Category: Speciality Sprayers

Main category for all sprayers that are not horizontal booms

  • Developing Criteria for the Ideal Agricultural Spray – a Biologist’s Perspective

    Developing Criteria for the Ideal Agricultural Spray – a Biologist’s Perspective

    Originally published in: Wolf, T.M. and Downer, R.A. ILASS Americas, 11th Annual Conference on Liquid Atomization and Spray Systems, Sacramento, CA, May, 1998

    Note to reader: It’s been nearly 23 years since we wrote this paper at the invitation of organizers of the Institute for Liquid Atomization and Spray Systems Conference. At the time, custom operators, not farmers, bought self-propelled sprayers. Air-induction tips had just been introduced. Pulse-Width Modulation was only beginning to be available. GMO crops were available but not widely adopted. Buffer Zones were more rumour than policy. How badly out of date are the thoughts we mulled over?

    Abstract

    The goals of an agricultural spray application are to provide effective control of the pest at low cost without adverse environmental impact.  A spray must transport effectively from the atomizer to the target, be intercepted and retained by the target, and form a biologically active deposit.  Improvements in efficiency are elusive because of interactions between successive stages in dose transfer.  Progress will depend on atomizers providing increased control over droplet size and velocity spectra without sacrificing mechanical simplicity: (a) elimination of the interdependence of flow rate and spray quality, (b) control over size span at any given nominal diameter, (c) reversal of present relationship between droplet size and velocity.  Such an atomizer would drive a new research thrust to improve spray efficiency

    Introduction

    Polydisperse sprays provide consistent results yet suffer from inherent inefficiencies in dose transfer.  Drift potential and poor spray retention at the extreme ends of their spectra are classic examples of this inefficiency, and environmental aspects of spray application have been criticized as a result (Pimmentel and Levitan, 1986).  Yet, despite ongoing research, efficiency breakthroughs remain elusive (Hislop, 1993).  Due to the interdependency of the factors governing dose transfer, progress in one area (i.e., greater retention with finer sprays) has often been at the expense of spray drift, and vice versa (Young, 1986).  Theoretical improvements in efficiency with monodisperse sprays (Controlled Droplet Application, CDA) have not translated into widespread adoption due to drawbacks in consistency and robustness of the results.  After 50 years of research, the same compromises which have been discussed since the early days of spray application are apparently still unresolved. 

    Nozzle designs have certainly improved – wider pressure ranges, improved spray patterns, more options for achieving various spray qualities, better quality, longer wearing materials, and lower costs are all important for the end-user.  But the basic atomizer – the hydraulic flat fan nozzle generating a polydisperse spray – has hardly changed over the years. 

    A New Start?

    The questions posed in this paper are:  If a biologist could design the ideal spray, what would it be?  What are the criteria for achieving the best result in the most efficient manner?  Such a discussion represents a unique opportunity to think about what we know about sprays and their biological impact, providing atomizer design information to meet our future needs. 

    Unfortunately, biologists still know relatively little about the impact of kind of spray quality on efficacy.  General statements can be made relating spray quality to herbicide, insecticide, or fungicide effectiveness, but for the most part, the ideal spray or subsequent deposit has still not been defined for most situations (Hislop, 1987). The situation reflects the lack of choice in spray atomization, creating a catch-22:  not being able to easily produce customized sprays has made it difficult for biologists to identify (without confounding factors) the ideal spray for any particular situation.  Further, the need by the industry for a simple, reliable, and standard application system has inherently hindered efforts to optimize the system.  All stakeholders will need to be flexible to present a fertile environment for improvements to take hold. 

    “Integrated Spray Management”

    In this era of integrated pest management, cropping systems are optimized to provide the most effective pest management strategy on a case-by-case basis with minimal crop protection agent (CPA) use.  This underlying philosophy can be extended to spray application.  When CPAs are used in such systems, they, too, must rely on diverse strategies to make them more efficient.  Within this philosophy, a single standard application technique for all pests will not be acceptable.  Two developments are needed to put such a development into action:  (a) an application system capable of delivering a wide variety of spray qualities (droplet sizes, spans, velocities) at a range of carrier volumes; and (b) the knowledge to utilize specific spray qualities under identifiable conditions. 

    Application Objectives

    During the development of such a new application philosophy, the objectives for spray application must remain clear.  They are to deliver a CPA in its most effective form to the pest, with no off-target effects, at the lowest possible cost, i.e., effective, economical, and environmental. 




    Figure 1: Typical droplet number and volume spectra for an agricultural hydraulic flat fan nozzle

    The status quo for most post-emergent CPA applications is the hydraulic flat fan nozzle.  As we know, such a nozzle produces a heterogeneous mix of fine and coarse droplets (Figure 1) with a droplet speed and size relationship (Figure 2).  This nozzle has frequently been criticized for inefficiency because only a small portion of spray is optimally targeted (Adams et al., 1990).  At the same time, it has been applauded for consistency because a portion of its size and velocity spectrum (although not necessarily the same one) is usually appropriate for the pest complex at hand.




    Figure 2: Typical velocity spectrum for an agricultural hydraulic flat fan nozzle.

    The most frequently documented drawbacks of the hydraulic nozzle are driftability of fine, and poor retention of coarse components.  An additional drawback is interdependence of flow rate and droplet size for any given nozzle, i.e., at the same work rate, lower carrier volumes are applied with finer sprays.  Research into droplet size effects has been difficult because no variable can be held constant while another changes.  Keeping dose constant, studies of carrier volume have to accept a simultaneous change in travel speed or droplet size, droplet size studies have to contend with changes in droplet density, and droplet density studies must alter active ingredient concentration.  Given the complexity of the problem, few researchers commit themselves to solving these dilemmas. 

    Maximizing Effectiveness – No Easy Answers

    For any given spray mixture, an atomizer controls spray pattern, droplet size, and droplet velocity.  Spray patterns determine spatial uniformity.  Droplet size and velocity in turn affect spray fate by controlling canopy penetration spray interception, spray retention, spray coverage, evaporation rate, etc.  Considering the variety of active ingredients, formulations, concentrations, environments, pests, and plant canopies present, it is not surprising that the scientific body of evidence is often contradictory (Knoche, 1994).  It should also come as no surprise that there is no single “best” droplet size to optimize these factors. 

    Basic principles:  In order to better understand why a single ideal spray cannot exist, a brief review of the principles of spray drift, interception, and retention are appropriate. Larger droplets are driven mostly by inertial and gravitational forces (Spillman, 1984).  As such, they tend to have vertical trajectories from which they cannot easily be displaced.  This makes them a good choice for drift reduction, and also for canopy penetration into vertically oriented canopies, such as cereal grains. Collection efficiency by a target is a function of target size and orientation – horizontally oriented, larger objects will be favoured by larger droplets.  Spray retention is a function of leaf surface wettability and microstructure, as more difficult to wet species will be more likely to reflect larger droplets (Hartley and Brunskill, 1958). 

    Small droplets, on the other hand, are more subject to viscous drag, have shorter stop distances, and can therefore move with local air turbulence to reach shadow regions (Nordbo, 1992).  Thus the finer sprays have a propensity for displacement from their flight path by air turbulence, but they also are better able to penetrate dense broadleaf canopies because they can move around larger objects. Small, vertically oriented objects such as stems and petioles have the best collection efficiency for small droplets. 

    Upon depositing successfully on a target, the deposit must be in a form which exerts the desired biological effect.  Given the same spread factor, deposits with greater volumes remain wet longer, providing more opportunities for uptake into the leaf.  Small droplets provide more efficient coverage per unit volume, but dry rapidly, which may limit their uptake. 

    Uptake and translocation of active ingredients by biological targets are physical processes driven by concentration gradients.  Concentrations of active ingredients and surfactants per unit leaf area are a function of carrier volume, droplet size, and spread factor.  Less than optimum concentrations can result in reduced uptake and translocation (Wolf et al., 1992).

    Further complications arise due to the heterogeneous nature of weeds.  Individual regions of weed plants have unique anatomical and physiological features that can affect retention, uptake, and translocation processes on a spatial level.  For example, Merritt (1982) showed that for wild oats (Avena fatua), younger leaves and the basal region of leaves absorbed more difenzoquat than older leaves.

    All these factors conspire to complicate the quest for optimization in a field setting. 

    The Ideal Spray

    Based on the previous discussion, it may be obvious that a single droplet size cannot meet all demands within such a complex system.  Therefore our focus must shift from a theoretical optimum solution, as was the basis for controlled droplet applicators (Bals, 1980) to one which emphasizes flexibility. 

    One advantage of speaking on behalf of biologists is that one can feign complete ignorance about atomization, and propose seemingly ludicrous ideas.  Perhaps a prerequisite to a fresh approach is such ignorance. 

    Biologists need a spray to not only implement optimum application, but as a means with which to learn how to optimize the process in the first place.  Further, since there is no single optimum spray quality to meet all application scenarios, the most important feature in a spray is flexibility.  The following features will be important:

    Spray quality independence:  The first criteria is the ability to adjust spray quality easily, without affecting carrier volume or droplet velocity, and vice versa (Figure 3).  A shift towards a coarser or finer spray can then be achieved without introducing other confounding effects.  Some progress has already been made in this area (Giles and Comino, 1990).

    Figure 3: Shift in droplet size spectra from medium to fine or coarse qualities, achievable without a change in carrier volume.

    Relative span factor flexibility: The relative span factor of the spray should be adjustable (Figure 4).  It will be important to narrow the broad spectrum sprays produced by flat fan nozzles to determine the importance of specific droplet sizes.  While such research was conducted during the 1970s and 1980s with controlled droplet applicators (CDAs), the unique droplet velocity associated with such atomizers would question results if they were to be applied using hydraulic atomizers.

    Figure 4: Narrowing the span of the droplet size spectra, while preserving its polydisperse nature,  will be useful to strike a balance between specific droplet sizes and spray heterogeneity.

    Velocity control:  The third criteria is for improved droplet velocity control.  The droplet velocity dependency on size has meant that in the absence of air assist, smaller droplets are always moving slower.  This factor has reduced the efficiency of their collection and made them more drift prone.  Additionally, the larger droplets, being faster moving, were more likely to rebound from targets.  Acceleration of small droplets is a strategy for reducing spray drift and enhancing collection efficiency, but greater velocity for larger droplets may reduce the efficiency of their retention by the target.  If the droplet size – velocity relationship were reversed, then smaller droplets would be less drift prone and larger droplets would be less likely to rebound (Figure 5). 

    Figure 5: Droplet velocity spectrum for a typical agricultural hydraulic spray, accelerated with air assist to reduce drift potential of smaller droplets, and with smaller droplets travelling faster than larger droplets to maximize transfer efficiency.

    Spray heterogeneity:  Spray heterogeneity will remain important in an optimized system, especially in the absence of specific knowledge on droplet function by size class.  In this sense, a polydisperse spray does more than provide insurance for changing conditions, it adds diversity to static conditions which strengthens the overall effect.  While a quantitative dose-based approach to CPA delivery is often appropriate, it under-emphasizes the role of deposit structure and spray redistribution, where quality is more important than quantity (Wolf, 1996).  For example, let us assume that canopy penetration is maximized with a spray of 400 µm VMD, with a relative span factor of 0.7.  In such a spray, fine droplets contribute relatively little to overall dose.  However, their ability to redistribute in the canopy, targeting areas left untouched by the larger droplets may be more important than their total dose contribution would suggest.  In this way, they provide benefits which are total dose independent.  A heterogeneous spray would ensure that these benefits remain. 

    Deposit uniformity:  Efforts at optimizing dose transfer are compromised if spatial dose uniformity cannot be maintained within the treated area.  High deposit variability has been associated with reduced control of insects (Uk and Courshee, 1982; Cooke et al., 1986).  As such, uniformity remains a fundamental requirement for spray application and should not be compromised with new atomizer designs. 

    Environment as a Priority

    Spray must land on the intended target, be it a plant, insect or ground, and in some cases on the optimal pest part, i.e., specific leaves, leaf sides, stems, etc.  Off-target placement not only represents inefficiency, but also undesired environmental input.  With any application system, an important criteria is the ability to manage off-target impacts. 

    Past solutions to spray drift or droplet rebound has been two-fold:  (a) eliminate those droplets which do not impact on the target efficiently, or (b) protect them from displacement.  For spray drift, the elimination of small drops through production of coarse sprays has been successful (Edwards and Ripper, 1953).  The challenge is to provide drift protection without compromising the advances made in the previous exercise of maximizing effectiveness.  The protection of fines with barrier (shrouds) is an effective strategy for reducing drift, and provides the advantage of maintaining a spray quality established to meet separate criteria (Wolf et al., 1993).  Another successful strategy has been to assist transport of fines with an external energy source (air or electrostatics).  This also allows the preservation of an optimized spray quality, with the added advantage of modifying the droplet velocity spectrum in favour of canopy penetration. 

    Nozzle design may offer some opportunities for the reduction of rebound.  Novel atomization systems such as venturi or twin-fluid nozzles, which offer air-inclusion in droplets, may reduce rebound of larger droplets.  If larger droplets are required, but retention is of concern, such approaches may be useful.  Spray adjuvants can also play important roles in this area (Downer et al., 1995)

    Economical Considerations

    Underlying any attempt to provide effective pest management is an economical consideration.  The producer must see a benefit in making a technical investment.  Any atomizer solution must therefore not only meet the technical requirements for optimizing dose transfer, it must also be a cost-effective and practical system.  A system which is complicated to use is not likely to be widely adopted.  Without strategies for implementation by the end user, innovations in delivery are merely theoretical exercises. 

    Putting it into Practice

    During a typical work day, an applicator may be called to treat crops for a range of pests with broad-spectrum products.  These pests will likely be present in a range of densities, some above and others below an economic threshold.  There may also be a range of canopies present, some broadleaves, others grasses, some dense, others sparse.  Depending on the area, there may have been a range of environments under which pests became established, or during which application is made.  Each field will also have a range of bordering ecosystems with unique trespass sensitivities. 

    There will obviously be a limit to the degree of customization that is possible.  But some efforts will be rewarded.  The applicator uses GPS technology to collect or recall relevant data – sensitive areas, high and low infestation levels, or changes in canopy structure.  With the new atomizer, the applicator can emit the most effective droplet size, velocity, span, and dose appropriate for the pest or canopy on a site-specific basis.  The use of spray quality classification systems such as those developed by the BCPC and ASAE will guide optimization efforts, but in the end, these classification systems will be too broad to fine-tune the system.  A higher resolution, multi-parameter scheme which is sensitive enough to represent the criteria laid out in this paper will be necessary. 

    Possible difficulties emerge when the system resists optimization.  For example, it would be comparatively easy for the applicator to control a broadleaf weed in a grassy canopy, as the size spectrum which optimizes grass canopy penetration is also likely to target the broadleaf weed effectively.  If a tank mix is used to control both grassy and broadleaf weeds in this canopy, the applicator now needs a more heterogeneous spray, where a finer component targets the grassy weed, and the coarser component still effectively transfers dose to the broadleaf weed.  As the situation increases in complexity, the simultaneous optimization of several criteria will be increasingly difficult. 

    Conclusions

    Only an integrated approach involving all stakeholders (engineers, chemists, biologists, etc.) can result in improved application of CPAs.  Individual goals and concerns must be communicated and reconciled in new design efforts.  This paper represents a wish list from biologists’ perspectives.  While greater flexibility and control are important objectives in our opinion, consideration must also be given to mechanical complexity and cost, possible interactions with formulations exhibiting a range of physico-chemical properties, biocontrol agents, and practical strategies for adoption.  A continued willingness to establish and maintain lines of communication and cooperation between these disciplines will be pivotal to success.

    Acknowledgments

    The invitation by the ILASS Program Organizing Committee to make this presentation is gratefully acknowledged. 

    Citations

    1. Adams, A.J., Chapple, A. C., and Hall, F.R.. 1990.  Agricultural sprays: lessons and implications of drop size spectra and biological effects. In: L.E. Bode, J.L. Hazen, and D.G. Chasin (eds.) Pesticide Formulations and Application Systems, ASTM STP 1078. American Society for Testing and Materials, pp. 156-169
    2. Bals, E. J.  1978.  The reasons for C.D.A. (Controlled Drop Application).  Proceedings of the 1978 British Crop Protection Conference – Weeds, pp 659-666.
    3. Downer, R.A., Wolf, T.M., Chapple, A.C., Hall, F.R., and Hazen, J.L.  1995.  Characterizing the impact of drift management adjuvants on the dose transfer process. In: R.E. Gaskin (ed.) Fourth International Symposium on Adjuvants for Agrochemicals. New Zealand Forest Research Institute, Rotorua, NZ, pp. 138-143.
    4. Edwards, C.J. and Ripper, W.E.  1953.  Droplet size, rates of application and the avoidance of spray drift. Proceedings of the 1953 British Weed Control Conference, pp. 348-371.
    5. Giles, D.K., and Comino, J.A.  1990.  Droplet size and spray pattern characteristics of an electronic flow controller for spray nozzles.  J. Agric. Engng. Res. 47:249-267.
    6. Hartley, G.S. and Brunskill, R.T.  1958.  Reflection of water drops from surfaces. In: J. F. Danielli, K. G. A. Parkhurst, and A. C. Giddiford, eds., Surface Phenomena in Chemistry and Biology, Pergannon Press, London, pp. 214-223.
    7. Hislop, E.C.  1993.  Application technology for crop protection:  an introduction. Pages 3-12 In: G.A. Matthews and E. C. Hislop (eds.) Application Technology for Crop Protection. CAB International, Wallingford, UK.
    8. Hislop, E. C. 1987. Can we define and achieve optimum pesticide deposits? Aspects Appl. Biol. 14:153-172.
    9. Knoche, M.  1994.  Effect of droplet size and carrier volume on performance of foliage-applied herbicides.  Crop Prot. 13:163-178.
    10. Merritt, C.R. 1982.  The influence of form of deposit on the phytotoxicity of difenzoquat applied as individual drops to Avena fatua. Ann. Appl. Biol. 101:517-525.
    11. Nordbo, E. 1992. Effects of nozzle size, travel speed and air assistance on deposition on artificial vertical and horizontal targets in laboratory experiments. Crop Prot. 11:272-278.
    12. Pimentel, D. and Levitan, L. 1986.  Pesticides: Amounts applied and amounts reaching pests. BioScience 36:86-91.
    13. Spillman, J.J.  1984.  Spray impaction, retention and adhesion: an introduction to basic characteristics.  Pestic. Sci. 15:97-106.
    14. Wolf, T.M.  1996.  Spray application into standing stubble – an exploration of physical and physiological components.  Ph.D. Dissertation, Department of Agronomy, The Ohio State University, 192 pp.
    15. Wolf, T.M., Grover, R., Wallace, K., Shewchuk, S.R., and Maybank, J.  1993.  Effect of protective shields on drift and deposition characteristics of field sprayers.  Can. J. Plant Sci. 73:1261-1273.
    16. Wolf, T.M., Caldwell, B.C. McIntyre, G.I., and Hsiao, A.I.  1992.  Effect of droplet size and herbicide concentration on absorption and translocation of 14C‑2,4‑D in oriental mustard (Sysimbrium orientale). Weed Sci. 40:568-575.
  • Perspective on Rates, Volumes and Coverage

    Perspective on Rates, Volumes and Coverage

    This short article is a thought exercise designed to give some perspective on chemical rates, carrier volumes and the foliar area we expect them to protect.

    Imagine we are spraying the fungicide Captan on highbush blueberry. In Canada, the label rate is to apply 2kg/ha (28.5oz/ac) of planted area. Captan is 80% active ingredient, so a quick unit conversion tells us our objective is to apply 160mg of active ingredient per m2 of planted area. Let us suppose we will use 500L of carrier per hectare (53.5 gal/ac), which converts to 50mL/m2.

    Now let’s say the blueberry patch is mature and well pruned. Each plant has a footprint of 1.2m by 1.2m (4ft by 4ft) and is 1.5m (5ft) high. The Leaf Area Index (LAI) is the one-sided green leaf area per unit ground surface area (LAI = leaf area / ground area) in broadleaf canopies. Assuming a conservative LAI of 2, that’s 2.88m2 (65ft2) of leaf surface area per plant. We double that figure since we want to spray both sides of the leaves, and then assuming the bushes are planted on 3m (10ft) alleys we arrive at a total foliar surface area per planted area of 3.25m2/m2 (3.25ft2/ft2).

    A grower with his mature, well-pruned blueberries. 4′ x 4′ on 10′ alleys.

    Let’s take these figures and convert them to something we can picture. An average grain of rice weighs 29mg and there are 15mL in a single tablespoon. What this means is that a sprayer operator’s goal is to dissolve active ingredient with a weight equivalent to 5.5 grains of rice in 3.5 tbsp of water and distribute it evenly over 3.25m2 (35ft2) of surface area!

    Now that’s perspective.

    This photo shows how much foliar surface area exists in a square meter of mature highbush blueberry. In the centre is the typical amount of active ingredient and water that must be distributed over that area. It’s amazing what we ask of an air-assist sprayer.
  • What’s the Relationship Between Vapour Drift and Inversions?

    What’s the Relationship Between Vapour Drift and Inversions?

    Drift symptoms can take a few weeks to be discovered, and to figure out the cause, people need to reconstruct the conditions during the application in question. Wind direction is the easiest. But when we consider factors like inversions, volatility, calm conditions, and others used to explain the movement of pesticides, it can quickly become quite confusing.

    Let’s review how and why pesticides move.

    There are about six main ways that pesticides can move off-target.

    1. Droplet drift at the time of application;
    2. Vapour drift at or after the time of application;
    3. Pesticide movement in water (precipitation or runoff) after application;
    4. Dislodgeable residues from plant surfaces after application;
    5. Pesticide-containing soil movement after application;
    6. Pesticide residue in sprayers applied to another site.

    Whenever we find pesticides in a place where they do not belong, usually first indicated by plant symptoms specific to that herbicide, we need to find out the possible reasons and take steps to prevent that from happening again. We’ll focus on the first two items from the above list because those two are the most common.

    Droplet Drift: Sprayer nozzles produce droplet sizes ranging from 5 to 1000 µm, some up to 2500 µm. All nozzles, even the venerable low-drift tips recommended for dicamba application, will have a fraction of their volume in driftable droplets, say, less than 150 to 200 µm. For the low-drift sprays, that fraction is indeed very low, only a few percent of the total spray volume. For conventional nozzles, the driftable fraction may be 10 to 20% or more if high pressures are used.

    Tiny droplets have no energy of their own and move with the air mass they’re released into. If it’s windy, they move downwind. If the air is turbulent, they move up and down. If the atmosphere is stable, the buoyant fraction stays aloft and concentrated. So in order to understand their movement, we need to understand the atmosphere.

    Vapour Drift: Some chemicals are inherently volatile. This means they convert from the liquid or solid phase to a vapour phase on their own in accordance with temperature. Water is a great example, it is highly volatile. It is also able to sublimate, which means it can convert from a solid directly to a vapour without going through the liquid phase. An example of that is freezer burn, in which ice cubes shrink due to water escaping as a vapour.

    Volatile pesticides can also sublimate. On landing on a leaf or soil, a significant portion of a droplet is absorbed or adsorbed. Some fraction may dry on the leaf surface. This remaining solid can volatilize (form a vapour) for hours or days after application. The rate of evaporation is driven by two factors, (a) the background vapour pressure of the substance in the atmosphere, and (b) the surface temperature of the object the chemical is resting on. For water, the atmospheric vapour pressure can be expressed as relative humidity. Droplets evaporate slower when the atmosphere is already full of water.

    Pesticide evaporation is driven primarily by surface temperature. The background concentration of pesticide in the air is much lower than saturation, and has no effect. Pesticide evaporation is not directly affected by relative humidity because vapour pressures are independent of each other. In other words, most active ingredients will evaporate at the same rate whether the RH is 30% or 100% (it’s actually a bit more complicated than that. See the Note on Evaporation at the bottom of this article). This will be on the test, kids.

    Vapour losses can be minimized by choosing low-volatile pesticides and also by making the application on cooler days. We also need to watch the forecast and avoid spraying when tomorrow or the day after is forecast to be hot.

    Sometimes a rainfall can affect vapour losses, prompting a release of pesticide into the atmosphere. This behaviour can be predicted by the Henry’s Law Constant of a chemical.

    Inversions:  There are two types of turbulence, mechanical and thermal. Mechanical turbulence results from air encountering friction as it moves across a landscape. Taller objects and stronger winds result in greater mechanical turbulence. This turbulence creates small eddies that allow different layers of the atmosphere to communicate with each other and transfer momentum and contents up and down. More mechanical turbulence means more mixing and more sedimentation and dilution of a contaminant. In other words, the downwind impact of drift particles is reduced with greater mechanical turbulence. Mechanical turbulence happens whenever it’s windy, day or night, and it tends to counteract thermal turbulence.

    Thermal turbulence is more powerful than mechanical turbulence for dispersion of pollutants. Driven by the solar heating of the earth’s surface, that causes the lower atmosphere to be much warmer than the air higher up. The atmosphere normally cools the higher you go (at about 1°C/100 m, called the dry adiabatic lapse rate), but when it’s sunny, the gradient is greater. In other words, it cools faster because the air at the ground is warmer.

    Thermal effects move large parcels of air up and down, and we call this an unstable  or a turbulent atmosphere. When parcels of air rise and fall great distances, we get a powerful diluting effect which is usually associated with a breeze but can also happen under calm conditions. An unstable atmosphere is great at dispersing drift, minimizing its downwind impact. This can only happen during the daytime, is most powerful when it’s sunny, and almost never happens at night.

    By the way, a neutral atmosphere occurs when the rate of air cooling with height equals the adiabatic lapse rate described above. A neutral atmosphere can occur on cloudy days just before a rain, or on windy nights. There are no thermal effects in a neutral atmosphere, and the only dispersion occurs due to mechanical turbulence (windy conditions).

    A stable atmosphere (inversion) happens when there is no solar heating of the soil. In other words, it can only happen when the sun is low in the sky or at night. In this case, soil cools off and the cold soil cools air near it. As a result, the air temperature rises with elevation. Since it’s normal for air to cool with elevation (at the dry adiabatic laps rate mentioned earlier), the temperature profile is now…inverted. Hence the name “inversion”. To be clear (write this down kids, it’s on the test), an inversion describes an atmospheric condition in which (potential) temperature rises with elevation. That’s it. It rarely happens during the day, but is common on clear calm nights. (btw, “potential temperature is the temperature adjusted by its normal rate of cooling with height. To have thermal effects, the rate of cooling needs to be different from this rate.)

    The atmosphere is called stable because there is no thermal mixing. Air parcels stay put. Suspended particles such as tiny droplets stay put. Drift clouds stay concentrated, potent. If you make a fire, smoke hangs around. Cool, dense air is near the ground, and moves laterally very slowly, and might run downhill, like water, in a sloped setting. This situation is dangerous because it can move pesticide particles or vapours great distances without them becoming diluted or dispersed. An additional danger is that relative humidity is higher at night, delaying evaporation of water from the droplets. They stay potent.

    In Summary: Pesticides move in the atmosphere and are rapidly diluted by mechanical, and especially thermal, turbulence. That is why we like to see spray applications on sunny days with a nice breeze, which moves the product in a predictable direction and dilutes any drift rapidly along the way. We minimize particle drift through the usual measures such as the use of low booms, protective shields, slow travel speeds, and coarser sprays. We avoid spray application of volatile pesticides on or preceding hot days to minimize the risk of vapour drift. We do not apply pesticides when the atmosphere is stable (inversion), which usually means from just before sunset to just after dawn on a clear night.

    OK, that’s the basics. Now let’s explore some common questions.

    1. Can all pesticides move as particles and vapours? All pesticides that are atomized through a nozzle can move as particles. Only pesticides that are considered “volatile” can form significant amounts of vapour and move in that form. Dicamba is volatile. New dicamba formulations such as Xtendimax, FeXapan, and Engenia are much less volatile than older formulations, but they’re still capable of moving as vapours. Glyphosate and many other pesticides are not considered volatile and are not known to cause vapour drift.
    2. Are inversions only a problem for dicamba? Inversions affect droplet drift from all pesticides equally. The key difference is the amount of harm that any given droplet or vapour cloud can impart. Dicamba can harm conventional soybeans, many vegetable crops, and many trees and shrubs at extremely low doses. That means that even a weak inversion or a small amount of drift can cause great harm for long distances. In comparison, most other products are not as harmful to most of our crops in such small doses (I’m generalizing, forgive me). Tiny amounts may never be noticed, but they are there. Dicamba lets us notice these tiny amounts.
    3. Does vapour drift move only by inversions? No, although its movement is more damaging under inversions. Vapour drift clouds form above a recently sprayed canopy on hot days when leaf or soil surfaces contain a volatile product. On a sunny day (no inversion), this vapour will likely disperse rapidly downwind, causing diminishing damage with increased distance in relation to the sensitivity of the non-target plant. But towards evening, the dispersion (caused by thermal turbulence) ends as the sun sets and the atmosphere becomes stable. Now, the residual vapour cloud above the crop is no longer diluted, and may move in an unpredictable direction based on the slope of the land or a very gentle evening breeze. This movement may be significant, extending for miles in some cases, and potentially causing harm along the way.
    4. How long after application can vapour drift occur? Under most conditions, vapour losses diminish rapidly and will likely be gone within a few days as the pesticide is taken up by plants, metabolized, converted to a non-volatile form, etc. For some products, a light rainfall event can release a new wave of vapour drift because these products would rather be vapours than be dissolved in water, in accordance with their Henry’s Law Constant.
    5. Do some products drift further than others? Yes and no, but mostly no. Spray drift is a physical process governed by the behaviour of droplets in the atmosphere. Droplet diameter determines its mass and this mass controls the time it takes the droplet to sediment to the ground. The substance dissolved or suspended in that droplet has no bearing on this behaviour. But there are two key exceptions to consider. First, we know that some formulations generate more fine droplets than others even when atomized through the same nozzles. The greater abundance of small droplets will create more drift damage at any given distance, and also extend further downwind. And secondly, some formulations change the rate of water evaporation from the droplets. As a result, droplets moving downwind may shrink faster, in effect making them more drift prone and causing them to move further downwind. The same droplet size drifts the same distance, but droplet size changes. Question for the final: If you spray dicamba and glyphosate on the same day using the same nozzle, and the formulation has no impact on droplet size or evaporation, which one drifts further over a soybean crop? The answer is at the bottom of this article.
    6. Do calm conditions indicate an inversion? Inversions are defined as a temperature profile, not a wind condition. But the two are associated. An inversion is most pronounced and persists the longest under calm conditions, and because it suppresses atmospheric mixing, an inversion does prevent a windier upper atmosphere from reaching the ground. But it can be calm in the middle of the day with an unstable atmosphere. The calm condition eliminates mechanical turbulence, and therefore reduces the dispersion of the spray cloud. Calm conditions are also undesirable because the winds that follow a calm period are often unpredictable in direction, force, or duration. So it’s not a great idea to spray when it’s completely calm, even on a sunny day.
    7. Can inversions occur during the day? Yes, but it’s rare. Sometimes a large cold air mass moves into an area, say from a cool body of water, pushing warm air above it. So technically the air at the ground is cooler than the air above it, suppressing dispersion through that cap. Another situation is an inversion layer that forms at the top of a transpiring plant canopy. The air at ground level is warm, and cools suddenly where the crop evaporates water from its leaves. Air temperature rises with elevation above this transpiring layer, then cools again in accordance with an expected profile. So we have a thin layer in which vertical mixing is suppressed. This is most common in dense, thick canopies with adequate soil moisture on hot days.
    8. Is there an inversion every night? No. Cloud cover suppresses the rapid cooling of the soil, and the air at soil level stays warmer longer. Wind mixes the cold air layer into a warmer air layer, returning a more neutral condition. Inversions are most likely on clear nights with little wind. Recent data in inversion frequency from Missouri and North Dakota shows that inversions occur on the majority of nights, but the frequency depends on the location.
    9. Can drift be eliminated? Yes, we can eliminate spray drift by atomizing the spray in droplets (or, for dry soil-active products on carrier particles) large enough to resist movement in wind. We would need to be sure that absolutely no fine droplets or particles are produced, and that they don’t dislodge after application. That will require different atomizers and significantly more water volume and possibly new adjuvants. Some will argue that drift can also be eliminated by protecting the fine droplets with shields or air assist, but again, the protection would need to be 100%. Drift control has not been a high enough priority for these technologies to be developed and made available to applicators. Vapour drift can be eliminated by not applying volatile products.

    Pesticide movement in the atmosphere is complicated. But pesticides don’t just move as a result of vapour or droplet drift. Consider all the options when investigating an affected field. And let’s all work together to better understand pesticide movement and to prevent it.

    Answer: Both drift equally. But assuming the beans are susceptible to both herbicides, the dicamba damage will appear further downwind due to the greater sensitivity of the beans to this herbicide. This does not mean it drifted further.

    Note on Evaporation: There is some discussion about the role of relative humidity on vapour loss. Although we stated that RH plays no direct role in pesticide volatility, we need to qualify that.

    (a) Many pesticides dissolve in water. More water moves to plant or soil surfaces during periods of low RH, and this can carry dissolved pesticide with it. The supply of pesticide that can evaporate is thereby replenished.

    (b) Evaporation is driven by temperature and the concentration gradient between the source and the atmosphere. In still air, the air layers closest to the evaporating surface are most concentrated with evaporated pesticide, slowing further evaporation. Air movement will remove these layers, increasing the rate of evaporation.

    (c) co-distillation may occur for some pesticides. This means that the pesticide dissolved in water may evaporate with water, liberating it into the atmosphere. When co-distillation occurs, low RH would increase pesticide losses as well.

    We still have much to learn about these phenomena, especially as it affects new formulations.

  • Categorizing air-assist sprayers by air-handling design

    Categorizing air-assist sprayers by air-handling design

    Air handling systems

    Air handling systems can be specialists or generalists; some are designed to do one thing very well while others are more adaptable but not as precise. Fan type plays a big role in determining a sprayer’s abilities. Their native characteristics make them better suited to certain scenarios.

    This may seem contradictory, but we are not saying that the fan alone defines or limits the entire sprayer. Fans operate within a larger, engineered air handling system. Also, the operator has control over how that sprayer is configured and used. This means it is equally important to consider how the air exits the sprayer – not just the fan type that generated it.

    Fan types

    • Radial fans: Radial fans produce high volumes of moderately turbulent air, and relatively low static pressures. They are often associated with fixed vanes and straighteners inside the fan housing to reduce initial turbulence.
    • Turbines: Turbines may look like radial fans but they’re designed to spin faster and they have blades designed to compress air. They are used in sprayers that have ducts, towers, cannons, or other more complex volutes.
    • Straight-through axial fans: These fans produce high volumes of the most turbulent air. With their comparatively short throw and wide air wash, they should be positioned close to the target.
    • Tangential (aka Cross-flow) fans: Tangentials produce the most laminar air, forming a very high volume, low velocity jet sometimes called a “curtain” or “knife”. They have a comparatively long throw and rely on the canopy to induce turbulence.
    • Centrifugal (aka Squirrel cage) fans: Centrifugal fans have a side-discharge arrangement that turns air 90 degrees. They can produce high pressures and are nearly always paired with an air-shaping volute.

    We are proposing defining air-assist sprayers for perennial crops according to their air handling systems. Ultimately, the defining characteristic of each design is the net vector of the air they generate. We have provided silhouettes for clarity, but these generic designs are not intended to imply a manufacturer.

    Low profile radial

    The oldest and perhaps most recognizable air handling design, the Low Profile Radial (LPR) sprayer generates air in a radial pattern from one or more axial fans or a volute connected to some other fan style. This is the classic airblast sprayer.

    Defining characteristics

    • Wide range of adjustable air energies from virtually zero to high.
    • Minor adjustability of air vectors via deflectors and moveable outlets.
    • Net air movement is lateral and upward.

    Cannon

    The Cannon (CN) sprayer generates and channels air through a single volute and delivers the spray as a compact, point-source jet. 

    Defining characteristics

    • High air energy characterized by high velocity and low volume.
    • Extensive adjustability of air vector via a vertical duct with positional outlet and deflector(s).
    • Usually a single-sided sprayer used to spray over and through multiple rows.

    Fixed tower

    The Fixed Tower (FT) sprayer generates air from one or more axial fans, multiple straight-through radial or tangential fans. It may employ flexible tubes, tapered bags or solid ducts to redirect air laterally from a fixed central tower. It may feature additional flexible ducts or adjustable deflectors at the top of the tower to spray over and beyond the adjacent rows. 

    Defining characteristics

    • Wide range of adjustable air energies from virtually zero to high.
    • Minor adjustability of air vectors via deflectors and moveable outlets.
    • Net air movement is lateral compared to LPR sprayers.

    Targeting tower

    Similar to the FT, the Targeting Tower (TT) sprayer can focus air vectors with a wider range of adjustability, shaping the lateral air output more precisely to the canopy. TT generates air from one or more radial fans or multiple tangential or straight-through axial fans. It may employ flexible tubes or solid ducts to redirect air generally laterally. 

    Defining characteristics

    • Medium to high air energy.
    • Moderate to high adjustability of air vectors. Airflow can be subdivided into individually-adjustable sections.
    • When the tower exceeds canopy height, net air movement is lateral to slightly downward.

    Wrap-around

    The Wrap-Around (WA) sprayer surrounds the target rows with air sources. This creates multiple converging and/or opposing airflows within the row. 

    Defining characteristics

    • Straight-through axial fan systems are either electric or hydraulic with a wide range of air energies.
    • Low to high adjustability of air vector via deflectors, moveable air outlets, or fan position adjustments. May also have an adjustable frame.
    • Net air movement is ideally neutral to slightly downward.

    Summary

    In adopting this system of classification, we believe the process of optimizing sprayer configuration and calibration can be made less complicated. A universal language facilitates clear communication between growers, industry and consultants/specialists.

    We acknowledge that there may be rare sprayers that don’t fit these categories. There are commercial examples of air-assist sprayers that combine features from these air-handling designs (e.g. hybrids of LPR and FT designs)… but let’s keep things simple.

  • Nozzle Selection for Boom Sprayers

    Nozzle Selection for Boom Sprayers

    Picking the correct nozzle for a spray job can be a daunting task.  There is a lot of product selection, and a lot of different features.  We try to break the process down into four steps.

    1. Identify Your Needs

    Before making any assumptions about the right nozzle for you, review your needs and objectives. Are you trying to reduce drift? Do you want better coverage? Are you moving towards more fungicide application? Do you need a wide pressure range?

    It’s always a good idea to review your experience with your previous nozzle. What, if anything, would you like to change?

    2. Identify Flow Rates

    Most spray operations fall into one of three categories, (a) pre-seed burnoff (3 to 7 US gpa); (b) in-crop early post-emergence (7 to 10 US gpa); (c) late season application to mature canopies (10 – 20 US gpa).

    To find the right nozzle size, you need to know the application volume, the travel speed, and the nozzle spacing. Most sprayers have 20” nozzle spacing, but some have 15” spacing. Use these metric or US units charts to find the right flow rate for common nozzle spacings. Various on-line calculators from Hypro, Greenleaf / Agrotop, Lechler, or Wilger or their apps, are also helpful.

    If you use our chart, the top row lists water volumes. The columns contain travel speeds. Travel speed is somewhat flexible and can change throughout the field.

    Let’s assume the water volume is 7 gpa, and the desired application speed is 13 mph. Move down the “7 gpa” column, searching for 13 mph. You will encounter 13 mph about 5 times: 02 nozzle @ >90 psi, 025 nozzle @ 60 psi, 03 nozzle @ 40 psi, and 035 nozzle @ 30 psi (the 035 size is only offered by some manufacturers) and the 04 nozzle at about 25 psi.

    Nozzle chart, in US units, solving for 7 gpa at 13 mph. Five nozzles can produce the required flow, each at different pressures.

    Note that for the smaller nozzle sizes, the spray pressure is perhaps too high, and for the larger sizes, it is too low. Select a size that allows optimum nozzle performance and travel speed flexibility. In this example, the 025 size is optimal, producing an expected pressure of about 60 psi. The column for the 025 nozzle can now be used to predict the travel speed range from 30 psi to 90 psi, about 9 to 16 mph. For the 03 nozzle, the minimum speed would be 11 mph, too fast for some.

    For Pulse Width Modulation (PWM), slightly different rules apply. See here for instructions.

    3. Select the Nozzle Model

    For general spraying, we recommend intermediate spray qualities ranging from Medium to Very Coarse.

    These intermediate spray qualities offer good coverage at reasonable water volumes and good drift control. Their spray quality can be tailored with pressure adjustments to suit specific needs. For images, see here. In alphabetical order:

    Air Induced:

    There is plenty of selection in this popular category, all manufacturers offering similar specs and performance.

    Pulse Width Modulation:

    PWM nozzle selection is improving, but some gaps in availability remain.

    All nozzles should be operated near the middle of their pressure range, for air-induction this is 50 to 60 psi or higher, a bit less for non air-induced types. This allows maximum flexibility when travel speeds change or when spray quality is adjusted with pressure.

    For fusarium headblight, consider a twin fan nozzle.

    Keep your booms no more than 15” to 25” above the heads for best results.

    Air Induced:

    There is an excellent selection of twin fans from most manufacturers.

    Pulse Width Modulation:

    Relatively poor selection, limited flow rate ranges or spray qualities available for some models.

    For finer sprays (lower water volumes), simply increase spray pressure or consider a non-air-induced design.

    There has always been a large selection of finer sprays on the market, remnants from a time when drift was less important. Very few offer flow rates above 06 or 08, decreasing utility for PWM systems.

    Notice that conventional flat fan tips and most pre-orifice tips are absent from these lists. These nozzles are not recommended for herbicides because they produce sprays that are too fine for acceptable environmental protection (ASABE Fine and Medium). The added coverage afforded by such sprays only has value with low water volumes, and in those instances is more than offset by their higher drift and evaporation. An exception is the use of insecticides with contact mode of action targetting small insects such as flea beetles or aphids. In thes cases, finer sprays (ASABE Fine or Medium) may be required to provide effective tragetting.

    Very high flows are sometimes needed (11010 and above, usually for PWM). When this occurs, conventional flat fans have merit because the higher flow rates of any nozzle usually create coarser sprays, and even conventional tips will create sufficient coarseness to prevent drift.

    For the best drift protection, consider these tips.

    The advent of the dicamba-resistant trait in soybeans has spawned interest in very low drift tips that comply with the label requirements for these products. Although superior for drift control, they are not well suited for low volume or low-pressure spraying, nor for contact herbicides or grassy weeds, as spray retention and coverage may be poor. But they are very valuable when drift control is paramount and when higher volumes can be used to maintain adequate coverage.

    The following advice is based on the rules at the time it was written. These may be suitable for 2,4-D application in Australia under the newest APVMA guidelines (check spray quality to be sure it is VC or coarser). Many are also suited for Dicamba in Canada (must be XC or coarser), or dicamba in the US (must be on approved lists such as this one for Xtendimax or this one for Engenia, but caution is advised, some low pressure limits make them impractical. Always check that spray quality can be achieved at pressures that offer travel speed flexibility.

    Air Induced:

    Excellent selection. This market has received much attention in recent years.

    Pulse Width Modulation

    Before making a selection, check the nozzle’s recommended pressure range and the spray qualities within that range from the manufacturer info. The target pressure for these tips may differ from your expectations.

    4. Tweak and Confirm

    Under field conditions, the spray pressures which produce the desired water volumes can vary from the charts. Make sure you trust your pressure gauge reading and know the pressure drop from the gauge signal to the nozzles, particularly with PWM, where the solenoid adds additional drop. Add the pressure drop to your target pressure reading. If using a rate controller, use the pressure gauge as your speedometer to ensure optimal nozzle performance. Adjust travel speed until the nozzle pressure meets with your spray quality and pattern goals. If that speed is too slow or fast…you have the wrong size nozzle and/or water volume.

    Spray pressure is more important than travel speed – make your pressure gauge your speedometer.