Author: Tom Wolf

Pulse Width Modulation For Newbies
I was recently asked to describe Pulse Width Modulation to a non-farming audience. My instinct was to send them back to what we’d already written about the topic on Sprayers101, here and here. But on reviewing the material, I soon realized that most of our posts assume a certain amount of basic knowledge and understanding. What about people who are new to the business, or just curious? Not that helpful.
This is the first in a series of articles that cover off topics which may be too basic for many, but are nonetheless important for others. More to come. And suggestions welcome.
Sprayers are used to apply crop protection agents to fields, and as with all crop inputs, it’s important to apply the correct dose. For boom sprayers, dose is a product of the swath width, the sprayer travel speed, and the flow rate of spray liquid through the nozzles. Of these three factors, swath width is taken as constant, whereas travel speed and flow rate are variable. If travel speed changes, flow rate also needs to change to maintain the target application rate.
The vast majority of nozzles come in fixed sizes. As a consequence, the only way to change their flow is with spray pressure. In a modern sprayer, a computer known as a rate controller takes care of the math and the adjustments. For example, if the sprayer speeds up, it will need to deliver more liquid to keep the same application volume per acre. The rate controller knows the swath width (entered by the user) and senses travel speed (using radar or gps) and liquid flow rate (using a flow meter). If the travel speed increases, the rate controller causes the spray pressure to increase until the flow rate sensor shows that the flow is enough to maintain the target application rate.
The problem with this approach is that sprayer nozzles are very sensitive to spray pressure. Too low a pressure will cause the spray pattern to deteriorate, resulting in poor coverage. Too high and the spray will become too fine, creating drift problems. As a result, traditional sprayer operators have to stay within a very specific, narrow speed range. This may not always be possible if, for example, the terrain is hilly or the soil is wet.
One solution to this problem is to control flow rate differently. A fairly new way to do it is with Pulse-Width Modulation (PWM). This is a fancy term that describes a well established way that liquid flow rates are controlled in a number of other tasks such as fuel injection or hydraulic oil systems.
With PWM, each nozzle body is equipped with an electronic solenoid (shut-off valve). The valve turns on and off ten or more times every second, creating an intermittent, pulsed spray. The number of times the valve cycles on and off per second is called the frequency, measured in Hertz (Hz), cycles per second. The proportion of time that the valve is open, called the pulse width or duty cycle, is related to the liquid flow rate passing through the nozzle. Duty cycle can be electronically controlled.
For example, each nozzle can operate at its full rated flow (100% duty cycle) or a fraction of its flow (say 20% duty cycle). At low frequencies (about 10 to 15 Hz, common in PWM systems) duty cycle is proportional to the flow rate of the nozzle. At 20% duty cycle, the nozzle delivers about one fifth of the flow compared to 100%. The pulses are so quick that it doesn’t affect overall coverage or droplet size. With this system, as a sprayer speeds up or slows down, the duty cycle changes automatically to match the flow rate requirements calculated by the rate controller.
What does this mean in practice? For one, the sprayer no longer relies on a pressure change to influence the nozzle flow rate because duty cycle has taken over that job. In fact, the operator can set the pressure to whatever is necessary for best coverage or best drift control, whatever is most important. A change in travel speed caused by a hill or a slippery spot doesn’t affect pressure any more. The end result is a spray application that is not only more accurate, but also more consistent over varying conditions.
Drift control is easier with a PWM system. A common way to reduce drift is to make the spray coarser, and this can be achieved with lower spray pressure. But lowering the pressure results in less liquid flow, and the operator has to slow the sprayer down if the same application rate is to be maintained. With a PWM system, the operator simply lowers the pressure. The system makes up for the lower flow by internally increasing duty cycle, allowing the same travel speed to continue and therefore not affecting the work rate.
An added side benefit with a PWM system is that it provides opportunities for site-specific management of application rates. Parts of the field needing less or more product can receive what they need. All the operator does is change the rate, via duty cycle, according to a prescription map.
A further bonus is the highly resolved sectional control that can be achieved. With any wide agricultural implement, overlaps are inevitable. With an advanced version of a PWM system, individual nozzle solenoids can be shut off or turned back on as required, thereby preventing double applications at these overlaps.
In short, PWM systems give operators much more control over their spray operation. And that’s good for everybody.
May 29, 2017
Storing Pesticide Mix Overnight
Not being able to finish a tank due to weather or any other reason happens to just about everyone. Is it OK to simply leave the sprayer as is, and resume spraying later after some agitation?
In many cases, the answer is yes. Most pesticide mixtures are stable in short term storage. On resuming spraying, an agitation could be all that’s needed to get back to where you started a day or so earlier.
But there are three important exceptions.
When the active ingredient is formulated as a suspension. Suspensions are typically wettable powders and flowables, and rely on a clay carrier to distribute the active in the tank. Because clay is denser than water, these formulations settle out quickly after agitation stops. Sure, they can be brought back into suspension with vigorous agitation. But in lines and booms, boom ends and screens, dislodging a settled clay carrier is much more difficult. It’s also hard to tell if the cleaning has been successful because the problem spots are hidden.
The best solution is to flush the spray boom with water before materials can settle and lodge. A visual inspection where access is possible, such as strainer bowls and boom ends, is part of the process to ensure the formulated product has been removed.
Learn to identify which formulations are suspensions. There’s lots of jargon out there. Look for terms such as DC, DF, DG, DS, F, Gr, SP. Even EC formulations are suspensions (oil in water) and require agitation.
When the active ingredient is chemically unstable. Some pesticides can degrade in the tank, usually due to alkaline (high pH) hydrolysis. The effect is very pesticide specific, but in general, insecticides (particularly organophosphates and carbamates) are more susceptible than other pesticides. This fact sheet by Michigan State University describes the impact of pH on a the half-life of a large number of pesticides.
Note that in the examples in the MSU fact sheets, pesticide half lives are typically days and weeks, and only rarely hours. Also note that while high pH is most often problematic, low pH can lead to faster breakdown in a small number of products.
Ensuring tank mix stability requires a pH meter or paper, and possibly a pH modifier such as citric acid. But do your research first! Here’s an article on pH and water quality.
When the tank previously contained a product known to harm the current crop. This situation is most common and most difficult to address. Some examples from western Canada are Group 2 modes of action sprayed prior to a canola crop. Why are Group 2 products implicated? Many are formulated as dry products on a clay base, and these can settle in boom ends, adhere to tank walls, or get stuck on screens. Their solubility is pH dependent, as we explain in this article.
Canola is particularly sensitive to this mode of action, and the most common canola herbicides, Liberty and glyphosate, are formulated with strong detergents that act as tank cleaners.
Even when applicators think that their tank is clean, they can’t actually be sure and can’t do much about it at that stage. The stripping of tiny amounts of residue off the tank walls, filter screens, or plumbing, can happen during a mid-day stop or an overnight break. Applicators eventually find out that this happened, usually about two weeks after spraying.
Our advice is:
After spraying a herbicide to which a subsequent crop may be sensitive, with the classic case being a Group 2 and moving to canola, be extra diligent with cleaning and pay attention to the tank walls, all screens, and boom ends.
The best way to solve issues is to avoid them in the first place. If the weather looks unsettled and may interrupt your spray operation, consider mixing smaller batches that can be sprayed out completely even if conditions change quickly. This allows you to rinse the tank and spray water through the boom, thus avoiding a contamination problem developing overnight.
If that’s not possible, at least do not let a tank mix sit in the boom overnight. Instead, use your clean water tank to push water through the boom prior to storage and double check the screens. The following day, prime the boom with your tank mix as usual and resume spraying the crop.
If you’re not sure that your sprayer can draw from the clean water tank and push through the booms (the wash-down nozzles are, after all, the intended destination for that water), decipher your system and add the necessary valves that make this possible.
A useful design that helps flush and prime a boom quickly is the recirculating boom offered by some aftermarket boom manufacturers. These booms are also more common on European sprayers. A nice feature of such designs is that the tank contents can be pumped through the entire boom assembly without actually spraying. This ensures that the boom is primed without any soil contamination. It also dilutes whatever residue there may be in the boom plumbing with the entire tank, likely reducing its concentration enough to be of little concern.
An additional feature of recirculating booms is that many offer stainless steel tubing throughout most of their feed and return length, minimizing the black rubber hose products that often adsorb, and later release, herbicide contamination.
Even if a wholesale boom or sprayer change is impractical, consider switching to steel boom lines and tanks tank to minimize residue carryover.
As is often the case in the spraying business, prevention is easier and less costly than solving a big problem later. Spray mix storage is one of those examples where a small amount of extra effort at the beginning can pay big dividends later.
May 26, 2017
How Do Hydraulic Low-Drift Nozzles Work?
Low drift nozzles have become the standard way to apply pesticides from a boom sprayer. In order to use them properly, we need to understand how they are designed and how they are intended to work.
Sprayer nozzles have three functions on a sprayer.
1. Metering flow
2. Atomizing liquid
3. Distributing liquid uniformly
Accurate metering of the flow is done through precise machining or molding of the nozzle.
Atomization of a liquid occurs by imposing some sort of force on the liquid that causes it to break up from a stream or a sheet into droplets of the desired spray quality.
Distribution is done by generating a pattern that, in combination with adjacent nozzles, produces similar dosages in appropriate droplet sizes and densities, along the target area.
All three of these functions are confirmed by the nozzle manufacturer, but the properties are likely to change with wear.
Atomization
Atomization forces could be air-shear (used in some aircraft, airblast, or twin-fluid nozzles), centrifugal energy (used in rotary atomizers), electrical energy (used in some electrostatic sprayers), or hydraulic pressure (used in the most common nozzles, the flat fan or hollow-cone tips).
Typically, the higher the applied energy, the greater the break-up of the spray. More air-shear resulting from faster aircraft or fan speeds, faster rotation of a cage, or more hydraulic pressure all have similar effects: they create finer sprays.
Most nozzles produce polydisperse sprays, comprised of a large number of different droplet sizes. For hydraulic flat fan nozzles, droplets ranging from 5 to 2000 µm can be produced. The exact distribution of the volume in these droplet sizes depends on the nozzle design, the spray liquid, and the pressure. Here are three examples, representing approximately Medium, Coarse, and Extremely Coarse sprays.
Droplet size distribution by number and volume from a Medium spray. Note the majority of the droplets are small, but the majority of the volume (dose) is in somewhat larger droplets.
Droplet size distribution by number and volume from a Coarse spray. Like in the Medium spray, the majority of the droplets are small although there is fewer of them. The majority of the volume is in intermediate sized droplets.
Droplet size distribution by number and volume from a Very Coarse spray. While the majority of the droplets are small as in the finer sprays, their overall number is sharply reduced from the finer sprays. The volume is now in the largest droplet sizes.
Let’s focus on hydraulic nozzles, by far the most common in agriculture.
Spray Pressure
Spray pressure is a useful tool for controlling droplet size from any hydraulic nozzle. Need a finer spray? Add pressure. It is also the basis for the age-old recommendation that lower pressures are a good tool for reducing drift.
We impose practical limits on the upper and lower range of recommended pressures based on several other factors, chief among them the spray pattern.
Spray patterns of a certain width, or angle, are required for proper pattern overlap. The convention is to space hydraulic nozzles at 15 or 20 inch intervals along a boom, and operate them at about 20” above the target. Boom height values will depend on the fan angle of the nozzle and the degree of overlap required. For low-drift flat fan tips, a minimum 100% overlap is best. With 100% overlap, the spray pattern width at target height is twice the nozzle spacing. With this approach, at any point under the boom, the target receives droplets from the closest two nozzle patterns.
Pattern angles are published by manufacturers, but in practice, angles often differ from those values and can vary with spray formulation. Importantly, they tend to become narrower at lower pressures. The exact pressure at which this happens depends on the tip design, but experience shows that pressures below 20 psi for conventional nozzles, and 30 to 40 psi for low-drift nozzles, result in poor (too narrow) patterns. Narrow patterns reduce overlap, resulting in poor distribution.
TeeJet AI11003 at 20 psi
TeeJet AI 11003 at 80 psi
We might also limit pressures at the upper end, based on drift potential. Most conventional flat fan nozzles, for example, drift excessively at pressures above 60 psi or so, hence that limit.
Low Drift Nozzles
Low drift nozzles were quickly adopted by applicators due to their ability to reduce drift and thereby widen the window of safe spray application. They work by using a two-stage design (often called “pre-orifice”) to reduce the internal operating pressure of the tip. The pre-orifice, the original liquid inlet, is round and sized for the nominal flow of the tip. The exit orifice is eliptical in shape and has a larger flow capacity than the pre-orifice, by about 1.2-fold to 2.5-fold. The larger exit creates an internal pressure drop, so the pattern formation produces larger droplets as though the operating pressure had been reduced. Most modern low-drift tips also introduce air into the nozzle via a built-in venturi. This further suppresses the formation of driftable droplets and introduces air into the interior of the nozzle, adding some pressure back to the system.. The Albuz AVI nozzle schematic below explains the venturi design.
Cross-section of the Albuz AVI venturi nozzle.
The tapered channel inside the nozzle is a venturi, which draws air into the nozzle via integrated ports. When low-drift nozzles are operated beside conventional nozzles at the same pressure, low-drift nozzles produce much fewer driftable fines, and also more larger droplets.
But while the two-stage design is useful for managing drift, it also conceals the actual operating pressure of the exit orifice in these tips. The exit orifice is important – it is the part of the nozzle that does the atomizing and that forms the pattern.
Let’s illustrate the pressure inside a low-drift tip by operating an air-induced low-drift nozzle at 60 psi. This nozzle has a pre-orifice size of 03 (0.3 US gpm at 40 psi, blue) and an exit orifice size of 06 (0.6 US gpm at 40 psi, grey). The operator sees 60 psi on the gauge. What is the exit orifice pressure?
The exit tip has twice the flow-rate of the pre-orifice, and therefore operates at one quarter the pressure, or 15 psi. Recall the square-root relationship between flow rate and pressure.
The relationship between spray pressure and flow rate. Doubling the flow rate requires a quadrupling of pressure
That’s not the whole story. The internal venturi is drawing additional air into the nozzle chamber, and depending on the operating pressure, this could be from 5 to 15 psi. The amount added depends on the specific nozzle, its flow rate, and its pressure. Let’s add 10 psi in this case. The exit tip is actually at 25 psi.
Now let’s assume the pressure gauge reads 40 psi, and that the venturi generates 5 psi additional pressure. The actual exit orifice pressure is now only 15 psi. This is at the lower limit at which a spray is atomized, and at which a good pattern can form.
Our general recommendation with venturi-style low-drift tips has been to avoid pressures below 30 or 40 psi for that reason. We’re trying to prevent the spray becoming too coarse for adequate coverage, and also to prevent the spray pattern from collapsing.
The upside of this design is that the same principle allows for much higher-pressure operation without creating excessive drift. These types of nozzle can, in fact, be operated at 70 to 90 psi without becoming very drift-prone because the pressure at which the spray liquid is atomized is likely only 30 or 40 psi (the actual exit pressure and drift potential will depend on the nozzle and the formulation).
Speed Range
A low-drift nozzle with a pressure operating range from 30 to 90 psi (i.e., 3-fold) would have a flow rate range of 1.73 (i.e., the square root of 3 due to the square root relationship of flow rate and spray pressure). This means that the fastest travel speed (at 90 psi) would be 1.73 times the slowest travel speed (at 30 psi).
A conventional nozzle operating between 20 and 60 psi would have the same travel speed range. So why don’t we just do that? The main reason is that the two-stage design lowers the overall amount of drift substantially, something a conventional nozzle can’t achieve even at very low pressures.
A second reason is that even at high pressures, a two-stage design will likely drift less than an conventional nozzle. This is still the case if the conventional nozzle is operating at low pressures. Any spray quality chart comparing spray qualities of conventional and low-drift tips will demonstrate that.
Pulse Width Modulation
PWM uses a solenoid to intermittently shut off nozzle flow, between 10 and 100 times per second (Hz) depending on the manufacturer. This has implications for nozzle design because the nozzle must not leak liquid during the brief off-cycle. If it does, the small amount of liquid leaving the nozzle will not only not atomize properly, it will also cause a pressure drop within the nozzle which must be replenished with the next on-pulse. This will mean the on-pulse will operate at a lower initial pressure, affecting pattern development and atomization. For this reason, venturi-style low-drift nozzles have not been recommended with PWM. The venturi provides an alternate exit for air or liquid, compromising nozzle performance.
And yet, some venturi style nozzles do, in fact, produce acceptable patterns with PWM according to the nozzle manufacturers. This goes to show that nozzle design can continue to evolve to provide the best in drift reduction technology with PWM. Design for PWM suitability should be at the top of nozzle manufacturers’ agendas.
Nozzle design continues to evolve. But in the foreseeable future, spray pressure will continue to control pattern width and droplet size. That’s why understanding the pressure limits of any specific nozzle type, and maintaining pressure within those limits, is so important in any spray operation.
April 4, 2017
Three Features that Should be Standard on all Sprayers
One of my main activities in the winter is public speaking. Attending producer meetings gives me the privilege of meeting many farmers, learning about their operations, and sharing my research results.
I enjoy providing practical solutions to problems. But there are three issues that always come up to which I wish I had better answers. Here they are:
1. The Correct Spray. We’re stuck with compromises in this area. We need small droplets for coverage. We need large droplets for drift control. We need to keep application volumes moderate for productivity. We’ve basically asked the nozzle to shoulder the entire burden of our application needs, seeking a spray that hits all the right notes. Not too fine. Not too coarse. Able to work with fast and variable travel speeds and high, variable boom heights.
Based on our research in field crops such as wheat, canola, corn, lentils, etc., we can be confident that Coarse, even Very Coarse sprays, coupled with a reasonable water volume, are appropriate for most modes of actions and target situations. These sprays contain enough small droplets for good coverage, and their larger droplets work surprisingly well in most cases. Sure, a finer spray could save some water. And a coarser spray would reduce drift even more. But we need a compromise spray, combined with some lucky weather, to get the job done.
And yet we usually make spray quality recommendations with caveats, because droplet size alone isn’t enough. Drift is always a possibility, no matter how coarse we go. Coverage is not guaranteed, especially if the canopy is dense. Finer sprays will get deeper into a broadleaf canopy, but then we may have drift or evaporation to deal with. The nozzle size, volume, and travel speed relationship has to be just right so the spray pressure is in the correct range. And on it goes.
I’d like to give the overworked nozzle some help. We used to use shrouds to protect fine sprays from drift. Now it’s time to let air assist take over that task.
Air assist booms can accelerate (i.e., add kinetic energy to) small droplets so they’re less prone to off-target movement. Properly adjusted, air assist can carry these droplets deeper into the canopy and enhance their deposition.
A good air-assist system allows the user to select the strength and direction of the airblast to match canopy, boom height, and travel speed conditions.
Air assist is the workhorse of most fruit-tree and vineyard spraying. It has to be done right to provide all the benefits I mentioned, and certain approaches should be rejected. For example, there are some companies using air assist to promote very fine sprays with very low volumes. That’s the wrong use of the technology, and invites a backlash.
Instead, we need systems that work with existing spray practice to address some of its classic shortcomings, such as drift management, deposit uniformity, and canopy penetration.
Let’s see some products. It’s time to bring air-assist to the mainstream of agricultural spraying.
1. Boom Height, Level, Sway and Yaw Control. Boom height is so fundamental it’s almost boring. We’ve long said that it’s important to set the boom at the right height for proper nozzle overlap and drift control. It was easy with wheeled booms. But over the last 15 years, suspended booms coupled with fast speeds have caused booms to rise again (RISE OF THE BOOMS!).
Fact is that there are some tasks we’re asking of nozzles that they simply can’t achieve without level, low booms. Drift control is one such thing. Low booms are surprisingly effective at reducing drift, not only because winds are lower closer to the canopy, but also because droplet velocities are faster closer to the nozzle.
Angled sprays for fusarium headlight control are another thing that is more effective with low booms.
Spray droplets released from an angled spray soon slow down and get swept back by air resistance and begin to fall vertically, or move with wind currents, reducing their intended benefit. Low booms can prevent that.
Uniform and low booms also keep deposit variability more manageable. They can save energy needed for air-assist systems. The shorter the path to the target, the less air-velocity will be needed to get it there.
So how about it? Can we have boom linkages and suspension systems, coupled with sensors and hydraulics, that are stable and maintain 20” above canopy at 16 mph on uneven ground? Can we have systems that do this reliably enough that we’re prepared to invest in, say, expensive nozzle bodies? It’s possible.
1. Sprayer Cleanout. One of my favourite questions about cleanout is: “When do you know that you’re finished cleaning the sprayer tank and booms?” Inevitably, someone from the back yells: “In two weeks!” And we laugh, knowingly.
We have a terrible system of sprayer decontamination. It’s a process that is awkward, imperfect, and time consuming, often leading to poor practice. I’ll ask a group of producers what they do with their pesticide waste. The response is silence. I don’t blame them for not telling me that they dump the remainder on the ground somewhere, but I’d rather they didn’t. Sprayer designs don’t help.
What we need is a system that guarantees results. To start, a tank gauge that is reliably accurate to the nearest gallon would remove some of the filling guesswork and help minimize leftovers.
We need a remainder volume (volume left in the non-boom plumbing after the pump sucks air) that is known and small, because that remainder can’t be expelled and needs to be diluted. The smaller it is, the easier it is to dilute.
We need pumps that can run dry, so nobody has to fear spraying the tank out completely.
We need a wash system that requires little volume and works quickly, like continuous rinsing.
We need plumbing that is easy to understand and whose inside surfaces do not absorb pesticide, or hide it in corners and dead ends. Perhaps it’s a recirculating system. Perhaps it hasn’t been invented yet.
We need pesticide formulations that clean up easily. We need an easier way to inspect and clean filters. And we need a safe place to put any waste that can’t be sprayed out in a field.
I’d like to see a sprayer that can be decontaminated in 10 minutes without the operator leaving the cab, and without any spillage of spray mixture. Clean enough to spray conventional soybeans after a tank of dicamba. Clean enough to spray canola after a tank of tribenuron. I know it’s possible.
I also know what many of our European readers are thinking right now. Much of what I’ve discussed exists in the EU in some form or another. Why does the North American, and to a lesser extent the Australian market, not have these features?
Part of the reason is federal standards and regulations. Some European countries test and approve products for remaining tank volume, boom stability, and spray drift, for example. Others have sprayer performance criteria that must be met to be eligible for sale in that country. An increasing number have mandatory sprayer inspection.
These requirements serve to protect the producer and the environment. They’re an example of useful government actions. Despite, or perhaps because of, stricter rules, the entire EU marketplace is very competitive, with about 75 sprayer manufacturers. Bottom line: producers benefit.
We need leadership, preferably from a combination of government, industry, and producers, to achieve better sprayer designs. Our market has room for products that make it easier to prevent drift, protect water, and protect yields.
As they say, a rising tide lifts all boats. And it will certainly make my job easier.
March 21, 2017
Fungicide Application in Cereal, Pulse, and Oilseed Crops
Get ready for a busy fungicide season. If your growing conditions have been good, your crop is dense and vigorous, and soil moisture is adequate, you have yield potential to protect. A bit of moisture and warm temperatures at a critical time, and disease is likely to develop.
Before we delve into how to apply fungicides, let’s review the basics.
1. There is no substitute for an informed decision about whether to spray or not. Seek the advice of a professional to make sure you understand your crop’s genetic susceptibility to disease, the conditions conducive to its development, and the parts of the plant canopy that are affected and therefore need protection. How much yield or seed quality is actually at risk? What do the disease forecasts say for your area?
2. Identify the best fungicide product for your disease situation. Consider inherent efficacy, but also the longevity of the protection and the fungicide’s off-target toxicity (less toxic products can be sprayed in windier conditions without harming susceptible ecosystems). Remember that most fungicides are not curative and must be present on the plant foliage before infection takes place. Also remember that most fungicides are not easily translocated and are at best “locally systemic”. This means that fungicide deposit must cover the plant part that requires protection with an adequate droplet density. If the fungicide is systemic, these deposits must be absorbed through the plant cuticle and will only migrate a small distance within the plant tissue, usually in the transpiration stream, from the point of application.
3. Make proper timing the priority. Disease control is usually only effective if the fungicide is applied in a narrow time frame in which the crop or disease is at a certain developmental stage. A great application at the wrong time is less valuable than mediocre application at the right time. The use of low-drift nozzles should be considered an agronomic tool that permits the correct staging even under marginal wind conditions.
Let’s now review the major highlights of fungicide application in the major cereal and oilseed crops.
Wheat
In wheat, the early growth stagings for foliar fungicides are usually done to protect from leaf spot diseases such as tanspot, septoria nodorum blotch and septoria tritici blotch. Because these diseases are trash-borne, they tend to migrate up from the bottom to the top and good canopy penetration of the spray is important.
Better canopy penetration can be achieved the following ways:
- Higher water volumes. This is probably the most powerful tool in an applicator’s arsenal. More water usually delivers higher doses of active ingredient deeper into the canopy, and whatever dose does get deposited will be present in higher droplet densities. So in short, for any given spray quality (droplet size), more water provides better coverage. We all intuitively know this.
- Slower travel speeds. Moving slower imparts less of a forward velocity on the spray cloud, particularly in the larger droplets. As a result, these droplets move more vertically. In the case of a cereal canopy, more of the spray will reach the lower leaves. The finer droplets in the cloud tend to deposit with the wind direction regardless of travel speed.
- Backward pointed nozzles. If a droplet moves backwards at the same speed as the spray boom moves forwards, then it is basically standing still relative to the crop. It will have a greater chance of moving down towards the lower canopy than a droplet that’s moving forwards. The latter droplet will likely be intercepted by something vertical, like a wheat head or stem.
A single nozzle oriented back, applying a water volume that is at least 10 to 15 US gpa, will be sufficient to get good canopy coverage for leaf spot and rust protection.
Fusarium Headblight (FHB), caused by Fusarium graminearum, is a special case. It infects the wheat head at anthesis, and fungicide must be present on the head, at the glumes where the anthers emerge, at the time of infection. So we have a relatively large vertical target that is at the very top of the canopy. Initial work at North Dakota State University, followed up by work at AAFC in Saskatoon and the University of Guleph at Ridgetown, found the following:
- Angled sprays are essential. Field and lab studies showed that angled sprays were much more effective at depositing the fungicide on heads than vertical sprays. Backward pointed angled sprays provided additional help at targetting the other side of the wheat head. Twin nozzles are available from most manufacturers.
- Use Coarse sprays when angling. Angled and twin sprays have their challenges. The angle at which the spray is released dissipates quickly, particularly for smaller droplets. As a result, more aggressive angles and coarser droplets were found to be more effective. Larger droplets were able to maintain their initial trajectory for a longer distance, increasing the chance that the droplet hit the head from the side rather than passing it by vertically.
- Maintain low boom heights. Even coarse sprays are deflected by air resistance and will eventually stop moving in the direction they were first emitted. In fact, this happens within a short distance. Low booms, less than 25″ if possible, help.
- Watch wind speed and direction. Field observations show that even a moderate wind can over-ride the application practices described above, resulting in most of the spray deposited on the windward side of a target regardless of its initial release.
- Awns intercept small droplets. Many of our modern wheat cultivars are awned, and these fine structures are excellent collectors of small droplets. In early studies with durum, we found a large proportion of the spray volume on awns, where it served no useful purpose. The best way to minimize this awn interception is to ensure coarse sprays and sufficient water, no less than 10 gpa.
It’s important to maintain realistic expectations with FHB. Fungicide chemistry is improving but still offers only suppression. Crop staging is variable. Excellent application practices place the odds more in favour of disease control, but can’t change these facts.
Pulse Crops
Lentils and peas are increasingly important crops. They appear spindly in their early stages of development and are poor weed competitors. But under the right conditions, lentils soon form an impressive set of leaflets that creates one of the most impenetrable barriers in our cropping systems.
Here are some pointers for fungicide application in pulse crops:
- Understand the disease in your crop. Do you need to protect stems (anthracnose), leaves and stems (ascochyta complex, mycosphaerella), or senescing leaves or flowers (sclerotinia)? This is where the spray needs to go.
- Understand the time of disease development.
  - Trash-borne diseases like anthracnose and ascochyta will start at the bottom of a lentil canopy, and early treatment before canopy closure will be important to arrest or at least delay disease development as long as possible.
  - Late season diseases like sclerotinia and botrytis push the application timing towards a closing or closed canopy. Success of such sprays is more elusive because of the rapid development of new biomass.
- Take a bird’s eye view of the canopy.
  - If you can see the target you need to spray, the job is pretty straightforward and conventional water volumes and nozzles will work.
  - If the targets are hidden from view, it will take more water and slower travel speeds to get the required coverage. Consider the higher end of the recommended water volumes (15 gpa in most cases), slower travel speeds (10 mph).
- - When a canopy has many layers of cascading leaves, it is very difficult for a spray to get past these “umbrellas”. We’ve observed many times that a leaf is a very effective shield for anything below it. Large droplets have a hard time changing direction because of their mass and resulting momentum. But small droplets, especially those below 100 microns, can move with slight changes in air movement and get around these obstacles. Use higher pressures (to generate the finer sprays) or select finer nozzles to improve canopy penetration.
- Look at the size of the plant part you need to target. Large targets like leaves can capture almost any droplet size, but small targets like petioles or vertical targets such as stems may benefit from finer sprays, especially if they’re hidden in the canopy.
Generally speaking, dense pulse canopies will require higher water volumes and finer sprays than their cereal counterparts. Although twin fan nozzles have not been shown to provide an advantage in our studies on chickpeas, higher water volumes proved very effective at improving deposition and disease control.
Canola
Canola has two main diseases for which foliar sprays are used. A small number of producers choose herbicide timing for control of blackleg. Because the crop canopy is small and the spray targets are exposed, general herbicide application guidelines (Coarse sprays from a venturi nozzle, 7 – 10 US gpa) will provide good targeting and adequate coverage.
Sclerotinia control requires that the spray reaches buds and petals of canola that is between 20 and 50% flowering. Work at AAFC in Melfort compared conventional and low-drift sprays at two pressures, and showed that droplet size had no effect on disease control. In fact, the Fine spray produced by hollow cone nozzles at high pressure did not significantly improve sclerotinia control compared to a venturi nozzle at its recommended pressure of about 60 psi.
Subsequent lab work showed that the proportion of the applied spray that was retained by petals and buds was statistically identical for all tested sprays.
Water volumes may need to be increased for modern canola hybrids that have significant biomass at flowering. Such cultivars may grow over 1.5 m tall and present a large range of canopy positions in which buds and petals appear. As with the other crops, when a spray needs to cover more area, and especially when this area presents itself in layers, more water volume is appropriate.
Fine Sprays for Coverage
Conventional wisdom says that fungicides require finer sprays for coverage and best effect. This is certainly true in some cases, particularly where the leaf area index is high and leaves are arranged in cascading layers. But it’s time to retire this notion as general advice and adhere to research results for guidance. For FHB, the recommended angled sprays benefit from being applied in coarser, not finer sprays. And in pulses and canola, research showed that there was no benefit from finer sprays. In fact, finer sprays can impair proper timing because of their propensity for drift and rapid evaporation under dry conditions.
Modern coarse sprays produced by air-induced nozzles are less susceptible to these environmental conditions and therefore offer an important advantage: they allow for better timing accuracy. For this reason, I view them not so much as drift control tools, but rather as agronomic tools.
There is a downside to the coarser sprays – they do require more water. Volumes should always be above 10 US gpa, and many recommendations go to 15 gpa if the canopy is dense. In some cases, 20 gpa may be beneficial. These higher volumes are a reasonable price to pay to protect a valuable crop, and we certainly have the equipment to make this price bearable.
Aerial Application
Aerial application is an important way to apply fungicides. An aircraft’s chief advantage is to cover large areas with no crop trampling, and can do so even in wet conditions. As a result, they offer the timing advantage we so often mentioned in this article.
A producer hiring an aircraft for spraying ought to have a conversation with the pilot and discuss water volume and droplet size. Aircraft, out of practical necessity, apply less water and distribute it in finer sprays to offer the required coverage. Although this has been shown to be effective, it creates drift and evaporation potential. It is worthwhile to ask for higher water volumes if it means that the spray can be applied somewhat coarser, creating less drift.
The rotary atomizers on many aircraft produce fairly uniform droplet sizes and do a good job of eliminating the larger droplets. This makes even more droplets available for coverage. However, even with this technology spray drift still matters and all steps to prevent it should be taken. This means using larger average droplet sizes and increasing water volumes accordingly to their label recommendations.
June 24, 2016