PWM is gaining popularity, and there is an ever-increasing number of first-time users that need to make nozzle selections for their system. We’ve written about it here, here, and here.
Recall the PWM replaces spray pressure with Duty Cycle (DC) of a pulsing solenoid as the primary means of controlling nozzle flow. The solenoid shuts off the flow to the nozzle intermittently, between 10 and 100 times per second depending on the system. The Duty Cycle is defined as the proportion of time that the solenoid is open, and for low-frequency systems, DC is more or less linearly related to flow rate.
The first rule of PWM nozzle selection is to understand that under average travel speeds, we’d like to see the duty cycle of the system at between 60 and 80%. This means that the nozzle solenoid is open about 2/3 of the time. This value also describes the flow rate as a proportion of the full capacity that nozzle.
The reason for this 2/3 duty cycle rule is to enable four key features of PWM:
It’s ideal for turn compensation, allowing the outer nozzles to increase their flow 20 to 40%, and the inner nozzles to decrease flow about three-fold, in accordance with boom speed.
It allows speed flexibility, providing some additional speed, but more importantly, reduced speeds should conditions require it, without a change in spray pressure.
It compensates for pressure changes so that spray quality can be adjusted without requiring a speed change. Less pressure reduces nozzle flow, and increasing DC recoups accordingly.
It allows for customized higher flows of certain nozzles, perhaps behind wheels, to address reduced deposition in their aerodynamic wake (available on some PWM systems).
The best tool for selecting the right nozzle size is Wilger’s Tip Wizard. This site asks for your desired average speed ( although it calls this “Max Sprayer Speed”), and reports the expected DC for a host of nozzle size solutions and pressures. It also reports maximum and minimum travel speeds and other useful information such as spray quality.
Fig 1: The Tip Wizard is a useful tool for sizing nozzles on any PWM system. Sizing information applies to any nozzle. Spray quality information is for Wilger ComboJet nozzles only.
Although intended for Wilger nozzles, the site’s sizing feature works for any nozzle brand. It asks the user which PWM system they have for the purpose of calculating the documented pressure drop across the solenoid.
Fig 2: Tip Wizard results for the Wilger SR11006 tip at 10 gpa and 15 mph. Look for a solution that provides 60 to 80% Duty Cycle (DC).
If you don’t have access to the site, a basic calibration chart can still work with a simple trick. Recall that we use the top row to identify the desired water volume, and the table’s interior values are speeds, as described here.
Below are two solutions for someone wanting to apply 10 gpa at 15 mph without PWM. The correct choice depends on the required pressure to produce the needed spray quality.
Fig 3: A conventional calibration chart, solving a 10 gpa application for 15 mph.
If you want to apply the same 10 US gpa using PWM, simply solve for a larger volume that offers the right DC. For example, choosing 13 gpa will over-apply by 3 gpa, or 30%. The PWM system adjusts by running at 100-30=70% DC. If the chart doesn’t offer 13 gpa, go nearby, to 14 gpa, as we did below:
Fig 4: By pretending to require 14 gpa instead of the actual 10 gpa, the conventional calibration chart is tricked into solving for a nozzle size that will work with PWM at 60% Duty Cycle.
Now solve for the same target speed, 15 mph. The solution will run at 60% DC. Again, there is more than one choice, and that will depend on the spray pressure needed.
Fig 5: Two possible solutions for achieving 10 gpa at 10 mph. An 06 nozzle at intermediate pressure or an 08 nozzle at low pressure.
We’ve developed a template, in US or metric units, that can be customized for any water volume. Here is the same chart with 13 gpa added:
Fig 6: A conventional calibration chart with the 13 mph speed added.
The best solution for 10 gpa at 15 mph is the 06 size nozzle at 50 psi. This is not engraved in stone. One of the nice things about PWM is that it has inherent flexibility. Make the nozzle pressure a priority to get the correct spray quality. It really doesn’t matter whether the resulting DC is 65 or 80%, the system will still work well. Simply avoid extremes that take you below 50% or above 90%, they will limit the system’s capabilities.
It can handle any water volume or nozzle spacing by filling in the blue cells. Two additional worksheets in the file automate the process, simply enter the desired application volume, travel speed, and nozzle spacing (yellow cells), and the solution that offers the optimal duty cycle range will be highlighted in light green.
We’ve had dire warnings about possible pesticide shortages and price hikes for 2022. Price hikes are one thing. But if the products we need simply won’t be available, we have a tougher challenge. It’s time to plan pesticide conservation.
But first, what’s behind the product shortage?
Emily Unglesby of dtnpf.com provided an excellent overview of the issue here and here. She said the reasons for the shortage are many-fold and came together in a perfect storm. Starting about 2017 or so, pesticide manufacturers tried to reduce the overall inventory of products to improve logistical efficiencies. That effort was rewarded in 2019 when a wet spring in the US dramatically reduced seeded area to a low of 165 M acres. The resulting lower demand again provided incentives to reduce inventories. At the same time, US trade sanctions against China in the form of tariffs impacted production and shipment of many active ingredients to US markets. When Covid-19 happened, it affected both production and shipping of many goods, including pesticides. Container shipment costs increased sharply, and the ability to move them to and from ports was hampered. This then coincided with record seeded area in the US of 180 M acres in 2021, creating higher than usual demand. By that time, very little buffer remained in the system. The growth of Enlist E3 and Xtend Flex has placed additional pressure on glufosinate.
Then two further events occurred. Hurricane Ida forced a shutdown of Bayer’s Louisiana glyphosate plant. And China, in late 2021, legislated a temporary 90% reduction of yellow phosphorus production in Yunnan Province in anticipation of the 2022 Olympic Winter Games. With phosphorus as a fundamental ingredient in glyphosate, glufosinate, and some fertilizers, this loss of production places significant strain on many products. The usual habit of returning unused pesticides to the retailer also became less common amidst shortage news, adding difficulty to planning inventories and demand.
Shortages of popular herbicides like glyphosate, glufosinate, and clethodim will put demand on alternatives. Spreading out risk by implementing pre-emergent products where possible will pay dividends. But the ability to ramp up production of minor products is just as dependent on the supply chain, and these alternatives may therefore not offer reprieve if ordering is left to the bitter end. Planning ahead and staying in touch with retailers about your plans and your own inventory will assist the entire system in managing production and redistributing existing stocks.
Safe to say products will be more expensive, and possibly impossible to obtain. Here are some things to consider to minimize the impact.
1.Grow crops that require less pesticides. Crops which have good genetic resistance to insects and disease will be more likely to cope without a protective spray. Some crops are inherently competitive early on and give less time and space for weeds to become established. Remember that the relative time of emergence is important for crop yield loss from weeds. If crops emerge before weeds, they have the upper hand and will maintain higher yield potential. Crops that can be seeded early will prevent weeds from occupying that niche.
A competitive crop is the best herbicide.
2. The most powerful herbicide is a competitive crop. Use agronomic tools that favour good seedling establishment. The usual advice of seeding into a warm, firm, moist seedbed, should we be fortunate enough for the weather to cooperate, applies here. There is value in higher seeding rates to help outcompete weeds. Use fertilizer placement that favours crops, not weeds, such as side banding.
3. Sample the spray water source and have it professionally tested. After a record drought in western North America, aquifers are low and surface waters have receded. Water quality will probably not be the same it has historically been. Use water conditioners to reduce effect of hard cations and bicarbonates. Ammonium Sulphate (21-0-0-24) at 1% w/v is a general treatment, harder water may require up to 3%.
Conduct a water test in 2022 and condition spray water if necessary.
4. Do not use untested mixes of pesticides with specialty foliar fertilizers. These may impact herbicide performance, or worse, result in an incompatible mix.
5. Use the lowest label rate of product that is relevant for the pest you’re trying to control. Many products have a range of rates depending on the weed species and stage. Scout your fields and take advantage of the lower rate option if you can.
Invest in logistics and be prepared to respond to a good spray day to get the timing right.
6. Spray herbicides early. It’s been shown that crop plants can sense the presence of weeds before they compete for resources, causing a physiological adjustment that results in irreversible yield loss. The shorter the time that weeds and crops co-exist, the better. Also, smaller weeds are easier to control. Weeds that escape this early application will need to compete with an established crop and won’t thrive or impact yield as much.
Smaller weeds are easier to control and may allow a lower label rate.
7. It’s not advisable to reduce product rates from the one recommended on the label. Although label rates contain a margin for poor conditions, the risk of selecting for polygenic resistance exists. Polygenic resistance occurs when some weeds happen to be slightly more tolerant to the herbicide than the rest of their cohort. These weeds may survive a lower rate, and go on to produce seeds. If these outcross with other survivors, their more tolerant offspring will increase in relative abundance. With further such selections in subsequent generations, weeds become even more tolerant and eventually dominate.
8. Apply the spray as uniformly as possible. Make sure the spray nozzles are within a 5% flow rate tolerance along the entire boom. If the set is older, consider a wholesale replacement. But the biggest enemy of uniform deposition seems to be turbulence created by wind and driving speed around the tractor unit. Slower wind and travel speeds help somewhat. Variable deposition means that some regions receive up to 50% more than intended, and others receive 50% less. This means weeds in the lower deposit regions may survive the application. The more variable the application, the higher the rate that is needed for acceptable control.
Slower travel speed reduces variability of the spray deposit, limiting escapes.
9. Use finer sprays whenever the tank mix contains a contact mode of action product (e.g., Group 1, 6, 10, 14, 15, 22, 27) or targets grassy weeds. Both situations require smaller droplets for best performance. The use of finer sprays may mean fewer hours in the day when drift is acceptable, and as a result, investment in efficient tendering and cleaning as well as overall time management pays dividends.
10. Make efficient use of the product in the tank, preventing waste. The amount of product being discarded can range to over 10% of the total needed to treat a field, but this can be reduced to 3% with the proper steps. The following are areas where improvement is possible:
(a) Prime the boom efficiently using sectional shutoffs or better yet, a recirculating boom. These can be primed without any spray leaving the nozzle or boom ends.
Conserve product by eliminating priming waste with a recirculating boom.
(b) Measure the spray mix of the last tank accurately, minimizing leftover. Consider the AccuVolume system that weighs the tank contents to the closest gallon. Tanks can be filled with the exact amount, and rates can be adjusted as the leftover becomes apparent on the last passes.
Accurate measurement of tank volumes prevents leftovers.
(c) Invest in individual nozzle shutoffs to improve sectional control resolution. These are part of any Pulse Width Modulation system but can also be obtained as air-actuated valves that are very affordable. Such capability is necessary for recirculating booms.
Nozzle sectional control can save 4 to 5% product use.
14. Consider an optical spot spray system such as the WEEDit Quadro, the Trimble WeedSeeker, or the John Deere See & Spray Select, available for 2022. These systems are “Green on Brown”, meaning they selectively spray just weeds in a burnoff or chem-fallow. This can save about 70% of the spray depending on weed density. More such systems are on the way, some even offering “Green on Green” that selectively identifies weeds among a crop. The return on investment of these systems is directly related to the pesticide cost, meaning in a year with high pesticide prices they pay off faster. If shortages of product become a reality, a spot sprayer may be the only way that some fields get treated at all.
Pesticide shortages will not be fun. Unfortunately, their appearance coincides with higher fertilizer prices, meaning crop establishment will need to overcome that factor as well. But there are tools to minimize the impact if we’re willing to implement them. Just as necessity is the mother of invention, scarcity is the father of conservation.
We’ve heard it often: calibrate your nozzles to be sure your boom output is uniform across its entire width. The downside of poor uniformity is obvious: strips of over- or under-application causing problems with pest control or crop tolerance. A graduated cylinder held for 30 s under each nozzle is the approach of choice. Several electronic versions exist to make the job easier, for example the Spot On.
But there’s more to the story. Nozzle calibration only ensures volumetric uniformity from nozzle to nozzle. It serves to identify worn, plugged, or damaged nozzles, and little else.
After release, the spray is atomized and distributed across a wider area with a properly developed pattern. An operator adjusts boom height or spray pressure to generate proper overlap for a given fan angle at the target height. Unfortunately, the uniformity of this pattern can’t be measured with a graduated cylinder, so we’ve traditionally used a “patternator”, a flat collector placed under a few nozzles that uses a series of channels to show the peaks and valleys of the volumetric distribution. Both calibration and patternation are done with a stationary spray boom. Nozzle manufacturers employ both methods to ensure their products meet international uniformity standards before marketing.
A spray patternator determines the uniformity of a stationary boom’s spray distribution (Photo: TeeJet)
Burt even that isn’t enough. We can have good volumetric distribution but still have inconsistent coverage in places. To identify those regions, we need a way to measure small amounts of spray deposit under a moving boom, ideally in the canopy we intend to treat. Here we have a few options. We can place a tracer (dye, salt, etc.) in the tank, and collect spray on small collectors placed throughout the area to be treated. We collect the samples, wash them, and analyze the solvent for the tracer. This requires special equipment and takes time. It’s useful, but only measures dose, not droplet size or density.
Plastic straws can act as collectors of sprays under field conditions.
Monofilament strings can be used to collect spray over long distances.
A faster way is to use water-sensitive paper, about which we’ve written here and here. Using WSP is fast and easy, and it can provide additional information such as the number of droplets per unit area, or the total percent of the area covered, or even the size of the deposits, with the right equipment. We call this “coverage”, and believe this to be one of the two components of good pest control (the other being “dose”, the total amount of material deposited). Because the world isn’t fair, WSP isn’t great at quantifying dose.
Water-Sensitive paper provides a quick visual indication of the deposit, not just amount but also qualitative aspects such as droplet size and distribution.
The industry has done a good job of identifying the dose required for good control, and this is reflected in the rate recommendations on a label. But there are a few gaps. They don’t tell us, for example, what “good coverage” is, despite often telling us to “ensure” it.
Back to Deposit Uniformity
We quantify deposit uniformity by calculating the Coefficient of Variation (CV) of a series of measurements. The CV is defined as the standard deviation of these measurements, expressed as a percent of the mean value.
Because it’s hard to measure, it’s easy to ignore. But here are a few basics our research has told us: (In the first three examples, deposits were measured under a spray boom using petri plate or drinking straw samplers. There was no interference from a canopy. The last example was taken from within a canopy.)
When measuring the deposited dose, the CV under a boom tended to rise with increased wind speed. This is no surprise, as it reflects that more wind has a greater chance to displace spray from its intended destination.
Spray deposit uniformity, observed during various spray drift studies, tended to decrease with higher wind speeds.
Higher booms and increased travel speed also tended to increase deposit CV.
Faster travel speeds during spray drift studies tended to decrease uniformity.
Finer sprays tended to increase deposit CV. This makes sense, as the finer droplets are more easily displaced by air movement.
Coarser sprays created more uniform deposits possibly because they were more resistant to turbulent displacement.
Deposits were reduced and became more variable deeper in a broadleaf canopy. Again this makes sense, as there are a lot of obstacles to clear and canopies themselves are by no means uniform.
Deposit amount was lower in the canopy, as expected. But the lower deposit was also more variable.
Also note that the CV in the canopy was quite a bit higher (40 – 60%) than for the exposed targets (10 – 20%). That’s another challenge.
To recap, the best uniformity was achieved with low booms (as long as patterns overlap sufficiently), slow speeds, low winds, and coarser sprays. It’s easy to see that current spray practice isn’t always conducive to uniform deposits.
Deposit variability as captured by a 2 mm diameter string with two sprayer configurations.
So What?
Why does uniformity matter? It matters because more variable deposits are less efficient. They require higher doses for the same effect as uniform deposits. Here’s why:
The figure below shows a typical dose response curve for a herbicide. On the y-axis, we see weed biomass, on the x-axis herbicide dose. At low pesticide doses, not much happens. (In fact, we often see a slight increase in biomass with very low herbicide doses.) As we increase dose, biomass begins to decline, and as dose increases further, the effect begins to taper off. At a certain dose, no further biological response is possible.
A typical dose response curve for a herbicide.
In the next figure, we see that application of a uniform dose “a” results in biomass “y”, about 20% of untreated.
A dose response curve represents the weed biomass that resulted from any applied dose.
Next, we apply the same average dose, but we do it non-uniformly. At some locations under the boom, the deposit may be 40% higher or lower than average. The result is response “z”. Weed control is worse, as bad as it would have been at a lower uniformly applied dose (effective dose “b”).
A variable dose across a field results in many individual weed biomasses because of deposit variation. The net result is lower control.
This effect only happens when the effective dose is near the lower inflection point of the dose response curve. Perhaps we’re shaving rates. Perhaps the weather is challenging the herbicide’s performance. Or perhaps the weed is difficult to control. Under those conditions, any gain in performance with a higher dose is less than the penalty from a lower dose.
There are two ways to correct this performance loss. One is to apply a higher herbicide rate. It’s commonly done, as insurance against – you guessed it – variability, and it’s one reason why label rates have some flexibility. The second way is to improve deposit uniformity. In effect, better uniformity allows for rate reductions.
Label rates are typically in the flat region of the dose response curve to allow for variable conditions in weed susceptibility, weed growth stage, growing conditions, and deposit variability.
Take Home Message
Uniform spray deposition improves overall control. Our examples used herbicides, but the same is true for fungicides and insecticides. It’s true for field crops as well as fruit and vegetable sprays.
Uniformity is especially important when the application is done under adverse conditions in which the pesticide performance is challenged. It’s a fundamental part of good application practice.
It’s not always easy to improve uniformity. But at least it should be measured. Without measuring it, an applicator may never know how much product is being wasted. Have a look at the Crop Adapted Spraying approach Jason is using, it’s a template for all sorts of applications.
What can you do? The easiest task is to record the flow from each nozzle. The results might be surprising. Ensuring proper and consistent boom height is also important. Using water-sensitive paper to visualize the quality of the job would be icing on the cake. And adjusting application method, with uniformity as a goal…that gets you a gold star.
Spot spraying promises to dramatically cut herbicide use. Data from Green-on-Brown (GoB) sprays suggest at least 50% and possibly 90% savings are possible, depending on weed density and the system employed. These savings are significant. But system performance depends on the nozzle selection even more than for broadcast sprays. What are the issues?
Pattern Width
Spot sprays represent a unique mix of single nozzle banding and multiple nozzle broadcasting on the same boom at different times and locations, depending on what the weedy spots require. Both need to be optimized to get the best performance and savings out of such a system.
Even (Banding) Nozzles
Let’s say the spot spray boom has a spacing of 10” (25 cm) and is carried by wheels to ensure consistent height. An operator would want the spray pattern to have a very similar width as the nozzle spacing. A 30 degree even fan angle would create a band of about 10” wide at a boom height of 19” (48 cm, download a worksheet that solves this for any fan angle and boom height here). Assuming a travel speed of 12 mph (20 km/h) and a pressure of 40 psi (2.75 bar), an 03 sized nozzle would apply 14.9 US gpa (139 L/ha) in these 10” wide bands.
But most applicators would be uncomfortable with zero overlap, and would prefer to raise the boom to allow, say, 20% overlap. This would ensure targetting of taller weeds that appear exactly between two sprays, for example. At 22” (56 cm) boom height, the pattern would be about 12” (30 cm) wide and affording 1” overlap on either edge.
Spot spray booms activate any number of nozzles depending on the weed locations.
Because the application is diluted by the extra pattern width, the applied volume is now 12.4 US gpa (116 L/ha), about 20% less than before. This change is easily accommodated by mixing the product more concentrated in the tank. The downside is that the overlap in banded sprays receives twice the dose, and this is less than ideal.
Tapered (Overlapping) Nozzles
A possible solution is to employ tapered flat fans that are the standard type on broadcast booms. These produce more of their volume in the centre, diminishing at the edges, to allow for overlapped patterns and thus functioning better when more than one nozzle is activated. In addition, the extra coverage from a wider pattern is not as wasteful as it is from an even pattern type since it comprises less volume. A single nozzle spray, however, would have a higher dose in the centre than at the edges, since a single pattern has a bell-shaped volume distribution. (note: a single nozzle moving through air loses some of its volume from the centre and places it at the edges, due to aerodynamics of the fan shape. That levels out the bell shape somewhat.)
Broadcasting
When more than one nozzle is triggered by the sensor, the spot spray of that region is just a small section of a broadcast boom. The average dose is now related to the nozzle spacing, not the actual band width as it was for a single nozzle. The wider the section of nozzles that are activated simultaneously, the less inefficiency a wider individual pattern creates because it’s only wasted on the outside edges of the outside nozzles.
Clearly, a sprayer that sometimes functions as a single nozzle spot spray, and at other times as a broadcast boom requires some compromises. Monitoring the activation of nozzles and learning from the relative frequency of single vs multiple nozzle activations will be useful to optimize the configuration. But when boom height is constant, a good compromise solution is possible.
Suspended Booms
A more challenging situation arises from suspended booms that do not hold a consistent height. Let’s assume a boom height variance of 10” (5” in either direction), and a wish to retain 20% overlap at the lowest height to avoid misses from a 30 degree nozzle. The lowest height would have about a 12” pattern width, achieved at 22”. The boom would be set 5” higher, 27” (69 cm). At this height, a 30 degree fan would produce a band width of 14.5” (37 cm), producing a 45% overlap. If the boom sways up to 32” (81 cm), the pattern width would be 17.1” (43 cm).
For multiple adjacent nozzles, boom height determines overlap, and a minimum overlap must be achieved even when the boom sways low.For single nozzles, boom height determines band width and therefore dose.
This is where it gets tricky. At suboptimal heights, the difference between a single band and a section of overlapping patterns increases. Do we calculate the tank mix for the rate a single nozzle delivers within its band, or for a set of nozzles activated simultaneously? If we knew that the majority of activations are for a set of two or more nozzles, we could opt to assume an application rate of a boom section with 10” spacing. An 03 nozzle at 40 psi and 10” spacing would apply 14.9 gpa (139 L/ha). But when a single nozzle is activated, the application volume in the 14.5” band is just 10.2 US gpa (95 L/ha), and the plants that triggered just a single nozzle would be under-dosed.
At the top of the sway (32”), a single nozzle’s wider pattern would deliver about 8.7 gpa (81 L/ha) , another 16% less spray volume than at 27”. At the low end of its sway, the band is 12” wide, applying about 12.4 gpa (116 L/ha) , 23% higher than the 10.2 gpa rate at the 27” boom height.
It’s clear that to take advantage of the potential savings of spot spraying, and to ensure good success with single nozzle activation, consistent and accurate boom heights are essential. I’m not sure how much more obvious a development priority can be.
Band Length
Spot sprays allow the user to select the length of band that the spray is activated for. Shorter band lengths require more targetting certainty. If booms and travel speeds are both low, an individual detected weed can be targetted accurately with relatively short band lengths because relatively little can happen to displace the spray during its short journey. But as booms and travel speeds are higher, the time that the spray arrives at the target is more difficult to predict and longer band lengths need to be programmed. For example, wind can push the spray off its target. Or the faster speeds impart more of a horizontal vector to the spray, causing it to land further away from the point of release.
The variances in where the spray lands along the direction of travel depend on droplet speed and boom height. A conventional flat fan nozzle produces an initial droplet velocity of about 20 m/s. These droplets slow at a rate dependent on their size and whether they’re entrained in the spray plume. At 45 cm below the nozzle, larger droplets are still moving at 10 m/s. Smaller droplets are only moving at 1 to 2 m/s.
Droplets take time to reach their target, and the spray band length must accommodate variance in this time arising from different from boom heights or droplet speeds.
Let’s assume an average droplet speed of 10 m/s for the journey. At that speed, the spray takes about 0.05 s to travel the 0.7 m (27”) from nozzle to target. During that time, the sprayer going 12 mph (5.6 m/s) moves about 0.25 m forward, as do the larger droplets from the released spray. If the boom sways down to 22” or up to 32”, the distance travelled by the sprayer is 0.2 and 0.3 m, respectively. In other words, the band length would need a buffer of 10 cm to accommodate the variability of the beginning and end of the band.
Overall Efficiency
Are these numbers such a big deal? You might say that we’re already cashing in on some big savings here, so why sweat the details?
It’s the principle and the resources. If we’re talking about individual nozzle band width and its change with boom height, accommodating boom sway means applying more than necessary on average to avoid under-dosing when booms sway high. The examples used here show a potential dose variance of 40% with a boom sway of 10”, a modest assumption. That’s a big number to leave on the table. If we had a constant boom height, we could decide what overlap we wanted and minimize these losses.
One of the features on most spot sprayers is to turn on all nozzles of a section that exceed a certain boom height. While this prevents under-dosing and ensures an area is treated even when the sensor is outside of its optimal range, it is possibly an unnecessary use of product.
If we’re talking band length, adding 10 cm to a band length of 50 cm is 20% over-application. That can also add up.
The key to being efficient with spot sprays is accurate and consistent boom height. We know we can do that with a wheeled boom. But show me a suspended boom that can deliver on this, and I see an instant industry leader in spot spray application.
