given the orientation of the target around the stem).

    The trend in droplet density from highest to least coverage is:

    1. Hollow Cone
    2. XR flat Fan
    3. Guardian Air
    4. Dual Flat Fan
    5. Air Induced Hollow Cone

    The percent coverage data was less clear. The top two nozzles for each position were:

    Top Target:

    1. Guardian Air
    2. All other nozzles approximately the same

    Middle Target (around the stem):

    1. XR flat Fan
    2. Hollow Cone

    Bottom Target:

    1. XR flat Fan
    2. Hollow Cone

    It can be argued that the target at the top of the canopy is easiest to spray, and therefore does not have as much importance as the middle and bottom targets. As such, it would appear that the XR flat fan and Hollow cone nozzles give the best overall coverage. It is debatable whether the higher droplet count from the Hollow cone is more important than the higher percent coverage of the XR flat fan.

    Does travel speed impact spray coverage?

    Hollow cone nozzles and XR flat fan nozzles were used to spray targets at two travel speeds and three volumes. Target coverage was determined both as droplet deposits per cm2 (see Figure 5) and total % covered (see Figure 6).

    Figure 5. Average deposits per cm^2 for Hollow cone and XR flat fan nozzles at 280 L/ha (30 US gpa) and either 8 kmh (5 mph) or 16 kmh (10 mph).
    Figure 6. Average percent coverage for Hollow cone and XR Flat fan nozzles at 280 L/ha (30 US gpa) and either 8 kmh (5 mph) or 16 kmh (10 mph).

    The variability in deposit density and percent coverage from medium/fine droplets created by the hollow cone nozzles make it difficult to determine statistical significance, but the trend suggests that higher ground speeds improve coverage in the middle and bottom of the canopy. This is likely due to the wake of the sprayer and the vortices created by its passage stirring fine droplets into the canopy.

    Overall recommendations

    The data suggest that coverage was improved when the sprayer travels at 16 kmh (10 mph) rather than 8 kmh (5 mph). Coverage was also improved at higher spray volumes, where 280 L/ha (30 US g/ac) provided the best overall coverage for all nozzles. As for the best nozzle, this depends on the application; the hollow cone created higher droplet densities than the XR flat fan, but the XR Flat fan created higher percent coverage. Higher droplet densities may be preferred when controlling disease with contact products, but spray drift becomes a significant concern. Higher percent coverage might be preferred with locally systemic products where complete coverage is less of a concern and preventing spray drift is a priority.

    Epilogue

    This work was performed in 2012. Since then there have been significant advances in sprayer design for spraying asparagus in fern. Dr. Torsten Balz (Bayer Application Technology Manager) kindly provided an example of such a sprayer (see below) and a video link to watch it in action. Drop arms that bring the nozzles closer to the target at all canopy depths are an ideal solution as long as the row spacing allows clearance without snagging the drops. Further, there have been developments regarding the use of hollow cones in an overhead broadcast application. Over- and under-laps in the hollow cone swath lead to double-dosing and gaps respectively that are referred to as “Technical Strip Disease”. Combined with considerable drift potential, hollow cones are not recommended.

    Air-assisted drop arms greatly improve coverage uniformity in asparagus in fern. Photo kindly provided by Dr. Torsten Balz.

    Special thanks to Max Underhill Farm Supply (Vienna, Ontario) for use of their sprayer and their assistance both spraying and placing water sensitive papers in the field. Thanks to Mr. Ken Wall of Sandy Shore Farms Ltd. (Port Burwell, Ontario) for providing the site and hosting the associated workshop, and thanks to TeeJet Technologies for their donation of parts and supplies.

  • Fundamentals of Spray Drift

    Fundamentals of Spray Drift

    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.

  • Methods for Applying Fungicides in Corn

    Methods for Applying Fungicides in Corn

    This work was performed with Albert Tenuta (OMAFRA) and David C. Hooker (University of Guelph, Ridgetown).

    Objective

    Gibberella ear rot is a significant disease that reduces the quality of grain corn, especially with the accumulation of mycotoxins (such as Deoxynivalenol (DON)) produced from the causal pathogen(s). Infection occurs through the corn silk channel when ideal temperatures (~27°C) and high humidity are present. Cool, wet conditions after pollination favour disease development and determine the degree of infection. With crop management practices providing only modest improvements in disease control, strategies to increase the efficacy of fungicides are important to investigate. Research has shown that the timely application of fungicide labelled to suppress the disease can reduce mycotoxins, but only by ~50%. We wondered if changes in the method of application could give better results.

    Gibberella ear rot

    It is reasonable to assume that improvements in spray deposit uniformity and increases in overall spray coverage (up to some threshold) at the infection channel (i.e. the silks) should result in improved efficacy. Water sensitive paper is an excellent tool for the qualitative evaluation of spray coverage. However, recognizing the complicated relationship between dose and coverage, we also looked at the deposition of copper sulphate as a surrogate for active ingredient .

    Our primary objective of this study was to compare various sprayer systems and nozzle configurations by evaluating both spray coverage and copper sulfate deposition at the silks.

    Experimental Design

    The test field of hybrid corn had a stand of ~80,000 plants/ha. It was located at Ontario’s U of G Ridgetown Campus and was managed similar to a grower’s field (e.g. fertility, etc.). In August of 2019 we evaluated nine sprayer rigs (or nozzle configurations) in a randomized block design.

    The ground rigs were calibrated to deliver a spray volume of 190 L/ha and the aerial systems to deliver 47 L/ha.  In order to achieve the target spray volume, the ground rig speed varied from 9.5 to 13 km/h, depending on nozzle configuration. The aerial applicators used the same nozzle configuration, travel speed and altitude as in their commercial field applications.

    SprayerNozzle SetNotes
    John DeereYield Center 360 UNDERCOVER drop pipes 75 cm (30″) spacing, each equipped with two Turbo TeeJet (TT) nozzles.Drop pipes were centred between corn rows with nozzles adjusted to spray ~horizontally and directly at the corn silks.
    John DeerePentair Hypro Guardian Air nozzles on 50 cm (20″) spacing.Boom positioned to create 100% spray overlap at tassel height.
    John DeereTurbo TeeJet Induction (TTI) nozzles on 50 cm (20″) spacing.Boom positioned to create 100% spray overlap at tassel height.
    John DeereTurbo TeeJet (TT) nozzles on 50 cm (20″) spacing.Boom positioned to create 100% spray overlap at tassel height.
    New Holland (front-mounted boom)Wilger 60 degree conventional flat fan nozzles on 40 cm (16″) spacing.Boom positioned to create 100% spray overlap at tassel height.
    New Holland (front-mounted boom)Wilger 60 degree conventional flat fan nozzles alternating with custom-made Wilger 40 degree conventional flat fan nozzles on 40 cm (16″) spacing.40 degree nozzles were positioned between corn rows (interrow) while 60 degree nozzles were positioned over the tassels.
    Hagie (front-mounted boom)Drop hoses terminating with TeeJet Duo Nozzle bodies equipped with Turbo TeeJet Induction (TTI) nozzles were alternated with TeeJet XR110 nozzles.Drop hoses were centred between corn rows but nozzles were not aimed directly at the corn silks (aimed down 45 degrees and spray parallel to ground rather than perpendicular). They alternated with the AI nozzles positioned over the tassels.
    HelicopterAir Induction TeeJet Turbo TwinJet (AITTJ) nozzles directed backwards.
    AirplaneCP-111T nozzle bodies with CP256-4015 40 degree flat fan tips on 15 cm (6″) spacing.Wingspan was 14.2 m with a 10.6 m boom width.

    The field was divided into four replicated blocks (REP 1-4 in the image below) which corresponded with a single pass of the sprayer. The sprayers alternated direction with each pass over the four blocks. Depending on the ground rig, a single pass through a block might include more than one set of nozzles. For example, in the image below, a John Deere sprayer carried a different nozzle set on each of four sections, leaving the centre boom section off. Therefore, each block was subdivided into four experimental units that corresponded with each nozzle set. Further, to account for variability, each experimental unit was further subdivided into five ranges. Four water sensitive papers (yellow rectangles) were oriented sensitive-side up and fastened to random corn plants directly on top of silks at each of the five intersections between range and treatment for a total 20 papers. This was replicated four times for a final count of 80 papers per treatment.

    The experimental unit covered by a nozzle set was four corn rows wide (~3 metres). Space was left between each boom section to provide a buffer and no nozzles were placed on the centre boom section. Four water sensitive papers (yellow rectangles) were fastened to random corn plants directly on top of silks at the intersection of each range and for a total 20 x 4 = 80 papers. The chevrons indicate sprayer direction.
    Test plots at University of Guelph, Ridgetown Campus

    Evaluating Coverage

    Each sprayer applied copper sulphate (Plant Products Inc., Leamington, ON) at 2 kg/ha as a chemical tracer. Agral 90 was added to the spray solution at 0.1% (v/v) to better emulate a typical fungicide application. After spraying, each water sensitive paper was allowed to dry, collected and then digitized using a DropScope (SprayX, Sao Carlos, Brazil). Droplet density and percent surface covered were evaluated within the detection limits of the equipment. Dose (represented by deposit volume) was more relevant to this study than percent surface covered, so a spread factor was used to convert area covered to volume. Once the papers were scanned they were subjected to flame emission spectroscopy (FES) (Actlabs – Activation Laboratories Ltd., Ancaster, ON) to determine the amount of copper deposited.

    DropScope digitizing water sensitive paper

    Results

    Deposit area and volume

    Note that papers were placed singly, oriented face-up. This was a missed opportunity to explore abaxial (down-facing) coverage and may have created a small experimental error wherein deposition from copper sulphate would be accounted for on both sides, but would only resolve on one side for area and density analysis. The results from evaluating water sensitive paper suggest trends and serve as quality checks for the experiment.

    The percent area covered on water sensitive papers was affected by nozzle configuration (P<0.0001). Ground rigs produced ~4.0-12.0% area coverage, while aerial produced ~0.7-1.0%. It is not appropriate to compare ground and aerial spraying using water sensitive paper. Water sensitive paper does not reliably resolve deposits under ~60 µm and therefore underestimates the deposits from aerial applications because their spray quality tends to be finer. Further, these figures have not been normalized to reflect the differences in sprayer volume (190 L/ha for ground versus 47 L/ha for aerial).

    The nozzle configurations with the highest percent area covered were produced by the 360 Undercover drop pipes and the TeeJet drop hoses (~9.5-12.0%). Coverage variability increased with percent area covered, but the lower 95% confidence limit with the pipes and hoses still exceed the upper limit of all overhead broadcast nozzles.

    Yield Center 360 UNDERCOVER drop pipes

    When area covered was converted to volume, estimated deposit volume on water sensitive papers was also affected by nozzle configuration (P<0.0001). The estimated volume calculated from deposit area showed fewer statistical differences across nozzle configurations compared to area data. However, once converted, there was no statistically significant difference in the volume deposited by drops or most broadcast methods.

    Copper deposition

    FES residue analysis (i.e. evaluating the amount of copper deposited on targets expressed as mass density) complements the water sensitive paper data. There are some differences that should be noted:

    • All applications sprayed the same amount of tracer per planted area. As such, depositions are more fairly compared with no need for normalization.
    • FES can resolve copper deposits as low as 0.5 µg/sample and may be more sensitive than the WSP method, which does not reliably resolve deposits under ~60 µm.
    • WSP will only resolve coverage on one surface. However, when these papers are subjected to FES, deposits on both sides of the paper will be accounted for, providing a more accurate result.

    As anticipated, there was no correlation between the area coverage or volume estimates and the FES-derived copper deposition data. Estimated copper mass density on water sensitive papers was affected by nozzle configuration (P<0.0001). Analysis showed 56% more copper deposited from the 360 Undercover nozzles (1.75 µg/cm2) compared to the next highest deposition (1.12 µg/cm2) which was from the drop hose configuration (P<0.05). We feel the TeeJet drop hose configuration would have performed better still had the nozzles been directed at the silks, and the alternating broadcast nozzles been omitted and flow redistributed to the nozzles on the drops (see below).

    Copper deposition from the airplane was similar to ground rigs with broadcast overhead nozzle configurations. The airplane deposited ~2x the copper as did the helicopter. It is assumed this is because the rotary atomizer nozzles on the airplane produced a much finer spray quality than the TTI nozzles on the helicopter. This increased the number of droplets considerably and has been shown to produce better coverage, particularly at such low sprayer volumes. Learn more about droplet size and behaviour here.

    Average copper deposition from the Guardian Air nozzle set was similar to all other ground sprayer overhead broadcast setups, but had the highest variability (Between 0.4 and 1.12 µg/cm2). Comparatively, the lower 95% limit of the 360 Undercover drop pipe deposited 3.4x the copper as the lower limit of the Guardian Air.

    Conclusions

    • The best deposition was produced from the Yield Center 360 Undercover drop pipes, followed closely by the TeeJet Duo nozzle body on drop hoses.
    • The deposition from ground sprayers with overhead broadcast nozzles was ~30% less than that of the two drop nozzle systems tested.
    • The deposition from Guardian Air and TTI nozzles were among the lowest of broadcast nozzle configurations with higher variability, but differences tended not to be statistically different (P=0.05) compared to other broadcast nozzles.
    • The deposition from the airplane was similar to the ground rig overhead broadcast applications, but the helicopter deposited the lowest amount of copper overall, likely due to droplet size (see image below).
    Helicopter with air induction TeeJet Turbo TwinJet (AITTJ) nozzles directed backwards.

    Next steps

    In the summer of 2022 we re-evaluated promising nozzle configurations from this study, as well as other application methods (see bulleted list below).

    • Include various RPAAS (remote piloted aerial application systems) designs.
    • Include the Agrotop Beluga drop hose (Greenleaf Technologies, Louisiana, USA) with two nozzle bodies to span the silking zone of the canopy.

    We used water sensitive paper as a qualitative indicator, but folded them to get adaxial and abaxial data. We also used copper deposition to indicate dose. Once the results are analyzed we’ll write a companion article to this one.

    In 2021 and 2022 a separate study was performed to evaluate the efficacy, ease-of-use and return on investment of the Beluga drop hoses in corn. An article describing that work can be found here.

    Thanks to the agrichemical companies, students, equipment owners and operators that donated their time and equipment to make this study possible.

    Bonus

    Watch these very cool slow-motion videos of the airplane and helicopter applications. Note that there is no difference in how the spray behaves once released; It deposits as a function of wind, gravity and momentum and is not “blown in” by the helicopter.