Category: Spray Basics

Electrostatic Spraying in Agriculture
Dear reader: This article is intended to provide basic information on how electrostatic sprayers work in an agricultural setting. The author does not sell or manufacture sprayers. If your interest is related to spraying disinfectant in private or commercial settings, please contact retailers or manufacturers of electrostatic sprayers.
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Electrostatic nozzles have been tested in agriculture since the late 1970’s. Predominantly used in aerial applications, they are sometimes employed on airblast sprayers in orchard and berry operations, and on horizontal booms in vegetable crops. To a lesser extent, they are even mounted on wands for low acreage applications.
Claims
Independent research, manufacturer claims and user testimonials are intriguing:
- Improved coverage uniformity (i.e. underleaf coverage, panoramic stem coverage and canopy penetration).
- Improved retention (>50% better than conventional) and/or potential savings of 50% spray mix.
- Reduction in losses to soil.
- Improved efficacy with both insect and disease control.
So it begs the question: “Why doesn’t everyone have an electrostatic sprayer?” We performed a study in carrot in Ontario’s Holland Marsh to explore some of the claims and to get a first-hand experience with the technology. That article might help answer the question. But first, read this article which explores the basic principles behind how electrostatic applications work.
Charging the Droplet
Spray is charged by a high voltage supercharger. Commonly, the charge is induced by an electrode positioned close to the atomizing spray plume as droplets begin to form. This is referred to as coronal discharge. An intense electric field imparts a positive or negative charge depending on the polarity of the DC power used. Think of it as high-voltage static electricity.
Sometimes the spray is atomized by a hydraulic nozzle (e.g. a hollow cone) and sometimes using an air-shear nozzle. The latter has the added advantage of blowing droplets away from the electrode and projecting them into the canopy.
Let’s consider a negatively-charged droplet (see diagram below). The droplet becomes polarized when it passes through the electric field. The field attracts electrons to the droplet surface and repels positrons towards the centre. The droplet now has its own field that electrically motivates it to land on neutral objects. As they approach such an object, the negative charge on the droplet surface repels mobile electrons on the surface of the target, which redistribute, creating a relative positive charge on the surface and attract the droplet.
Another style of electrostatic technology employs a highly charged plate along the air outlet of the sprayer, generally attached just inside the duct. The clearance between the droplets and the plates is quite large in relation to that in a twin-fluid atomizer, coil-type charging system.
Droplet Size
Droplet size is a critical factor. Droplets must be large enough to resist evaporation and drift but small enough that the charge can change their trajectory when it comes close to a target (I.e., the Charge-to-Mass Ratio). Most electrostatic nozzles produce ~50 µm droplets, categorized in agriculture as Very Fine. For comparison, a human hair ranges from 20 to 180 µm. Fog is about 5 µm. Such a small droplet means that the distance between nozzle and canopy is a determining factor for the spray depositing, or drifting.
Droplet Behaviour
Many forces influence droplet behaviour (E.g., inertia, wind, gravity, etc.). Very Fine droplets have a low terminal velocity causing them to fall slowly (~40 seconds to fall 3 m). This makes them highly drift-prone. However, simulations have shown that a charged droplet released close to a grounded target would be “pulled” faster than an uncharged droplet. Further, their trajectories would be less affected by air movement and they have the potential to move upwards against gravity towards the underside of a leaf.
Of course the droplets must reach the canopy before any of these potential advantages can be realized. Even with air-assist to project the spray into the canopy, it has been shown that the droplet must be within two centimetres of the target before attraction improves deposition. There are many physical phenomena that influence this process:
The Faraday Cage Effect can occur when spraying dense canopies. The spray deposits on the first grounded object it encounters. This is the outer surfaces of the canopy and the spray can be prevented from moving deeper into dense canopies. Regarding arable crops, there is often a naturally occurring negative charge on the earth’s surface that repels negatively charged spray. This may be why studies often report reduced loss to soil.
The Corona Effect is a very complicated relationship between the shape, density and spacing of the crop and it’s influence on charged spray. Research has shown that deposition is better for rounded targets than pointed. The gaseous exchange of charges between leaf tips and spray can neutralize or even repel droplets. This may be why electrostatic demonstrations so often include fruit or spheres.
The Expansion Cloud Effect (or cooler, the “Space Cloud” Effect) describes how charged droplets are repelled by objects with a like charge. Coulomb’s Law describes how objects with an opposite charge attract, but it also says objects with a like charge repel. Since the droplets all have the same charge, they repel each other. While this causes the plume to expand into the canopy and helps to distribute the droplets to give uniform coverage, it also causes droplets to expand upwards away from the crop, making them susceptible to drift.
Observations
The opportunity for reduced pesticide use is appealing and it may entice consumers to consider the electrostatic sprayer as a more environmentally-conscious choice. However, we have found very few studies relating to drift, and opinions are mixed whether electrostatic applications are any more drift-prone than conventional applications.
Considered collectively, electrostatic applications seem to perform well in controlled conditions, but the complications arising from variability in a natural environment coupled with the cost of equipment has slowed adoption. The current rules for practical adoption are poorly defined. More fulsome drift studies are required and coverage uniformity and canopy penetration (particularly from ground rig systems) must be consistent in real world settings.
Nevertheless, electrostatic applications have a lot of “potential”.
October 10, 2018
Diagnosing Airblast Coverage
Assuming there are no mechanical or maintenance problems, water-sensitive paper can be used to diagnose sprayer performance. Go here to read more about water-sensitive paper. Interpreting the results and knowing what changes to make is the critical part of the process. Observing no coverage, or a sodden paper, make for obvious conclusions… but what about everything in between? Here are the ground rules:
First: Only ever test coverage in environmental conditions you would normally spray in. Temperature, humidity and wind speed can make or break an airblast calibration.
Second: When altering sprayer settings, only make one change at a time for each test pass so you can isolate what’s wrong.
Third: Each pass requires a new set of papers located in the same place, oriented the same way, distributed throughout the canopy. Mark their locations with bright flagging tape and write the pass number and canopy position on the back of paper prior to placement. This helps you to compare the passes later on. Don’t collect papers until they’ve had an opportunity to dry a little, or they will smear and stick together.
Fourth: Pass down one alley first. Have a look at the papers without removing them. Then, spray the target canopy from the other side. Now the papers can be removed for analysis. This order is important because it reveals the impact of wind direction and the cumulative effect of spraying from both sides. In some cases, the sprayer operator may wish to travel an additional upwind alley to reflect the cumulative coverage on a typical spray day. Alternate row applications are not recommended.
This Turbomist has been outfitted with Gillison’s Sonic Spray sensors that detect the presence of a canopy. Each eye corresponds to a boom section, turning the section on and off as required and improving efficiency. If it’s not there, why spray it?
Once the papers are retrieved, it’s time to diagnose the coverage. The following situations are typical in calibrations, and possible fixes are suggested. Remember, this is a process that takes time. Several passes may be required before satisfactory coverage is obtained. Once the correct settings are determined for the block, continue to use them until there is a significant change in the crop staging or weather. At that point, repeat the process.
Seven Situations
Situation One:
<15% coverage and <85 Fine/Medium droplets/cm² at top of target (e.g. tall targets such as hops or trees). Suggested Fixes:
- Wind might be stealing fine droplets. Try Coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts and that they may fall out of the airstream before reaching distant targets.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
- Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
Situation Two:
<15% coverage and <85 Fine/Medium droplets/cm² deep in canopy – sometimes papers on outside of canopy are visibly wet. Suggested Fixes:
- Ground speed may be too high. Use flagging tape indicator on far side of target and see if air is getting through.
- Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
- Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
- Increase carrier volume.
Situation Three:
Papers are drenched, dripping or show channels of running liquid. Suggested Fixes:
- Reduce spray volume, either overall or in key locations on the boom corresponding to the drenched papers.
- Ground speed may be too low. Use flagging tape indicator on far side of target and see if too much air is getting through. If so, increase ground speed.
Situation Four:
Considerable overspray beyond target row. Suggested Fixes:
- Turn off upper nozzles until spray JUST clears target.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
Situation Four:
Considerable blow-through beyond target row. Suggested Fixes:
- Slow the fan speed by shifting to low gear, or using GUTD method
- Ground speed may be increased as long as coverage is not compromised. Use flagging tape indicator on far side of target and see if air is getting through.
Situation Five:
Ground under target row is drenched. Suggested Fixes:
- Rotate lower nozzles slightly upward, but do not shut them off. If ground remains drenched, turn them off entirely. Each hollow cone produces up to an 80º spray angle, so the next higher nozzle often compensates by spraying lower than expected.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
Situation Six:
<15% coverage and <85 Fine/Medium droplets/cm². Remember that this coverage threshold is only a point of reference, not a hard fact. It does not apply when using Coarser droplets. Suggested Fixes:
- Increase spray volume, either overall or in key locations on the boom corresponding to the under-sprayed papers.
- Wind might be stealing fine droplets. Try coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts.
- Ground speed may be too high. Use flagging tape indicator on far side of target and see if enough air is getting through. If not, decrease ground speed.
- Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
Situation Seven:
Inconsistent coverage on outer edge of canopy (e.g. one spot never seems to get spray.) Suggested Fixes:
- Nozzle spray angle may be too acute (e.g. full cones), and spray is not overlapping before reaching target. Try wider spray angles.
- Some tower sprayers have ‘dead spots’ in their air. Check for limp or flagging ribbons tied to nozzle bodies and/or deflectors. Deflectors may need to be adjusted, or adjacent nozzle body angles repositioned to compensate. Try an air induction nozzle in the dead zone.
- Canopy may be brushing against nozzles as the sprayer passes, temporarily blocking them. Canopy management required.
Some sprayers, such as Rears, Turbomist, FMC or this Durand Wayland have an option for electronic ‘eyes’ that detect spray targets. The boom will shut off completely if there is a gap in the planting. This can save a great deal of wasted spray. It is less applicable in trellised plantings where it has been known to be “fooled” by wires and posts.
If you still are unable to achieve satisfactory coverage, you may have to consider more extreme solutions. You may have an under- or over-powered sprayer. You may have to perform significant canopy management. Or, you may be trying to spray in poor weather conditions.
August 29, 2018
What is Delta T and why is it important for spraying?
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Humidity is important in spraying. With the average tank of pesticide being 90 to 99.5% water, evaporation plays an important role in both droplet size and active ingredient concentration. Low humidity causes droplets to evaporate faster, potentially increasing drift and reducing uptake. But relative humidity (RH) isn’t the best way to measure this effect because the same RH at two different temperatures results in two different water evaporation rates.
Instead, we present Delta T, also known as “wet bulb depression”. Delta T is an atmospheric moisture parameter whose use in spraying has made its way to North America from Australian operations. It is defined as the dry bulb temperature minus the wet bulb temperature, and provides a better indication of water evaporation rate than RH. Higher Delta T means faster water evaporation.
The recommendations from Australia are to avoid spraying when the Delta T is either too high or too low, with a range of two to eight being described as ideal.
Figure 1: Delta T chart used in Australia (Source: Australian Gov’t Dept of Meteorology)
Delta T is being reported on an increasing number of weather stations, and it’s time we took a closer look at what it means.
Measuring Relative Humidity
In the early days of weather reporting, relative humidity was calculated from psychrometric charts. All one needed was a hygrometer, usually a sling psychrometer. A sling psychrometer is two identical thermometers side by side whose bulbs could be slung in a circle, exposing them to moving air. One bulb was covered in a cotton wick moistened with distilled water, the other was left exposed and dry.
Figure 2: Sling psychrometer (Source: ScienceStruck.com)
As the bulbs met moving air, water evaporated from the cotton wick and that reduced the temperature of that thermometer. The dryer the air, the greater the evaporation rate and therefore the greater the temperature drop. The dry thermometer was unaffected by this movement.
On measuring the wet and dry bulb temperature, one consulted a psychrometric chart. This chart converted the two temperatures to total water content in the air, compared it to total water-holding capacity, and expressed it as Relative Humidity. Psychrometric charts are useful for many other air parameters such as dew point, vapour pressure, or enthalpy. (Pause briefly to give thanks that we don’t need to know what enthalpy is.)
Figure 3: Psychrometric Chart (Source: Carrier Corporation)
Turns out that RH is a poor measure of water evaporation rate. An RH of 24% at 20 C has exactly the same evaporation rate as an RH of 44% at 35 C. That’s why Delta T is the preferred measurement: it’s linearly related to evaporation.
Note: Modern electronic weather stations don’t need two thermometers to measure air moisture content, and use polymers whose capacitance or resistance changes with atmospheric moisture. Add an internal look-up table, and we have all the information we need.
Pros and Cons of Water Evaporation
It’s important to note that our Australian colleagues caution against spraying when water evaporation rate is both too high and too low.
Too High:
- Water evaporates rapidly, reducing droplet size and pre-disposing the smaller droplets to drift;
- Deposited droplets dry quickly, reducing pesticide uptake which is more effective from a wet deposit.
Too Low:
- Water doesn’t evaporate, maintaining the smaller droplets in a liquid state. These small droplets are already drift prone, but are now more potent because of more effective uptake. Overnight conditions that are inverted are usually humid, adding to harm potential from the inversion.
Delta T in North America
The addition of this parameter to our spraying weather lexicon has been useful. But it’s important to understand the context in which it was developed to properly judge its suitability.
Aussies started talking about Delta T because the use of finer sprays under the hot dry conditions found during their summer sprays resulted in significant evaporative losses, significantly greater drift potential, and potential reduction of product performance. The guidelines to avoid spraying when Delta T exceeds eight or ten originate there.
A few changes have happened since these guidelines were developed. Over the past ten to 20 years, we’ve observed greater use of low-drift sprays, with the coarser sprays’ larger droplets resisting fast evaporation. In the past five to ten years, water volumes have increased due to our heavier reliance on fungicides, desiccants, and contact modes of action. Both of these developments have helped reduce the impact of a dry atmosphere. We simply can’t say if a Delta T of 10 is too high with these new application methods.
Looking at it another way, if Delta T values are very high, increasing water volume and droplet size will mitigate that to some degree, as the Aussies state in their extension materials (linked earlier).
Formulation
Pesticide formulation can also play a role in evaporation. Once the water is gone, oily formulations may still have good uptake because the oily active ingredient stays dissolved in the oily solvents. This is both good and bad, helping on-target efficacy but also increasing the risk of more potent drift. Solutions, on the other hand, are more likely to leave their actives stranded on leaves as crystals once the water is gone.
Bottom Line
Delta T is definitely useful information when spraying. It will typically rise and fall with air temperature as the day proceeds, and it is wise to consider suspending operations when values are critical. Take note of the Delta T when spraying the same product throughout these hot days and learn from the experience. Remember, the atmosphere affects not just sprays but also plants and insects, and due to this complexity we may not be able to attribute success or failure to just one measurement.
June 19, 2018
Biobeds for Pesticide Waste Disposal
One of the most challenging aspects of a spray operation is the disposal of leftovers or rinsate containing pesticides. Let’s be honest, too much of it is drained onto the ground in a corner of the yard or the field. Nobody’s happy about that, nobody’s proud of it, but what are the alternatives?
Waste disposal is a skeleton in the closet of the pesticide industry. One of the problems is the time-consuming nature of sprayer cleaning, and the lack of clear guidelines on product labels that pass the buck. Too often, the applicator is asked to “act in accordance with provincial or state guidelines”, which is essentially a dead end.
Figure 1: Sprayer fill station
At Sprayers101.com, we’ve tried to tackle the problem by finding ways to generate less waste (Express End Caps, Accu-Volume), by disposing of the rinsate by spraying it out, or by installing an efficient continuous rinsing system. We’d now like to talk about another component, biobeds.
What is a biobed?
Simply put, a biobed is a place where it’s safe and acceptable to dump dilute pesticide waste. First implemented in Sweden about 20 years ago, a biobed typically consists of a 1-m deep pit measuring about 3 m x 6 m or so. The pit is filled with a biomix, a mixture of cereal straw, compost or peat, and soil. The biomix, when properly prepared, acts to absorb a large amount of moisture, adsorb the pesticide molecules, and provide an environment in which microbes break down the residues.
Figure 2: Canada’s first commercial biobed installation at Indian Head, SK, 2009 (Source: Murray Belyk, Bayer CropScience (retired)).
The effluent from a properly constructed biobed system contains 90 to 99% less pesticide than what was introduced, depending on the pesticide.
Biobeds have been extensively studied and are now found throughout Europe and many parts of Central and South America. Canada currently has 6 research biobed sites in the West, and a further 17 in Quebec. The systems have been researched by Agriculture & Agri-Food Canada (AAFC) in recent years, with promising results.
Figure 3: European biobed installations, 2016 (Source: Jens Husby, Biobeds.org).
Figure 4: Global biobed installations, 2016 (Source: Jens Husby, Biobeds.org).
Constructing a biobed
There are many possible variations of biobeds, some relatively simple and others engineered to address certain specific needs. A great deal of creativity can be used to customize a biobed for any operation.
A simple biobed
The following is a variation of the simplest biobeds, and these are the types first tested by AAFC in Saskatoon and Indian Head, Saskatchewan about 10 years ago. This design is based on the biobeds established in Sweden and the UK, and is a good way to learn about the system.
Note that this biobed has an impermeable liner, so it’s a closed system. Excess water that leaches to the bottom must be removed and cycled back to the top of the biobed.
- Create the biomix by blending two parts, by volume, chopped cereal straw or wood chips (not cedar), one part mature plant-sourced compost or peat and one part relatively coarse-textured soil (for optimal drainage). Add water as necessary as if making compost. Allow to sit for four to six weeks.
Figure 5: Biomix preparation.
- During this waiting time, the biomix will warm and form a white-mold complex. This is the microbial basis for its ability to break down pesticide residues. White mold will be visible on the cellulose portions of the biomix.
Figure 6: white mold (Source: AAFC).
- Identify a well-drained site easily accessible by spray equipment. Avoid low spots as water management becomes problematic.
Figure 7: Site selection and/or biobed covering are essential to avoid waterlogging (Source: Murray Belyk, Bayer CropScience (retired)).
- Dig a pit sized to suit your requirements. As a rule of thumb, 1 m³ can process about 1000 L of liquid in a season. Rainfall is included in this amount.
Figure 8: A nice looking pit.
- Line the pit with a geomembrane liner. 40 mil is plenty thick; any thicker and it gets hard to handle. Include a raised berm at the edge.
Figure 9: Liner creates a closed system that will require a way to remove leached water.
- Install weeping tile at bottom of pit, and extend it to ground level. This will be useful to determine water status and remove water if necessary.
Figure 10: Weeping tile to collect excess water.
- Cover weeping tile with pea gravel and a silt trap. This serves to make leached water freely available for removal.
Figure 11: Pea gravel over weeping tile.
- Fill pit with biomix, anticipating significant settling. Top up as necessary over next few weeks. Use extra biomix to create a slope away from berm.
Figure 12: Filled biobed.
- Establish a bromegrass cover by transplanting or sodding. This is an important way to remove excess water via evapotranspiration.
Figure 13: Early sod growth on biobed at Indian Head, SK.
- Introduce pesticide waste to biobed, managing moisture content to avoid waterlogging.
Figure 14: Pesticide waste entering biobed via drip irrigation.
Introduction of pesticide waste to the biobed
Moving pesticide waste from the sprayer to the biobed should be easy and trouble free. A simple pad built beside biobeds, either sealed with concrete or asphalt, or with a hardy geomembrane liner, works well. The sprayer is cleaned on this pad and rinsate flows into a drain. A sump pump lifts the rinsate to a storage tank from which it is introduced via gravity or pumped drip irrigation.
Figure 15: Biobed system in Simpson, SK. Rinsate from sprayer is collected in a sump, which is pumped to the black storage tank in background. Rinsate is introduced into biobed (blue tub) as needed (Brian Caldwell in foreground, left, Larry Braul, right).
When not in use, the sump drains freely to dispose of rain water.
Others choose to pump or dump rinsate directly into a holding tank, from where it can be pumped onto the biobed.
Figure 16: Holding tank at biobed in Outlook, SK.
Some European systems include driving supports on the biobed so the sprayer can be parked directly over top.
Figure 17: Steel beams can allow (light) sprayer access (Source: Eskil Nilsson via Biobeds.org).
A two-stage biobed
The same basic building principles apply as in the original simple biobed. However, instead of reintroducing the effluent to the top of the biomix as it collects on the bottom, it is instead pumped onto a second biobed. This biobed then degrades any remaining product. This system is more efficient at degrading persistent products, and allows for better water management.
Figure 18: Two-stage biobed system at Outlook, SK.
The principle has proven effective, helping degrade more difficult pesticides to acceptable levels.
Above-ground biobeds
One of the problems with below-ground biobeds in wet climates is the difficulty managing water. Above-ground biobeds can address this issue by eliminating the possibility of surface runoff being added to the biomix. Adding a rain cover would also be easier and more effective.
Above-ground biobeds can be edged with plywood, or placed entirely into plastic tanks whose tops have been removed.
Figure 19: Above ground biobed installation with plastic tub.
One potential problem with above-ground biobeds is the later spring warming of this installation compared to below-ground types. Cold temperature reduces the effectiveness of biobeds due to the reliance on microbial activity. Heat tape has been tested by AAFC and shown to be very effective at warming the biomix and stimulating initial microbial activity. Passive solar systems have also been studied but are more difficult to install.
Figure 20: Heat tape (Source: AAFC).
Figure 21: Passive solar biomix heating system.
Phytobac and Biofilters
European designs have utilized plastic containers to form of various designs, including the commercial “Phytobac” systems from France and developed with the support of Bayer CropScience.
Sequential biofilters have also been implemented. The leachate simply migrates through the biomix into the next container below. Eventually, adjacent biofilters containing plants act to remove the moisture.
Figure 22: Phytobac installation, cross-section.
Figure 23: Biofilter installation in Belgium (Source: Inge Mestdagh via Biobeds.org).
Biomix longevity
Swedish and UK research has suggested that biobeds require minimal maintenance aside from water management in closed systems. Biomix will settle over time and may need to be topped up. After five to eight years of use, it has been recommended to remove biomix and distribute it over a field with a manure spreader.
Canadian research results
Extensive analysis of pesticide degradation in five biobeds across Western Canada was conducted as part of a three-year study led by AAFC. Between eight and 51 products were analyzed per site, including herbicides, fungicides, and insecticides. Their results showed that single biobeds could remove about 90% of the introduced pesticide, and two in sequence usually removed more than 98%.
Pesticides that tended not to degrade rapidly were removed to a greater degree in the second biobed.
In the AAFC studies, three herbicides were more difficult to remove in the tested biobeds: clopyralid (e.g., Lontrel, Stinger), bentazon (Basagran, Storm) and imazethapyr (Pursuit, Arsenal). For these three, roughly 60% was removed in a two-biobed system.
Concentrated pesticides should not be introduced to a biobed as this will kill the microbial populations.
Some fungicides were shown to depress microbial populations but only temporarily. Microbial breakdown still occurred.
Biobed manual
AAFC has authored a comprehensive manual on biobed operation and installation based on research experience in Canada and elsewhere. It will be available here in late June 2018.
The future of biobeds
Research into biobeds remains active around the world. Different substrates for the biomix are being studied to suit local availabilities. Various systems, ranging from simple to highly engineered are being studied. Degradation effectiveness for various influents remains a topic of significant interest. Producer adoption and implementation are being reported.
Thanks to funded research projects, biobeds are up and working at Canadian institutional sites such as government research centres, and there are opportunities for county and municipal government sites. For biobeds to be a viable option on North American farms, their design needs to remain simple and their integration into established practices needs to be seamless. Producer experience and feedback are essential
Learn more
Valuable information on biobeds can be obtained from these two websites:
Voluntary Initiative (UK industry)
Biobeds.org (International research)
Note: Brian Caldwell and I first learned about biobeds from Eskil Nilsson (website) during a visit to Sweden in 2001, and obtained support for initial studies in Saskatoon and Indian Head from the Pest Management Centre as well as Bayer CropScience. Brian took a lead in our creative and technical efforts over many years. Dean Ngombe, under the co-supervision of Diane Knight at the U of S and myself, produced the first M.Sc. thesis, and with significant input from Allan Cessna, the first scientific publications in Canada on biobeds. Thanks for Larry Braul and many collaborators for leading the most recent AAFC study and generously sharing resources, and Erl Svendsen, Bruce Gossen, and Claudia Sheedy for editorial input.
May 20, 2018
Spraying in Dusty Conditions
Dusty conditions are common in spraying, and in dry springs they are often associated with a further challenge, drought-stressed plants. There is no magic cure for these problems, but here are a few guidelines:
1. Most products are not strongly affected by dust. But two important products are very dust-sensitive, glyphosate and Reglone. The active ingredients in both products are very “charged”, therefore they bind readily and strongly to soil particles, which includes not only dust on plant surfaces, but also suspended soil in spray water that gives the “turbid” appearance.
2. Dust can be viewed as similar to hard water cations, as a game of relative concentration. We try to get the herbicide concentration to be higher, essentially over-powering the antagonist. For glyphosate, two approaches are common: (a) reduce water volume; (b) increase herbicide rate. Reduced volume is tricky if the glyphosate spray contains a tank mix partner such as a Group 6, 14, or 15 to combat resistance. Those products require more water. For Reglone, low water is a bad idea for the same reason.
3. Some specialists recommend the use of higher water volumes to reduce the effects of dust. Although spray volumes are usually too low to actually wash dust off surfaces, the higher water volumes permit the use of larger droplets which may have better absorption characteristics in the presence of dust.
4. Another remedy is to increase the application rate in the spray swath where dust is most severe, usually behind the wheel tracks. Slightly larger nozzles in those regions are widely used by sprayer operators.
5. Even when dust is not a problem, roadside field edges may contain dust from traffic. Higher rates may be justified on the outside rounds for that reason.
6. A report in No-Till Farmer makes the following useful statements: “Greenhouse research conducted by researchers at North Dakota State University in 2006 found that control of nightshade species with glyphosate was reduced when dust was deposited on the leaf surfaces before, or within 15 minutes after, glyphosate application. If the dust was deposited later than 15 minutes after application, phytotoxicity was not reduced. Dust generated from silty clay soil tended to reduce glyphosate phytotoxicity more than dust generated from loamy sand soil.”
7. Several additional management opportunities exist for dusty conditions. Slowing down tends to reduce turbulence and dust generation. Although front-mounted booms apply the spray before the dust is generated, it will deposit before the spray is dry, limiting the benefit, as indicated by the NDSU study.
8. Don’t mistake aerodynamic turbulence for dust. Weed control may be lower behind the tractor unit or near the wheels because the spray is displaced by air currents. The use of water-sensitive paper can help identify if this is part of the problem.
One of the better references on dust and wheel tracks was produced by the GRDC in Australia, and can be found here.
May 15, 2018