In this installment, Dr. Tom Wolf, intrepid reporter, braves the unforgiving wilds of Saskatoon as he investigates claims of mysterious devices popping up all over the city. Colloquially referred to as “nozzles” these items are imprinted with obscure codes that scientists are struggling to decipher. Be the first to learn how to read a nozzle and nozzle table in our newest installment.
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.
When we consult a nozzle catalogue we are interested in the flow and droplet sizes produced at a given pressure. Perhaps we should also consider the effect of pressure on spray angle. We have several articles discussing the collective impact of spray overlap, nozzle spacing and boom height on coverage uniformity (Check here and here for example). However, we don’t really address the fact that fan angle is not a constant. This may be more relevant with the growing adoption of spot sprayers.
To illustrate the potential for fan angle variation, we assembled a collection of red, flat fan nozzles (‘04s) from several manufacturers. We plugged each nozzle into a spray pattern table, set the regulator at a given pressure, and photographed the spray angle and flow distribution. This process was repeated for each nozzle at seven different pressures within the manufacturer’s approved range of 20-80 psi. After digitizing the photos, we measured the spray angle using a digital protractor.
We anticipated a concomitant increase in spray angle as the pressure increased. This is not news. Anyone who has operated a sprayer has seen the spray pattern open up as the boom fills and pressurizes. Bear in mind this was only performed once (i.e. n=1), so while it illustrates trends it shouldn’t be mistaken for a rigorous scientific comparison. Further, this demonstrates a static situation and not a dynamic one where travel speed, wind conditions and the vortices from the sprayer it self will influence matters.
We saw similar trends with nozzles other than 110˚ fans, but let’s focus on 110˚s due to their current popularity.
Fan angles for five common 110 degree AI flat fans over their manufacturer-recommended pressure range
The spray angle for 110˚ nozzles ranged from 75˚ at 20 psi to approximately 143˚ at 80 psi. One nozzle failed to reach 110˚ at any pressure. Conversely, there was another that was over 110˚ at nearly all pressures. Ideally, spray nozzles should be operated around the middle of their manufacturer-recommended operating range. Three of the nozzles tested came close to 110˚ at that median pressure, but only the TeeJet AIC110-04 measured 110˚ at the middle of its recommended range (~50 psi).
Using that nozzle as an example, let’s look at the pressure, spray angle and subsequent distribution of flow along the swath at three different pressures. At 20 psi, the spray angle was 85˚. The yellow balls are floats that reflect flow as a series of cross sections of the swath. We see that aside from the tapered edges (which illustrate the need for 100% overlap between neighbouring nozzles) the distribution was fairly even. One of the priorities in nozzle design is to ensure a low coefficient of variability over the operating pressure range. In other words, the length of the swath may change, but the spray quality and uniformity in that swath is still within spec. At 50 psi the nozzle produced the expected 110˚ fan, and the spray distribution remained even. At 80 psi, the angle spread out to 125˚, spanning a greater distance, but it started to produce a less-even distribution.
Photographs of spray angle and distribution for the TeeJet AIC110-04 at the extreme low, middle and highest pressures of its recommended pressure range.
When fan angle changes with pressure, it can have significant implications. Nozzle spacing on a boom varies from sprayer to sprayer. Generally 50 cm (20 inch) centres are the standard in North America, but we’ve seen 15″ and even 10″. Nozzle spacing and boom height collectively determine the degree of spray overlap. Excessive overlap isn’t a problem, although additional nozzles do mean added expense, cleaning time and potential for plugging. Conversely, gaps in the pattern could lead to sub-lethal applications or flat-out misses. For example, in this soybean demo plot (below) we sprayed a contact herbicide at low pressure to collapse the spray pattern. You can see the alternating stripes of hits and misses that resulted from an incomplete overlap of spray.
Soybean demo plot sprayed with a contact herbicide using 110˚ air induction flat fans at 20 psi. The collapsed spray pattern did not overlap sufficiently to burn the entire crop down, leaving a striped pattern and demonstrating the poor coverage.
Nozzle manufacturers generally recommend a 100% spray overlap for flat fans. This creates sufficient overlap when the boom sways low to the ground. It also increases the degree of droplet size homogeneity under the boom as coarser and fewer droplets are generally found at the “horns” or edges of the pattern compared to the centre. In order to ensure this degree of overlap, sprayer operators should observe and consider changes in fan angle over their typical pressure range. Otherwise, the cost of poor deposit uniformity under the boom could be high.
Operate nozzles around the middle of the manufacturer-recommended pressure range. However, just because a nozzle is rated over a range of pressures does not mean the angle is constant.
Lower pressures are a greater concern than higher pressures. 30 psi is the absolute lowest pressure for operating a 110˚ air induction flat fan; the ideal operating range for these nozzles is 50-70 psi.
If nozzles are not maintaining the recommended 100% overlap at your preferred pressure range, then consider switching nozzle rates, and adjusting pressure and boom height.
This work was performed with Victoria Radaukas, 2015 OMAFRA application technology summer student.
We’ve often heard the adage “Coverage is King” but what does that mean, exactly? It means that in order for your spray application to yield acceptable results, a threshold amount of the active ingredient in your tank must end up on the target. But at what point have we achieved sufficient spray coverage without wastefully over-applying to the target? What does good coverage look like?
Let’s manage expectations right here at the beginning of the article: There is no single, definitive answer because it depends on the nature of the application. In other words, you have to understand which factors are relevant to your specific situation before you can understand what success looks like.
Let’s highlight some of those factors:
Transfer Efficiency, Catch Efficiency and Retention
This relates to the spray’s ability to span the distance from nozzle to target (transfer efficiency) get intercepted by that target (catch efficiency) and then deposit a biologically-active residue on the target surface (retention).
First, the spray must reach the the target location. This may be the soil, or it might be the underside of a leaf deep in a plant canopy. The degree of success will depend on the droplet size(s), distance to the target and the environmental conditions.
Then the droplets have to be retained by the target surface and not bounce or slide off. Difficult-to-wet surfaces such as fruit, stems and waxy vertical leaves may be more easily covered with finer droplets and/or formulations that include activator adjuvants (e.g. surfactants).
Then the deposit must stay wet long enough to be absorbed by the tissue, or leave a hardy residue on the surface that can withstand weathering (e.g. precipitation, sun, and even bacteria) long enough to encounter the pest. More on this below.
Mode of Action
This relates to where spray must deposit (or relocate to) in order for it accomplish it’s objective. Here are a few examples of how products might work. Read your pesticide label to determine your situation.
Some products require contact. Insects must touch them, either via a droplet landing on them or as they move through a deposit. Similarly, certain fungicides must contact fungal hyphae on the plant surface. A few products are designed to drench the target, as is the case with oil-based miticides.
Some insecticides must be ingested. That may be in the form of a surface deposit or in plant material that has absorbed the chemistry. Similarly, some fungicides are absorbed by plant tissue.
Many herbicides are mobile (i.e. systemic). They may be drawn up through the roots, or enter the cytoplasm via leaves and travel to the growing points on the plants, or move through the xylem. Others are contact, staying relatively close to the original deposit.
The sprayer operator should consider these factors when planning the application and when evaluating the resulting coverage. So how do we visualize coverage? Some operators look for the shine on leaves, or a cloudy residue once the spray has dried. That’s better than nothing, but we recommend water sensitive paper (WSP), which is still the most versatile and economical way to visualize coverage.
WSP can be purchased from most retailers that carry spray equipment. It is available in three sizes, of which the 1” x 3” size is the most common. It can be folded and clipped to a plant surface, or placed on the ground. We’ve written several articles on how to use it (such as here and here and in pretty much a third of the articles on Sprayers101).
There are two metrics that must be evaluated when assessing coverage on water sensitive paper:
the area of the target that has spray on it, and
the distribution of the droplets over that area.
Let’s use a metaphor to explain:
The Battleship® / Coverage Metaphor
Imagine the boats in this Battleship® game are the insect pests, and the board they’re on is a leaf. The white pegs represent the spray deposits. In this first image, we see 100% coverage and a very high deposit density. Sure, we got every boat, but this is literal and figurative overkill. There’s no need to completely drench the target in order to control most pests. When you spray a target past the point of run-off, you are not adding more pesticide to the target – you are displacing what was already there. The surface will not exceed the concentration of product you sprayed (with the possible exception of mixes that include certain adjuvants). While additional volume can improve coverage to a point, there is a diminishing return.
Unless the label specifically asks for a drench, this is too much coverage.
In this second image, we’ve covered about 15% of the target area, which is reasonable. However, note the lack of distribution. You can see that we’ve missed quite a bit of the leaf. If our pretend pests are sedentary and if this was a contact product, then we’ve missed. If this was WSP we would advise the sprayer operator to note how much space there is between the deposits. Could a pest such as an insect or small weed easily fit between the deposits?
15% coverage is good, but the distribution is bad.
In this third image, we are still covering about 15% of the target, but now the spray is distributed more evenly. Some of you are likely noticing that we missed a pest. That observation reminds me of one of my favourite exchanges from the movie “Christmas Vacation” where Clark finally got his house illuminated, but his father-in-law only sees the problems: “The little lights aren’t twinkling.” “I see that and thanks for noticing, Ed.”
15% coverage, distributed evenly. Deposits may have some pest activity beyond the edge of the residue (light red circles).
Yes, we still missed a pest, but spraying is playing a game of odds. You want enough spray to increase the odds of controlling a pest, but not so much to waste spray (and money and time). This image represents an ideal coverage situation. If this pest moves, or this pesticide redistributes even a little, it will affect the pest.
Plus, we should not discount the threshold of influence that lies around pesticide residue. Imagine a small circle around each droplet (illustrated here as light red haloes) where active ingredient may redistribute beyond the initial deposit to affect an adjacent pest. Perhaps even more importantly, deposits do not spread on WSP the way they do on actual plant tissue, so WSP always gives an underestimate of the potential coverage.
In this last image, we see that red deposits have been introduced. This represents a disease control program where an earlier (white) application retains some residual activity when next application (red) is applied. The second spray application almost never lands on top of the first, giving much more protection on the target. For those keeners out there, note that we got that last pest!
In the case of many disease management programs, subsequent sprays tend to fill in gaps left by previous sprays. If timing is prompt, residual activity will see you through.
If you Absolutely Need a Number…
So, what if you’ve read all this but still insist on a firm number to define adequate coverage? We’ll reiterate that there’s no universally-accepted threshold of deposit density or area covered. It would be nice if pesticide labels included this information, but they don’t.
We’ll stick out necks out and say that in general practice we see excellent results when we achieve 85 discrete deposits per cm2 as well as 10-15% surface coverage on at least 80% of the water sensitive papers in a spray application. If you can manage this, it should give satisfactory results in most situations.
Ontario Agriculture Conference – 2022
For a really in-depth conversation on the topic of coverage, check out our presentation from the 2022 Ontario Ag Conference. We tried to deliver a fun and memorable demo at the end of this presentation to show how different droplet sizes might contribute to coverage. Enjoy.
North American built boom sprayers have nozzle spacings of 20” (50 cm in the rest of the world), but other spacings such as 15” (37 cm) and 10” (25 cm) also exist. What are the reasons for these alternative spacings and do they offer any inherent advantages?
Why spacing matters
Nozzles are spaced along a boom to allow their fans (patterns) to overlap sufficiently at the target. In broadcast spraying, a uniform distribution of spray volume gives us the best chance for consistent coverage along the boom. Since flat fan nozzles produce a tapered pattern (i.e. the volume is highest in the centre and diminishes towards the edges), approximately 100% overlap (i.e. 50% from each neighbour) will produce a uniform swath.
Figure 1: Tapered flat fans that require some overlap are the default pattern type for agricultural boom nozzles. This is true of conventional and low-drift styles. Note that the flat fans are turned 15° to prevent the spray patterns from interfering with one another.
The 100% overlap isn’t just for volumetric distribution. Flat fan spray patterns tend to have more and finer droplets in the centre and fewer and coarser droplets at the edges. All droplet sizes contribute to coverage in different ways, so the overlap ensures both number and sizes are evenly distributed along the entire boom.
Figure 2: 30% overlap may achieve volumetric uniformity. But because the centre of the pattern contains the majority of the smaller droplets, low overlap may result in low coverage in the overlap regions, resulting in striping.Figure 3: Consistent droplet number distribution along the boom requires at minimum 100% overlap (50% from each neighbouring nozzle). This blends those regions of the patterns with high and low droplet densities.
The generic 20” spacing arose from long-held conventions about boom height, fan angle, and travel speed. Specifically, this spacing required a boom height of 20” to obtain good overlap of the once-dominant 80° fan angle. Combined with 0.15 to 0.3 US gallon per minute (gpm) nozzles and travel speeds of 6 to 8 mph, operators were able to apply 5 to 15 US gallons per acre (gpa) volumes. Using nozzles with smaller flow rates would generally result in nozzle blockages.
But what if we want to change any of those variables? How does this affect nozzle spacing? Figuring out the pros and cons of an alternate spacing requires a little math and some contingency management.
Boom Height Math
First the math. If the boom has 20” nozzle spacing and we need 100% overlap, the width of the spray pattern at target height must be two times the nozzle spacing, which is 40″. You must calculate the required fan angle and boom height to achieve this. Most nozzle catalogues have tables to help with this, or you can download a handy spreadsheet to calculate your own scenarios here.
For today’s standard 110° fans, a minimum boom height of 14” is needed to achieve 100% overlap. For 15” spacing, the height is reduced to 11”. For 10” spacing, we drop to a mere 7”. However, consider that most modern suspended booms are not operated at heights less than 24” to allow for sway. At that height, there’s plenty of overlap to go around for 20″ nozzle spacing. For those booms that are able to operate at a consistent height, narrower spacings permit lower heights that will reduce drift potential significantly. Every time we halve boom height, we also halve drift potential.
Figure 4: Using 110° tips with 20″ spacing, the theoretical height at which we achieve 50% overlap is 11″ above target.
By tilting the nozzles forward or backward from the vertical, we can reduce the boom height somewhat further and still get the same overlap. For example, for 20 and 15” spacings, angling nozzles forward or backwards by 30° allows us to drop the boom another 2” closer to the target.
Contingencies
A suspended boom hardly ever stays at a uniform height; It sways up and down with field conditions, topography, etc. This is why many operators set their booms above the minimum height – to prevent striping when the boom sways low. The penalty is that this increases the distance droplets need to travel, increasing drift potential and any turbulent displacement problems arising from the moving boom.
Assuming a 110° flat fan at 24” boom height, each nozzle achieves a theoretical pattern width of about 70”, which is an overlap of 70÷20=3.4-fold or 240% on 20” nozzle spacing. Given a minimally-acceptable overlap of 50% (25% from each neighbouring nozzle), the boom could be as low as 11”. For 15” spacing, the minimum height for 50% overlap is 8”, and for 10” spacing it’s 5”. This means the narrower spray patterns gain 3” to 6” in allowed downward boom movement.
Figure 5: Using 110° tips on 15″ spacing, the height for 50% overlap is 8″ above target.
A second contingency is that spray patterns are rarely the exact value that the nozzle catalogues specify. A so-called 110° nozzle may operate at only 90°, or up to 150°, depending on the nozzle model, the spray pressure, and the tank mix. Learn more here and here. Patterns also don’t continue to grow at their rated fan angle, as droplets slow due to air-resistance and fall more vertically due to gravity. For that reason, a visual check is recommended to ensure the expected overlap is achieved.
Figure 6: Fan angles indicate initial trajectories of droplets at the edge. With distance, gravity pulls these droplets downward, narrowing the pattern width from that achieved theoretically (figure adapted from image in TeeJet catalogue).
A third issue to consider is less related to boom height but nonetheless affects spray distribution. Small droplets move with air currents, and the turbulence created by large, fast sprayers creates enough turbulence to move these droplets significantly. A perfect pattern under static conditions can look quite different at a fast travel speed with a modest side wind. Low booms may help prevent some of this displacement because droplets spend less time in flight, and their average velocity is faster.
Figure 7: Spray deposition onto a 2 mm string to measure deposit uniformity for a fast travel speed and high boom and a slow speed, low boom configuration.
Flow Rate Math
Flow rate requirements per nozzle change whenever we equip a boom at an alternate spacing. The basic formulae are shown below.
Moving from a 20″ to a 15″ spacing would require a nozzle with 0.75 of the flow rate, approximately from a 02 to 015 size, or 03 to a 025 size, or 04 to 03 size, etc.
Pulse Width Modulation
The use of Pulse Width Modulation (PWM) has increased the overlap requirement. With PWM, alternate nozzles are on a 180° timing offset from their neighbours. This means that when running >50% duty cycle, when one nozzle is temporarily off, its neighbours are on. These neighbours’ patterns must now span the gap, and 100% overlap is the absolute minimum to achieve this. PWM users therefore select the wider pattern angles and some opt for >100% overlap.
Figure 8: Pulse Width Modulated booms require 200% overlap so that the entire boom receives proper coverage when the alternate set of nozzles is off. For 110° fans at 20″ spacing, the minimum boom height would be 21″
PWM Considerations
High flows (greater than 1 US gpm at the nozzle) that are common for fertilizer top-dressing may require higher-flow PWM valves.
Narrow spacings reduce the individual nozzle flow rates and can therefore support higher application rates before triggering a larger valve requirement.
PWM valves aren’t cheap and for example 15″ spacing compared to 20″ spacing adds 24 valves on a 120′ boom.
Banding
We noted that 20” nozzle spacing is a standard because it corresponds to what has traditionally been achievable with available boom heights and spray pattern angles. But things can change.
Narrower spacings such as 15” originate with row crops and planter row spacings of 15” or 30”. These spacings exist so the spray pattern can be placed either over the top of a crop row, or in between the rows for banding. Using narrower fan angles and/or lower boom heights, together with “even” (as opposed to “tapered”) fans, banding sprays can be applied over the top of, or between crop rows. Or drop hoses can reach between the rows for top-dressing or directed sprays into the canopy.
Canopy Penetration
With narrower spacing, it can be argued that a greater proportion of the boom length has spray directed directly downward (corresponding to the centre of the pattern). Whether or not this translates into better penetration of a canopy is a fair question. In laboratory trials, use of 10” or 20” spacing did not improve penetration into a broadleaf canopy. But if the lower boom height afforded by the narrower spacing was utilized, some improvements in the deposit of angled sprays onto vertical targets was observed.
Adjusting to Narrower Spacings
As we showed earlier, use of 15” or 10” spacing booms for broadcast sprays requires a smaller nozzle size to achieve the same spray volumes as the 20” spacing. If boom height remains constant, narrower spacings result in greater pattern overlap which provides more latitude for sway. Alternately, lower boom heights can be used.
Using smaller nozzles on narrower spacing presents some challenges. Generally, smaller nozzle size means finer spray quality. If an operator wants to retain the spray quality they had on a 20″ spacing, they may opt to use lower pressure (not advisable for non-PWM systems) or swap to different nozzle design that can produce the desired spray quality at the lower flow rate.
Smaller nozzles are more prone to plugging, so that needs to be managed with filtration, filling practices and water sourcing. Be aware of the the product formulations and their requirements for filter mesh size. Most dry products specify a 50 mesh filter (or coarser). Also, check size options for nozzles. The smallest size for most nozzle models is 015, but certain PWM-specific nozzles are only available in 03 or larger.
The marriage of narrow spacings with individual nozzle shutoff can result in a versatile system capable of producing high resolution banded sprays in narrow seeded crops. For example, consider a boom with a 10” nozzle spacing spacing that matches the seeder row spacing. The operator can shift from 10” to 20” or 30” from the cab if the valve control software allows it. With accurate guidance and good boom levelling, topdressing foliar products (e.g. nutrients, fungicides) can follow the crop row precisely.
Spot Sprays
Spot sprays present a situation where compromises are needed. Some, such as WEEDit, utilize narrower nozzle spacings to allow better treatment resolution and increase product savings. Any one nozzle or sets of adjacent nozzles may be triggered by the sensor. For single nozzle activation, to preserve the value of the better resolution a uniform, narrow band of spray needs to be created. This means a 30° or 40° fan angle from a banding nozzle will be necessary. For example, a 24” boom height will result in a 13” band with a 30° fan, and an 18” band with a 40° fan. In the latter case, the dose would be diluted by 80%, wasting much of the potential savings.
Figure 10: Boom height is critical for banded sprays and for spot sprays. Too wide a pattern on a single nozzle reduces dose, too narrow creates misses.
Frequently, a patch of weeds will trigger several adjacent nozzles. Now these individual bands need to work together to create a uniform swath. This will inevitably require some overlap to avoid gaps, but too much overlap will result in bands where twice the dose will be applied. A tapered fan may suit this situation better. As a result of these varying needs, tolerances for spot spray boom height are even more strict than for broadcast spraying. More thoughts on spot spray nozzle selection are here.
Conclusions
Narrower nozzle spacings on a broadcast boom allow somewhat lower boom heights and these can in turn reduce drift and improve deposition of sprays. Lower flow nozzles will be needed with narrower spacings, requiring management of plugging and potentially a more drift-prone spray quality. The value of narrower spacings depends on the availability of booms that control sway, allowing them to operate at uniform, low heights.
