Tag: drift

  • 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 How badly out of date are the thoughts we mulled over?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.
  • The Most Important Developments in Spraying

    The Most Important Developments in Spraying

    Some things have improved a lot. Others have lost ground.

    Some years ago, a few of us weed scientists sat around a table and debated the most important developments in agriculture in our lifetimes. It was a great discussion, and we arrived at a few that included direct seeding (for its soil and moisture conservation as well as improved fertilizer placement), GMO crops (for slowing Group 1 and 2 herbicide resistance), and the abandonment of summer fallow in much of western Canada. Let’s apply this exercise to spray application to see what we come up with.

    What follows are my version of the most important spray technology developments in the last 50 years.

    1. Low-drift Nozzles. Spray drift is the biggest time management challenge and also perhaps the biggest public relations battle. These nozzles reduce drift, making more time available for spraying and doing it safely and effectively.
    2. Rate Controllers. I both love and hate these things. On the one hand, a rate controller matches sprayer output to travel speed. On the other, it has allowed spray pressures to go wherever they need, even beyond the optimum, to match travel speed, and that can lead to nozzle performance issues.
    3. Pulse Width Modulation. The pulsing nozzle fixes the rate controller problem mentioned above. Now, travel speed and pressure are independent. Plus, of course, a whole host of other flow management options, such as turn compensation and rate boosting, become available.
    4. Optical Spot Spraying. Once you see these in action, you can’t go back. Why would you spray a whole field when weeds only cover 10% of it? Products like WEEDit and WeedSeeker are proven green-on-brown performers after years of field success around the world.
    5. GPS Guidance. Some of us grew up with foam or disk markers, others learned to aim for brave family members perched on headlands. Achieving accuracy was stressful, overlap was insurance, and misses were common. The importance of this development is probably under-estimated.
    6. Sectional Control. The ability to adjust the spray width in individual nozzle steps makes sense, and this can come with or without PWM. In fact, that alone can save 5% of an annual chemical bill compared to conventional sections measuring about 10 to 15 feet. And it’s definitely better than the left boom or right boom options from the 70 and 80s.
    7. Operator Comfort and Safety. The refuge of the cab makes longer days bearable for all equipment, but for spraying it dramatically improves safety as well.

    But we’re far from done. We still need work in these areas.

    1. Cleaning and Waste Management. I can’t imagine another industry where managing potentially hazardous leftover materials are left to the discretion and circumstances of the applicator. Let’s make it easy and fast to thoroughly clean the sprayer and safely dispose of leftovers. Step 1 is smarter and simpler plumbing.
    2. Boom Stability. Booms are too high, resulting in more drift and poorer nozzle performance, and adding to operator stress. The sole reason is unsatisfactory levelling. It’s possible to solve this, but it seems to not be a priority.
    3. Weight. The road to productivity seems to be paved with larger, heavier machines. The side effects are fuel consumption, compaction, getting stuck. Let’s get smarter with frame design and logistics and talk acres/h rather than tank capacity and power.
    4. Cost. All farm equipment has seen cost increases that far outstrip inflation or any reasonable accounting of productivity and features. Sprayers lead the way. Yes, it’s possible to spins this as a value proposition. But it shouldn’t be necessary.
    5. Drift Management. Sprayer design continues to ignore drift management. We need sprayers that produce less drift by design, and this requires consideration of tractor unit, wheel, and boom aerodynamics. It’s more than a droplet size issue.
    6. Direct Injection. Although very handy for single product application, the plethora of product formulations and mixes has limited the success of direct injection systems. The complexity of injecting at the nozzle, and the resulting lack of available systems, has stymied some very attractive options, such as site-specific rate or product use.
    7. Ergonomics. If you need training, or to call someone before using your new sprayer for the first time, something’s wrong. Interfaces need to be intuitive and simple. The golden age of spray monitors was the 1980s. Those featured a main power toggle switch, a pump power switch, boom section switches, an agitation switch, and a simple way to enter the important information which was basically desired application volume. The screen can still be pretty, and you can still paint and monitor or tweak all the functions if you like that. But let’s at least have different tiers so beginners can also use the machine. Make interfaces using the philosophy Steve Jobs instilled in his trusted designer Jony Ive with the first iPod: no more than three clicks to achieve any desired outcome.

    A few areas show promise and may suit certain niches.

    1. In-Crop Weed Sensing. The green-on-green sensing that has been made possible by machine learning has shown some encouraging early success. Continuing improvements will eventually bring its reliability to within commercially acceptable standards. There is significant activity below the radar in this area, as all players recognize the enormous upside of a breakthrough.
    2. Autonomy. While dispensing a pesticide adjacent to sensitive areas isn’t exactly the low-hanging fruit of autonomy, such field sprayers will have a fit in the temperate plains of North and South America, Australia, and Asia and may help solve the cost and weight problem.
    3. Drone Application. The rapid pace of advancement in remotely piloted aerial systems, along with a seemingly low barrier to entry of new companies, will put pressure on the industry to make a decision on this alternate application method. If it can be done safely, it will have a dramatic impact.

    If you want to improve your sprayer, don’t ignore the small things you can do in your operation. Although we’re conditioned to look for game-changing technology, the most sustained improvements don’t come from a single innovation, but from a period of persistent evolution. A lot of small improvements add up. Spray application is no different.

  • 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.

  • Don’t try this tempting shortcut

    Don’t try this tempting shortcut

    There’s a call that I’ve been getting for 20 years now. It came again this week. Someone has a twincap with two small air-induced tips, and they’re applying herbicides and fungicides with low water volumes, often 5 gpa, sometimes less. They call because they want to know how much wind they can spray in. Is 30 km/h OK? They want my blessing.

    I don’t need to hear much more. Some nozzles are sold entirely on the premise that they provide superior coverage – more droplets per square inch – and that this improved coverage permits the reduction of water volumes. Furthermore, the claim goes, when water is reduced, the spray concentration increases and the whole darn package just works a lot faster and better.

    This line of thinking is as old as spraying itself. Applicators seek pesticide performance as well as productivity, and this approach gives them both. The proponents are well aware of their customers’ desires, and sell into it. “Use these tips and cut back on water. Any more than this just runs off anyways. You’ll get better coverage and better performance, get more spraying done.” It’s a convincing argument. Get an edge on your neighbour, the person who’s not in on the secret and is wasting time and water.

    Why don’t I embrace it? There are a few reasons.

    First, it doesn’t tell the whole story. Invariably it involves a twin nozzle setup. Use two nozzles, get more droplets, right? If that were true, believe me, I’d be advocating for quintuples.

    Fact is that the only factors that change droplet numbers are droplet size (spray quality) and water volume. Want more droplets at the same water volume? Make the spray finer. Want to keep spray quality and add droplets? Add water (not nozzles).

    The easiest way to improve coverage at the same volume is to use a finer nozzle, or to increase spray pressure. Depending on how far you go, you could make the spray finer and cut water, and still have more droplets per square inch.

    The hardest way to improve coverage is to purchase a twincap and buy two nozzles, each of them half the size. True, within any given nozzle type, smaller sized tips usually generate finer sprays. But why bother with two tips? They’re more expensive and plug more.

    If someone asks me how to improve coverage without changing water volume, I usually tell them to speed up a few mph. The rate controller will increase pressure and the spray gets finer. If speeding up is not possible, get one size smaller nozzle and run at higher pressure, same speed. Or keep nozzle and speed, and add some gpa, pressure will go up. It’s that easy. No twins necessary.

    Second, the twin nozzle/low volume approach exaggerates the value of the twin nozzle for herbicides. With small plants and relatively open canopies in the early season, plus our high booms and travel speeds, the twin tips are not adding a lot, if anything at all, to coverage. It remains a sum of droplet size and water volume, the angle is not important at this stage. Deposit is by turbulence and wind, most of the time.

    Third, low volume believers ignore a few potential problems. Drift is a big one. Low volume, fine spray operators are surrounded by nervous neighbours. They have fewer hours per day during which drift is acceptably low. And they definitely should not be on the field when wind is at 30 km/h. Basically, they’re a bit uncomfortable (at least they should be) and get less done per day.

    Another potential problem is evaporation. Most sprays, even when applied at lower volumes, are still 90% or more water. The same volume of water evaporates much quicker when atomized into smaller droplets. This has two main downsides: On their way to the canopy, small droplets evaporate and become even more drift prone, and may not impact at all. Those that impact evaporate shortly thereafter. Research has shown that pesticide uptake is better from wet than dry deposits.

    When Delta T (dry bulb minus wet bulb temperature) is high, evaporation can be so strong that it reduces pesticide performance or causes solvent burn. Fine sprays make it worse.

    I also hear about the use of oily adjuvants to control evaporation from small droplets. This could be even more dangerous. Small droplets drift, and evaporation to dryness is actually helpful in reducing the impact of that drift. How? It makes the small droplets disappear, with their remnants dispersing into the turbulent atmosphere. With oily adjuvants, the small droplets stick around and stay potent and their drift damage is much worse.

    Lastly, the practice is possibly off label. Water volume and spray quality label statements are designed to offer good performance and acceptable drift risk. While that part of the label is often a bit dated, it does provide better support from the manufacturer should something go wrong.

    If you’re spraying under hot, dry and windy conditions, the low volume, fine spray approach is irresponsible. Use sufficient water (7 to 12 gpa) to allow low-drift sprays, at least Coarse to Very Coarse, in some case, even coarser.

    Agronomists provide the best possible information for their clients, based on scientific evidence and experience and in accordance with their professional code of ethics. Sometimes the news we deliver aren’t what the customer wants to hear. But we have to represent the interests of all of us, collectively. I find that pretty important.

  • Thermal Inversions for Sprayer Operators

    Thermal Inversions for Sprayer Operators

    In April 2014, NDSU extension published an excellent factsheet explaining what thermal inversions are, how to detect them and how they affect pesticide spray drift. That factsheet inspired this article.

    The Atmosphere

    The Earth is surrounded by a layer of air called the atmosphere. Think of it as a sheet of liquid percolating and flowing over the Earth’s surface. Seems a bit precarious, doesn’t it?

    We define “layers” of atmosphere based on their distance from the Earth’s surface (see image below). We’ll focus on the lowest part of the Earth’s atmosphere: the Surface Boundary Layer. As it drags along the Earth’s surface it experiences rapid changes in wind speed, temperature and humidity (on a time scale of an hour or less).

    The Earth’s Atmosphere. The illustration of the Earth is to scale, but the landscape is not. Our focus in on the Surface Boundary Layer.
    The Earth’s Atmosphere. The illustration of the Earth is to scale, but obviously the landscape is not. Our focus in on the Surface Boundary Layer.

    Atmospheric temperature

    In relatively calm, clear and dry conditions (e.g. a nice afternoon), air cools with elevation at a rate of about 1°C per 100m. This change is called the Adiabatic Lapse Rate and it’s caused by pressure changing with elevation. If your ears have popped when driving down a steep hill, you’ve experienced pressure change with elevation; there is more atmosphere overhead and the weight pushes down.

    With higher elevation, there is less atmosphere overhead. Less weight means less pressure and this gives air room to expand. Expansion takes work and work costs energy, which creates a cooling effect. See how simple thermodynamics are?

    In the graph below, the red line shows the Adiabatic Lapse Rate of air cooling with elevation. The blue line indicates wind stirring and homogenizing the atmosphere, reducing the degree of temperature change with elevation (more on that later).

    Day and night

    When we add the effect of daytime solar heating and nighttime cooling, the rate of temperature change is affected. Let’s consider how this works on a clear, relatively calm day:

    Early morning

    The morning sun emits short wave radiation, which is absorbed by the Earth’s surface. The surface conducts some of this energy deeper into the ground and also heats the air near the surface. This creates a temperature gradient wherein the surface is warmest and the air gets relatively cooler with elevation (remember the red line in the graph above).

    As the air near the surface warms, that energy causes air molecules to vibrate and push away from one another. Parcels of air become less dense and rise just like the gloop in a lava lamp. The cooler air around it falls to fill in the space left behind, and air begins to circulate in a Convection Cell. The rising parcel of air will eventually cool and shrink as it rises through the relatively cooler air above it.

    These convection cells create Thermal Turbulence, which is a very effective way for airborne particles, such as pesticide vapour, to be rapidly diluted. This is also how the atmosphere disperses pollution. More on the process of dispersion, later.

    Mid to late afternoon

    As the sun passes over and the wind starts to rise, the convection cells get disrupted by the wind and experience mechanical turbulence (remember the blue line in the graph above). So, mechanical turbulence also mixes warmer air near the ground with cooler air above it, but suppresses thermal turbulence.

    Mid-afternoon to night

    As the energy from the sun lessens, the soil begins to cool and so does the air next to it. Once the air cools enough to be colder than the air above it, we have the beginning of a Radiation Inversion, which is a specific kind of Thermal Inversion (see the green line in the graph below). It is called that because we now have the reverse of the typical day-time temperature profile. The height of the inversion (the ceiling) grows with time, and can reach a maximum of about 100m by sunrise. Within the inversion layer (before the green line bends back at 100m), turbulence is suppressed. We have a stable air mass. More on that below.

    How inversions affect dispersion

    The rising portion of a convection cell carries whatever particles are in the air with it. Suspended particles become much less concentrated at ground level thanks to the thermal turbulence.

    Thermal Turbulence allows particle-laden warm air to rise and clean cool air to fall. This disperses air-borne particles like dust or pollution.

    Now let’s imagine we are in a thermal inversion. The cooler, particle laden air near the ground cannot rise and the cleaner air above, which is now relatively warmer, cannot sink. Thermal turbulence is suppressed, and so is any vertical dispersion.

    Thermal Turbulence is suppressed during a Temperature Inversion. Particle-laden cool air at the surface cannot rise, and warm, clean air cannot fall. No dispersion occurs, and the concentrated, particle-laden air tends to move downhill or laterally with light winds.

    When spraying, the smallest spray droplets fall slowest, staying airborne for long periods of time. If spraying occurs during an inversion, those particles accumulate beneath the inversion layer. Remember we said our atmosphere behaves like a liquid? The colder, denser (pesticide-laden) air drains downhill into low-lying areas. It can also move laterally over great distances, in unpredictable directions, when light winds begin.

    Clouds

    If the morning were overcast instead of clear, the clouds would intercept much of the sun’s short-wave radiation, absorbing or reflecting it back into space. The Earth’s surface would still warm, but more slowly, suppressing thermal turbulence. As an aside, if clouds form in the evening, they reflect long-wave radiation from the Earth’s surface back down. This Greenhouse Effect is why overcast nights are warmer than clear ones.

    Therefore, extended periods of mostly clear skies in the evening or night means a high probability of strong temperature inversions. Conversely, cloud cover usually means a near-neutral atmosphere, so no strong inversion.

    Wind

    Inversions are only mildly affected by light wind (e.g. 6 to 8 km/h), but as the wind increases and mechanical turbulence mixes the air, the strength of the inversion will be reduced and the atmosphere will approach a neutral condition (see the blue line). In this condition, airborne particles are not dispersed by thermal turbulence, but some mixing will occur. So, there may not be a thermal inversion, but spraying would still be inadvisable if the wind got too high.

    Humidity

    Inversions form more rapidly when there is less water vapour in the air to absorb radiation. Once humid air has cooled to the dew point, water condensation gives off energy and warms the air a little. This slows the formation of the inversion. Be aware that inversion conditions can exist long before fog, dew or frost forms, so they are not a good indicator for the beginning of an inversion – you’re already in one!

    If you see fog, dew or frost, you’re already in an inversion. The air has become cold enough to condense or even freeze water.
    If you see fog, dew or frost, you’re already in an inversion. The air has become cold enough to condense or even freeze water.

    Soil conditions and topography

    This is a complex issue, but soil conditions that make inversions more intense include low soil moisture, freshly tilled soils, coarse soils, heavy residue and closed crop canopies. Topography matters, too. We’re discussing radiation inversions in arable regions, and the kind that form on mountains or deep valleys. Nevertheless, inversions in shaded areas (e.g., behind windbreaks) start sooner, and last longer. See the NDSU factsheet for more detail.

    Spray timing

    Inversions, once formed, persist until the sun rises and warms the Earth’s surface, or until winds increase and mix the stationary layers of air together, re-establishing a more neutral temperature profile.

    Sunset is not a good indicator of the beginning of an inversion – it can start a few hours before. Therefore, evening spraying may be just as risky as night spraying. Very early mornings (e.g. around sunrise) are not much better. Remember, at sunrise, the inversion will be at its maximum height.

    The rising sun will warm the earth and create turbulent conditions, starting near its surface (e.g. a few metres). Most inversions will have dissipated two hours after sunrise, which may be the best choice for spraying.

    Detecting an inversion

    The only sure way to know if you are in an inversion is to take two air temperature readings: one near the ground and one about three metres higher. If the surface air temperature is cooler, you are in an inversion. The magnitude of the difference indicates how strong the inversion is.

    Accurate measurements are difficult to manage with conventional thermometers, but SpotOn now makes a hand-held detection unit. If you have one, be sure to let it acclimate before you use it. Leaving it in a hot, or cold, truck or sprayer cab prior to use means it may give a false reading.

    Inversion forecasting is getting better, but it’s still location-specific and not entirely reliable. Sprayer operators should learn to watch for the following environmental cues:

    • Large temperature swings between daytime and the previous night.
    • Calm (e.g. less than 3 km/h wind) and clear conditions when the sun is low.
    • Intense high pressure systems (usually associated with clear skies) and low humidity where you intend to spray.
    • Dew or frost indicating cooler air near the ground (fog may be too late).
    • Smoke or dust hanging in the air or moving laterally.
    • Odours travelling large distances and seeming more intense.
    • Daytime cumulus clouds collapse toward the evening.
    • Overnight cloud cover is 25% or less.

    Note: If you suspect a temperature inversion, don’t spray.

    For more information on how weather affects drift, download this pamphlet from the Australian Government Bureau of Meteorology.