Tag: airblast

  • How to Calibrate an Airblast Sprayer Operator

    How to Calibrate an Airblast Sprayer Operator

    Checking coverage on water-sensitive paper with some of the Grape Growers of Ontario members in 2012
    Checking coverage on water-sensitive paper with some of the Grape Growers of Ontario members in 2012
    Press play to hear the audio version of this article.

    When an extension specialist, equipment retailer or consultant is asked to calibrate an airblast sprayer, they would be well advised to calibrate the sprayer operator as well.

    Consider this: you and the operator are each investing three hours (average) to optimize the sprayer for a specific set of circumstances: the crop dimensions, density, and the weather conditions at the time of calibration. Depending on the reason for the application, you may even account for the product(s) mode of action and the pest location. This means that once you leave, the circumstances will change and the benefits of your efforts will quickly diminish.

    Calibrations, like milk, have an expiry date.

    There are three possible outcomes from a single, stand-alone calibration:

    1. The operator manages efficacious applications throughout the season because the variability in weather, crop and pest isn’t significant. This is generally not the case.
    2. Not recognizing that sprayer settings need constant adjustments (or being unable to make the changes) the operator experiences only modest results and decides calibration isn’t worthwhile.
    3. The operator experiences failures and lays the fault with you (as the last person the touch the sprayer) and/or the agrichemical rep that sold the chemical. Few sprayer operators blame timing or spray coverage.
    Explaining how to place water-sensitive paper and ribbons in an apple tree
    Explaining how to place water-sensitive paper and ribbons in an apple tree

    The solution lies in the proverb “Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime.” It is the sprayer calibrator’s responsibility to involve the sprayer operator and ensure they understand what is being done, why it is being done, and how to do it when you leave. Otherwise, expect to calibrate that sprayer again… soon.

    Personally, I have had the most success educating and empowering sprayer operators to make their own seasonal adjustments based on a formulaic approach. Depending what you are trying to accomplish, you may not need all of the following steps, or you may perform some on your own and others as part of the education:

    1) You could be working one-on-one, or you may be presenting to a large group. When it’s the latter, I like to arrive the day before to meet the host or owner of the sprayer(s). You can scope out the operation and triage the equipment so you know what parts you might need the next day. It also helps to see the space you will be working in.

    2) Perform a pre-calibration inspection of the equipment with the sprayer operator. They know their equipment and can tell you about usage, history and maintenance. It also opens a dialogue between you and helps the operator to relax. Remember: from their perspective they may feel they are being judged and they will take criticisms and corrections personally. Do your best to reassure them that you are trying to make a good thing better – not to correct failings.

    3) If you’re working at a large operation, educate the manager (decision maker) and the operators (drivers) at the same time. If you teach the manager, they might not effectively communicate the lessons to their operators. Likewise, if you teach the operators, they may not be able to convince the manager to let them spend money, or time, on making changes to the sprayer program. Get everyone on the same page, at the same time.

    4) With the operator, perform a basic maintenance check. Specifically, confirm sprayer ground speed, evaluate pressure gauge accuracy and evaluate nozzles. Explain what you are doing, and ask the operator questions. This is where you learn about their attitude. Are they open-minded about changing how they do things? How has their efficacy been in the past? Will they spring for new parts? Do they need convincing that this process must be repeated regularly?

    Demonstrating how deflectors aim air, and spray, into the target using some scrap wood.
    Demonstrating how deflectors aim air, and spray, into the target using some scrap wood.

    5) With the sprayer in the crop, have the operator tie wind-indicator ribbons in the canopy (or better, use lengths pre-tied to springback clips). Explain what they are doing and why. Tell them these ribbons should be monitored, maintained and replaced season-long.

    Here’s a tip: If you are working with a large audience, keeping them focused is critical. Growers will take the opportunity to catch up with each other while you are occupied with the sprayer. They are also inclined to wander away to answer cell phones. If they are not focused, you are on a service call and are not really educating. If you feel you are losing control, single out the ringleaders or wayward students and give them jobs, such as holding tools, or placing/removing water sensitive papers. When they have a responsibility, they pay closer attention.

    A convenient, weather proof calibration kit for flagging tape, clips and water sensitive paper.

    6) Discuss where water-sensitive papers should go, and how they should face. Give the operator a latex glove and after you write on the back of each card (position and trial number) have them clip them in place. Tell them how much they cost, where to buy them and the benefits of using them regularly.

    7) Have the operator spray the target crop using their typical set-up (i.e. ground speed, pressure, rate, air settings, etc.) Have attendees and the operator watch the ribbons as the sprayer passes. Spray from one side with both booms on and then stop to discuss results. Then spray from the other side and explore the cumulative impact.

    8) The operator will be very surprised to learn they have drenched or missed the papers. They may or may not be surprised to have seen the ribbons stood straight out (indicating too much air). If you like, you can even set up papers in the next alley (or alleys) to show how much spray blew through the target. When the papers are dry enough, collect them and store them somewhere safe for later comparison. They tend to blow away, so stick them to a whiteboard with two-sided tape, or clip them there with paperclips. Explain that they can (potentially) save a lot of money and lost fill-time by improving their efficiency. Get them on-board for the big change to come.

    9) Optimize sprayer ground speed, air direction (i.e. deflectors) and air speed/volume (i.e. fan speed). Then re-nozzle the sprayer using brass disc and core tips to reduce output in areas that were drenched or increase output in areas of sparse coverage. Quite often, I turn off the lowest (and sometimes, highest) nozzle positions. A piece of water-sensitive paper at the top and bottom of the canopy will confirm the wisdom in this. Label a new set of papers and have the growers position them in the same locations. Spray again. This entire process should take about 1/2 an hour and is described in detail in the Airblast101 handbook.

    Tying flagging tape in trees to indicate prevailing wind and to calibrate airblast air settings.
    Tying flagging tape in trees to indicate prevailing wind and to calibrate airblast air settings.

    10) The goal is 85 medium droplets per square centimetre and 10-15% coverage on 80% of the target surfaces for most insecticides and fungicides. If there are still drenches or misses, or if you’ve gone too far in a few positions, correct them and try once more. This is iterative. Make sure the sprayer operator will not be spraying in particularly hot or windy conditions, or your calibration at the top of the target can be compromised. Once you are both satisfied, work out the new sprayer output per area (e.g. US gpa or L/ha). You will have to discuss whether the operator plans to concentrate the tank mix to maintain the labelled “per area” rate (not recommended by me) or will continue to mix the tank as always and simply drive further on it (recommended by me). The later is called “Crop-Adapted Spraying“. Don’t push because it’s their livelihood, and therefore their choice.

    11) The final step relies on how well you’ve earned the sprayer operator’s trust throughout this process. Once you have an output and spray distribution that you are both happy with, the operator should invest in molded ceramic tips that emit similar rates to replace the brass disc-core. Then, they must be willing to repeat the process on any crops that are significantly different to ensure they have the right settings. Sometimes only modest changes are required between blocks. Perhaps they will dedicate certain sprayers to certain blocks to reduce the number of changes required. In either case, they will have to revisit these settings as the season progresses to compensate for denser and/or larger canopies.

    A few examples

    The following figures illustrate three airblast calibrations in Ontario apple orchards from spring 2014. Some required one attempt; others required a few trial settings before we achieved reasonable coverage. In all three cases, the sprayer operators reduced per-area rates, bought new nozzles and planned to buy water-sensitive paper. Further, they indicated they would continue to monitor ribbons (as long as they could be seen) and would review coverage after petal-fall.

    Several nozzles shut off, spray re-distributed. Targets still drenched in two locations with a 24% savings in spray mix.
    Several nozzles shut off, spray re-distributed. Targets still drenched in two locations with a 24% savings in spray mix.
    Three successive re-calibrations were required. Output was reduced in the first trial, but poor coverage in position 3. Top nozzles turned off and spray re-distributed in trial 2, but a gust of wind reduced coverage at the top of the tree. Bottom nozzles turned off and spray redistributed to top nozzles for a 40% savings in spray mix.
    Three successive re-calibrations were required. Output was reduced in the first trial, but poor coverage in position 3. Top nozzles turned off and spray re-distributed in trial 2, but a gust of wind reduced coverage at the top of the tree. Bottom nozzles turned off and spray redistributed to top nozzles for a 40% savings in spray mix.
    Output reduced in all nozzle positions and sprayer fan speed reduced. The high humidity greatly reduced droplet evaporation and increased the spread on the papers. In this case, it was decided not to reduce output any further to account for anticipated growth and the high humidity. There was a 27% savings in spray mix.
    Output reduced in all nozzle positions and sprayer fan speed reduced. The high humidity greatly reduced droplet evaporation and increased the spread on the papers. In this case, it was decided not to reduce output any further to account for anticipated growth and the high humidity. There was a 27% savings in spray mix.

    Conclusion

    So, the next time you calibrate an airblast sprayer, be sure to teach the sprayer operator (and audience) what you are doing and why. Involve and engage them. Answer their questions. Encourage them to perform the same calibration for each significantly different block and make mid-season changes. With luck they will only call back to report success and savings, and not to condemn your efforts, or worse: to ask you to re-calibrate their sprayer!

  • How Airblast Spray Droplets Behave (or Misbehave)

    How Airblast Spray Droplets Behave (or Misbehave)

    Listen to article here.

    Some pesticide labels require or prohibit certain droplet sizes to reduce the potential for drift. But, even when labels are silent about size restrictions, operators should be aware of the potential for droplet size to affect coverage. In the case of airblast, droplets should be:

    • large enough to survive evaporation between nozzle and target.
    • small enough to adhere without drifting off course.
    • plentiful enough to provide uniform coverage without compromising productivity (e.g. affecting refills and travel speed).

    Once spray leaves the nozzle, the operator has no more control over the application, so it’s important to plan for as many contributing factors as possible. Deciding which nozzles to use (and yes, you have alternatives beyond disc-core), requires an understanding spray quality symbols and basic droplet behaviour.

    Spray Quality

    Droplet diameter is measured in microns (µm). For a given pressure, a nozzle creates a range of droplet sizes which are described by the American Society of Agricultural and Biological Engineers (ASABE) standard S572.3 (Feb. 2020) In North America, these spray quality ratings range from “Extremely Fine – XF” to “Ultra Coarse – UC”. For interest, the scale is based on the British Crop Protection Council (BCPC) system, which is slightly different.

    To make sense of the spray quality rating, we must first understand that not every droplet produced by a hydraulic nozzle is the same size. We noted that a single nozzle produces a range of droplet sizes. Spray quality captures that span using a few key metrics. The first is the Volume Median Diameter (VMD) or DV0.5. Think of it this way: Let’s say you have a hollow cone nozzle that breaks a volume of liquid up into droplets. Let’s arrange them from finest to coarsest as in the following graph.

    The DV0.5 refers to the droplet size where half the spray volume is comprised droplets smaller than the DV0.5, and the other half is comprised of larger droplets. But we need more to understand the variation in the population. In other words, are they all the same size, or do they vary a great deal?

    That’s why we also assign a DV0.1 which tells us the droplet size where 10% of the spray volume is comprised of smaller droplets, and a DV0.9 which indicates that 10% of the spray volume is comprised of larger droplets. Let’s add them to the graph:

    With all three numbers, we can calculate the Relative Span (RS) by subtracting the DV0.1 from the DV0.9 and dividing by the DV0.5. The smaller the resulting number, the less variation there is in the spray quality. Two nozzles might produce a range of droplets with the same DV0.5, but the one with the larger RS is more variable, and is more likely to drift. Since we don’t typically have access to the RS of each nozzle, we rely on the spray quality symbols in nozzle catalogues to alert us to potential drift issues.

    Relative Droplet Size

    Did you notice in the graph that there are a lot of Fine droplets compared to Coarse?  Disc-core (or disc-whirl) nozzles do not have spray quality ratings, and moulded hollow cones may or may not. This is, in part, because the standard was developed for flat fan nozzles, but mostly it arises from the nature of airblast spraying. No matter the original droplet diameter, the air shear from the sprayer and the distance-to-target reduce the DV0.5 considerably by the time spray reaches the target. It is safe to assume that the final spray quality will be much finer than the nozzle’s rating.

    Incidentally, this is a big difference between boom sprayers and airblast: Where the boom sprayer operator should be aware of how pressure affects droplet size, it’s of little consequence to an airblast operator. On an airblast sprayer, pressure really only affects nozzle rate.

    So, while shear and evaporation raise drift potential, shear also increases droplet count. Imagine the volume a nozzle emits as a cake. No matter how many slices you cut the cake into, you still have the same amount of cake. The finer the slices, the more people can have a slice, albeit not very much. Similarly, a single Coarse droplet can contain the same volume as many finer droplets. Mathematically, a droplet with diameter X represents the same volume as eight droplets with diameters of 1/2X. See the illustration below:

    The one to eight rule: Every time the median diameter of spray is doubled, there are eight times fewer droplets. Conversely, every time the median diameter of spray is halved, there are eight times more.
    The eight to one rule: Every time the diameter of a droplet spray is doubled, there are eight times fewer droplets. Conversely, every time the diameter of a droplet is halved, there are eight times more.

    Droplet Behaviour

    The droplets that comprise the spray behave differently from one another. Finer droplets have a low settling velocity, which means they take a long time to fall out of the air. Conversely, coarser droplets fall out of the air more quickly. Think of how a ping pong ball (the finer droplet) has much less mass than a golf ball (the coarser droplet). When thrown into the wind, the golf ball follows a simple trajectory before falling. The ping-pong ball behaves erratically, like a soap bubble. Wind, thermals, humidity and many other factors will change where it goes because it is too light to resist them. It may even land behind the thrower, blown by the prevailing wind.

    It is because of the behaviour of finer droplets, and the airblast sprayer’s inclination to create them, that we must be so diligent when we adjust the air settings.

    We once explored this at a nursery workshop. The operator was spraying whips, which are young trees with very few lateral branches. He used a cannon sprayer to cover 30 rows (15 from each side) and felt he would incur less drift if he just used pressure, not air, to propel the spray. Water sensitive paper exposed the erratic coverage that resulted. Coverage uniformity was greatly improved when air was used, even when only spraying from one side of the 30 row block. Of course, this was only to demonstrate a principle; we don’t recommend alternate-row-middle-spraying.

    Air-induction nozzles can be used to increase the median droplet size on an airblast sprayer. When used in the top nozzles positions, the coarser droplets that miss the top of tall targets will ultimately fall (reducing drift). They can also be used in positions that correspond to restricted airflow. In this case the operator relies on pressure to propel the coarser droplets where there is limited air to carry finer droplets.

    Conclusion

    The net result of all this is that the sprayer operator must choose a nozzle, pressure, and travel speed while considering the effect of distance-to-target and the weather. The resultant range of droplets should be fine enough to increase droplet count and be carried by sprayer air to deposit uniformly throughout the canopy. However, droplets should also be coarse enough to reduce drift if they miss.

    Hey, if it was easy, anyone could do it!

    Move ahead to 29:40 to watch a video describing how droplets behave an misbehave. Ahhhh Covid-hair. It was a thing.

  • Adjuvants in the airblast tank

    Adjuvants in the airblast tank

    Spray adjuvants are tank mix additives that either physically or chemically influence the efficacy, consistency or safety of pesticides. For example, adjuvants can improve the handling characteristics of a spray solution (e.g. water conditioners, de-foamers, emulsifiers). They can improve uptake into a target plant and/or improve the amount of contact between spray droplet and target surface (e.g. non-ionic spreaders). They can also modify droplets to reduce the potential for wastage from drift or run-off (e.g. anti-drift additives, stickers).

    Note how little of the droplet contacts a waxy leaf (above). This hydrophobic reaction between water and wax can be overcome using a non-ionic spreader. Similarly, note how the droplet gets hung up on the trichomes (hairs) on a leaf before it reaches the leaf surface (below). Again, a non-ionic spreader would reduce droplet surface tension allowing it to splash onto the leaf. Photo Credit – Dr. H. Zhu, Ohio.
    Note how little of the droplet contacts a waxy leaf (above). This hydrophobic reaction between water and wax can be overcome using a non-ionic spreader. Similarly, note how the droplet gets hung up on the trichomes (hairs) on a leaf before it reaches the leaf surface (below). Again, a non-ionic spreader would reduce droplet surface tension allowing it to splash onto the leaf. Photo Credit – Dr. H. Zhu, Ohio.

    Some pesticide labels require the use of adjuvants in the tank mix for the pesticide to work correctly. They are not formulated with the product because of expense, bulk, or product stability, and must be added during loading. In order for a pesticide to work as advertised, it is important to include any adjuvants required by the label. In some cases, we are encouraged to use adjuvants to improve an application, even though they are not on the label.

    There are potential benefits to introducing some unlabelled adjuvants, but there are also potential problems. The difficulty is that unless someone tests a specific tank mix combination for a specific crop, the results cannot easily be predicted. For example, when a tank mix is incompatible, an adjuvant could cause phytotoxicity, create more drift when used with the wrong nozzle, deactivate or enhance a tank partner, and/or potentially reduce spray coverage.

    We once conducted a trial to test a deposition utility modifier intended to reduce run-off and drift. Water-sensitive papers were placed in the canopies of a 40 year old McIntosh orchard, which was then sprayed from one side in late May. The papers in the left panel (dilute control) were sprayed with 600L/ha (~60 g/ac.) of water. Those in the right panel (adjuvant) were also sprayed with 600L/ha but included the label rate of 500 ml of adjuvant. The water-plus-adjuvant reduced drift and runoff, as advertised, but did not penetrate as deeply into the canopy or spread on the papers, which is a concern if the operator was performing alternate-row middle spraying or needed better coverage (e.g. for mites). It was an unexpected side effect.

    For better or worse, even small amounts of adjuvants can have a significant effect on spray coverage. Always test spray coverage when using a new adjuvant in a tank mix.
    For better or worse, even small amounts of adjuvants can have a significant effect on spray coverage. Always test spray coverage when using a new adjuvant in a tank mix.

    We also investigated the use of an anti-drift adjuvant in airblast sprayers, which you can read about here.

    There is no simple answer regarding unlabelled adjuvants; there are too many possible product/adjuvant/plant combinations. If you intend to experiment with an adjuvant, perform a jar test to test for physical incompatibility. Then spray a small volume of the tank mix on a few trial plants to ensure there are no unexpected chemical issues (e.g. phytotoxicity or inactivating tank mix partners) or coverage issues.

    It is highly recommended that every sprayer operator have a copy of Purdue Extensions’ 2015 “Adjuvants and the Power of the Spray Droplet – PPP-107”. This comprehensive handbook describes of how water quality and adjuvants affect the performance of pesticide applications. I consult it regularly.

    Here are two videos from Dr. H. Zhu, USDA-ARS Ohio, showing how adjuvants that affect surface tension can help improve the level of contact between spray droplet and target surface.

  • Diagnosing Airblast Coverage

    Diagnosing Airblast Coverage

    Assuming there are no mechanical or maintenance problems, water-sensitive paper can be used to diagnose sprayer performance. Go here to read more about water-sensitive paper. Interpreting the results and knowing what changes to make is the critical part of the process. Observing no coverage, or a sodden paper, make for obvious conclusions… but what about everything in between? Here are the ground rules:

    First: Only ever test coverage in environmental conditions you would normally spray in. Temperature, humidity and wind speed can make or break an airblast calibration.

    Second: When altering sprayer settings, only make one change at a time for each test pass so you can isolate what’s wrong.

    Third: Each pass requires a new set of papers located in the same place, oriented the same way, distributed throughout the canopy. Mark their locations with bright flagging tape and write the pass number and canopy position on the back of paper prior to placement. This helps you to compare the passes later on. Don’t collect papers until they’ve had an opportunity to dry a little, or they will smear and stick together.

    Fourth: Pass down one alley first. Have a look at the papers without removing them. Then, spray the target canopy from the other side. Now the papers can be removed for analysis. This order is important because it reveals the impact of wind direction and the cumulative effect of spraying from both sides. In some cases, the sprayer operator may wish to travel an additional upwind alley to reflect the cumulative coverage on a typical spray day. Alternate row applications are not recommended.

    This Turbomist has been outfitted with sensors that detect the presence of a canopy. Each eye corresponds to a boom section, turning the section on and off as required and improving efficiency. If it’s not there, why spray it?
    This Turbomist has been outfitted with Gillison’s Sonic Spray sensors that detect the presence of a canopy. Each eye corresponds to a boom section, turning the section on and off as required and improving efficiency. If it’s not there, why spray it?

    Once the papers are retrieved, it’s time to diagnose the coverage. The following situations are typical in calibrations, and possible fixes are suggested. Remember, this is a process that takes time. Several passes may be required before satisfactory coverage is obtained. Once the correct settings are determined for the block, continue to use them until there is a significant change in the crop staging or weather. At that point, repeat the process.

    Seven Situations

    Situation One:

    <15% coverage and <85 Fine/Medium droplets/cm2 at top of target (e.g. tall targets such as hops or trees). Suggested Fixes:

    • Wind might be stealing fine droplets. Try Coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts and that they may fall out of the airstream before reaching distant targets.
    • Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
    • Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.

    Situation Two:

    <15% coverage and <85 Fine/Medium droplets/cm2 deep in canopy – sometimes papers on outside of canopy are visibly wet. Suggested Fixes:

    • Ground speed may be too high. Use flagging tape indicator on far side of target and see if air is getting through.
    • Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
    • Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
    • Increase carrier volume.

    Situation Three:

    Papers are drenched, dripping or show channels of running liquid. Suggested Fixes:

    • Reduce spray volume, either overall or in key locations on the boom corresponding to the drenched papers.
    • Ground speed may be too low. Use flagging tape indicator on far side of target and see if too much air is getting through. If so, increase ground speed.

    Situation Four:

    Considerable overspray beyond target row. Suggested Fixes:

    • Turn off upper nozzles until spray JUST clears target.
    • Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.

    Situation Four:

    Considerable blow-through beyond target row. Suggested Fixes:

    • Slow the fan speed by shifting to low gear, or using GUTD method
    • Ground speed may be increased as long as coverage is not compromised. Use flagging tape indicator on far side of target and see if air is getting through.

    Situation Five:

    Ground under target row is drenched. Suggested Fixes:

    • Rotate lower nozzles slightly upward, but do not shut them off. If ground remains drenched, turn them off entirely. Each hollow cone produces up to an 80º spray angle, so the next higher nozzle often compensates by spraying lower than expected.
    • Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.

    Situation Six:

    <15% coverage and <85 Fine/Medium droplets/cm2. Remember that this coverage threshold is only a point of reference, not a hard fact. It does not apply when using Coarser droplets. Suggested Fixes:

    • Increase spray volume, either overall or in key locations on the boom corresponding to the under-sprayed papers.
    • Wind might be stealing fine droplets. Try coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts.
    • Ground speed may be too high. Use flagging tape indicator on far side of target and see if enough air is getting through. If not, decrease ground speed.
    • Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.

    Situation Seven:

    Inconsistent coverage on outer edge of canopy (e.g. one spot never seems to get spray.) Suggested Fixes:

    • Nozzle spray angle may be too acute (e.g. full cones), and spray is not overlapping before reaching target. Try wider spray angles.
    • Some tower sprayers have ‘dead spots’ in their air. Check for limp or flagging ribbons tied to nozzle bodies and/or deflectors. Deflectors may need to be adjusted, or adjacent nozzle body angles repositioned to compensate. Try an air induction nozzle in the dead zone.
    • Canopy may be brushing against nozzles as the sprayer passes, temporarily blocking them. Canopy management required.
    Some sprayers, such as Rears, Turbomist, FMC or this Durand Wayland have an option for electronic ‘eyes’ that detect spray targets. The boom will shut off completely if there is a gap in the planting. This can save a great deal of wasted spray. It is less applicable in trellised plantings where it has been known to be “fooled” by wires and posts.
    Some sprayers, such as Rears, Turbomist, FMC or this Durand Wayland have an option for electronic ‘eyes’ that detect spray targets. The boom will shut off completely if there is a gap in the planting. This can save a great deal of wasted spray. It is less applicable in trellised plantings where it has been known to be “fooled” by wires and posts.

    If you still are unable to achieve satisfactory coverage, you may have to consider more extreme solutions. You may have an under- or over-powered sprayer. You may have to perform significant canopy management. Or, you may be trying to spray in poor weather conditions.

  • Spraying Large Nut Trees – Part 2

    Spraying Large Nut Trees – Part 2

    This article continues from Part 1.

    Droplet size

    Droplet size influences droplet behaviour. The following table lists the pros and cons to changing droplet size when overall spray volume (e.g. L/ha) remains constant.

    Relative Spray QualityProsCons
    Coarser DropletsLower drift potential because they resist deflection by wind and evaporation from heat and low humidity.Lower droplet count may reduce coverage.
    Greater mass means they move ballistically, propelled at higher speeds by pressure for greater distance.May fall out of the spray before reaching the top or centre of the canopy.
    Coarser droplets do not penetrate dense canopies as easily as finer droplets.
    Redistribution due to bounce, shatter or run-off may either improve or compromise coverage.Redistribution due to bounce, shatter or run-off may either improve or compromise coverage.
    Finer Droplets Higher droplet count may improve coverage (if they arrive at the target).Higher drift potential from wind, and evaporation from heat and low humidity.
    Finer droplets penetrate denser canopies better than coarser.
    Finer droplets move unpredictably and require optimal air settings to direct them to the target. Sprayer design and air settings will determine if it is optimal for nearby or distant targets, but it is rarely if ever both.Finer droplets move unpredictably and require optimal air settings to direct them to the target. Sprayer design and air settings will determine if it is optimal for nearby or distant targets, but it is rarely if ever both.

    It is preferred to use nozzles that create coarser droplets at higher rates (to compensate for fewer droplets) in the higher boom positions. They are more likely to stay on course to the tops of the trees, and when they miss, many fall out of the air rather than contribute to drift.

    Learn more about strategies to reach the top of a canopy here.

    Finer droplets have very little mass and therefore very little kinetic energy. This means they slow quickly (imagine throwing a feather) and require entraining air to carry them to the target. Finer droplets also evaporate quickly, particularly on hot and dry days (i.e. unsuitable Delta T conditions). If employed, they should be distributed in the lower-middle portion of the boom where they have the least distance to travel and are most likely to be intercepted by canopy.

    Boom distribution

    Unlike a broad acre boom sprayer, where each nozzle emits the same rate, an airblast boom can distribute spray unevenly. For a curved (axial) boom, the rule of thumb is to produce 2/3 of the overall volume from the top 1/3 of the boom. This compensates for the distance and greater proportion of canopy it is intended to cover.

    A vertical (tower) boom positions each nozzle roughly the same distance from the target, and if that target is a hedged canopy, the spray can be distributed equally over the boom. Research has demonstrated that there is no appreciable advantage to one spray shape over another (e.g. flat fan, hollow cone, full cone) other than the spray quality they produce.

    In extreme cases, operators might elect to “fire hose” spray to the tops of canopies using high pressures. This is achieved by using streaming nozzles or removing the swirl/whirl/disc plate in a disc-core combination nozzle in the top few nozzle positions. Given the heavy demand on the pump and the inaccuracy of the method, this should only be considered when air fails to reach the tops of trees.

    Learn more about nozzling an airblast sprayer here.

    Spray coverage and diagnostics

    It’s well understood that spray coverage has a negative correlation with tree height. The irony is that in large nut trees the upper portion of the canopy produces much of the harvest. Taken collectively, this may explain why pest activity is also highest in the upper canopy. When choosing a spray volume and boom distribution, the metric is threshold coverage in the top 1/3 of the canopy. This requires us to define threshold coverage.

    If ribbons and leaf movement represent the feedback mechanism for air settings, then water sensitive paper (WSP) is the choice for spray coverage. Placement in tall trees can be tricky, given that we are most concerned with coverage at the top, but this can be overcome by mounting the WSP on telescoping poles. Papers can be oriented horizontally to represent a leaf, or curled around the pole to give panoramic coverage and emulate a nut. Beware over-blowing in the lower canopy, which creates a shingling effect where leaves cover one another (or the WSP) and block coverage.

    Fluorescent dyes and kaolin clay show spray coverage in situ, but there are drawbacks. Few growers will spray dye and come back at night with a black light to examine targets. Further, a target sprayed with dye or clay cannot be sprayed a second time, which means the grower can’t adjust the sprayer and try again in the same canopy. And finally, it’s very difficult to determine if there is more or less coverage with clay or fluorescent dye.

    Learn more about how to use water sensitive paper here.

    WSP is fast, cheap and effective. With the exception of drench applications, the most demanding spray application (e.g. contact fungicide) should produce a spray coverage pattern of 85 drops per cm2 and 10-15% total coverage on 80% of the targets. This threshold comes from collective research and experience in many horticultural crops, and should true hold for tree nut.

    Be prepared to make changes to your sprayer calibration to compensate for tree height, canopy density, and weather conditions throughout the season. The feedback from water sensitive paper is far more accurate than shoulder-checks and leaf residue. It takes some time and effort, but it’s well worth it. Coverage is King.

    What others have done

    Researchers like Brad Higbee (Paramount Farming Co.) and Ken Giles (UC Davis) have explored spray coverage and efficacy from different sprayer configurations to combat Naval Orange Worm in almond. What follows is a summary of their observations. This information comes from their presentations and conversations with Brad.

    Ten years of trials spanned travel speeds of 3-6.5 km/h, volumes of 1,400-2,150 L/ha, and sprayer-generated airspeeds (measured at source) of 80-290 km/h. They looked at efficacy, residue levels and WSP coverage both in leaves and on the nuts themselves. When comparing sprayer configurations, the target almond tree was divided into four levels:

    • Level 1 = 1.8 m to 2.5 m (Lower canopy)
    • Level 2 = 3.0 m to 3.7 m
    • Level 3 = 4.2 m to 4.9 m
    • Level 4 = >5.5 m (Upper canopy)

    Many configurations were tested, but the following figure shows the top four. Of those not shown, most notable are the Bell 206 helicopter (280 L/ha at 50 km/h) and the Curtec AC 1000 Cross-Flow tower.

    A. Air-O-Fan low profile axial D-240 (Also used Air-O-Fan 232).
    B. Progressive Ag two-head 2650 electrostatic air-shear with 4 m tower (Also used 4.9 m three-head and 5.5 m four-head).
    C. Blueline Accutech 10-head air-shear tower.
    D. Low-profile axial airblast with two Sardi-style fans on mast. Upper fans set to 70% overall fan speed and spray volume. Axial fan and nozzles set to 30%

    Here is a summary of their observations:

    • Spray coverage and residue deposition was weakest in upper half (Levels 3 and 4) of canopy. Tower sprayers tended to provide more uniform coverage across vertical levels. For low-profile axial sprayers, most of the residues were deposited in the lower half of the tree.
    • The Air-O-Fan low-profile axial had the highest overall residues. But, above 3.7 m there was severe drop off in coverage. PTO-driven sprayers seemed as effective as engine driven. Incremental improvements were observed on this sprayer when using multiple banks of booms, full cone and hollow cone nozzles.
    • The Progressive Ag tower provided the highest residue deposition above 3.7 m and modest deposition in the lower canopy. While tower sprayers tended to provide more uniform coverage, the Progressive Ag was not significantly better than the Air-O-Fan overall.
    • Aerial application (280 L/ha) combined with the Air-O-Fan low-profile axial sprayer (1,870 L/ha) did increase residues in the upper canopy, but did not result in greater damage reduction relative to the Air-O-Fan alone.
    • Slowing the Air-O-Fan low-profile axial sprayer from 4 to 3.2 km/h resulted in 30% more coverage and 47% higher residue deposition overall.
    • Electrostatic treatments did not perform well on WSP (small droplet size was suspected), but they were among the best in residue deposition at full volume and “delivered surprising residues at high speeds/low volumes”.

    Brad has done remarkable work studying the impact of several sprayer configurations. While many were tested, there are still more that might be considered.

    Canopy management

    When all else fails, we are left with only one alternative: canopy management. Hedging and pruning the trees to create sprayer clearance opens canopies to spray (and light and air) and is a critical part of crop protection.

    Learn more about the benefits of canopy management here.

    Topping trees to bring them to a manageable height to improve coverage and reduce drift may be the only viable option for protecting the crop. I acknowledge that a great deal of nut production takes place in the upper third of the canopy, and it is beyond the scope of this article to discuss production and yield economics. However, when the crop is left unprotected, the yield quality is negatively impacted and it has been shown that a reduction in harvest weight is offset by the improvement in overall quality.

    Where plants are very old and overgrown (such as macadamia), it is highly recommended that the orchardist engage a local crop expert and discuss a strategy for canopy management. There are many benefits, including:

    • Improved harvest quality
    • Fewer refills (saving time and water)
    • Less time to spray means more timely applications
    • Potential chemistry savings
    • Savings in gas, noise and equipment wear and tear
    • Potential for reduced off target spray drift

    Summary

    Spraying large nut trees is a challenging proposition. A number of inter-connected factors are involved and an operator must address all of them make spraying as efficient and as effective as possible.

    • Adjust sprayer air settings first, using canopy penetration as your guide to travel speed.
    • Distribute the 2/3 of the volume and coarser spray quality to the top 1/3 of the boom.
    • Consider an air-assisted vertical boom configuration to improve coverage uniformity and reduce drift.
    • Use water sensitive paper for critical coverage feedback and make changes based on that feedback.
    • Develop a canopy management strategy to improve spray coverage and yield quality.