Category: Spray Basics

Water Quality and Spray Application
Water is one of the main inputs into a spray operation. The amount of water applied per acre is closely related to spray coverage and pesticide performance. But water quality – a term encompassing its cleanliness and chemical composition – is also critical to the performance of pesticides. Ensuring good performance means testing water and understanding the results.
There are four main water quality indicators related to pesticide performance:
1. Water Hardness. Water hardness is caused by positively charged minerals, primarily calcium and magnesium, but also sodium and iron. These cations can bind to some herbicides (glyphosate is the best-known example, also 2,4-D amine), reducing its performance. Hardness is usually named “Total Hardness (calculated)”, based on the concentration of calcium and magnesium in the sample, and is expressed in ppm or mg/L of CaCO3 equivalent. Some tests refer to the older unit “Grains”, which is ppm divided by 17. Bayer suggests that total water hardness should be below 350 ppm (20 grains) for the low rate (1/2 L/acre equivalent) of glyphosate, and below 700 ppm for the higher rates.
2. Bicarbonate. Sometimes referred to as alkalinity, the bicarbonate ion can inhibit herbicide activity, and also make some herbicides more difficult to mix. The most commonly affected herbicides are members of the Group 1 modes of action, products like clethodim, sethoxydim, and others, as well as MCPA amine and 2,4-D amine. Definite guidelines are hard to find because the antagonistic effect of the bicarbonate ion depends on the presence of other ions such as sodium and calcium.
3. pH. This is a complex parameter because it is related to pesticide solubility, hard water antagonism, and pesticide degradation. In most cases, pH values between 4 and 7 are considered acceptable. But some herbicides, notably those in the Group 2 modes of action, have specific pH needs to dissolve properly. For example, the sulfonylureas (FMC products such as Refine, Express), triazolopyrimidines (Corteval products such as Frontline, Simplicity), Triazolones (Bayer products such as Varro, Velocity M3) and Sulfonylaminocarbonyltriazolinone (UPL products such as Everest) dissolve better at higher pH, whereas the imidazolinones (Odyssey, Pursuit, Ares) tend to require lower pH. Some Group 14 products such as saflufenacil (BASF products Heat, Eragon) also prefer higher pH values for solubility. Label directions are important, sometimes calling for specific adjuvants to adjust the pH prior to adding the pesticide. Some pesticides, particularly insecticides, can break down rapidly in higher pH water. The rate of breakdown is usually not of importance on a spray day but may matter if a mixed tank needs to be stored for many hours or days.
4. Cleanliness / turbidity. Water may contain suspended solids such as clay. Glyphosate and diquat (Reglone) are sensitive to this, as these chemicals are readily adsorbed to soil particles, and turbid water can reduce their effectiveness. This is also why dust generated by the sprayer can reduce these herbicides’ performance.
Ensuring good performance
Select clean water sources and conduct a water test to identify possible problems. Well water is more likely to be hard than surface water. If a laboratory water test is not available, then some quick home testing can provide the necessary guidance. First, use a conductivity meter to test the electrical conductivity (EC) of the spray water. Although this test does not identify the ions present, it shows if a potential problem exists. EC values less than 500 µS/cm are considered safe. For values above 500, a hardness test is necessary to confirm the presence of antagonizing cations. Paper test strips compared to a colour scale are a quick way to determine hardness.
If you have done a water test and want to know what all the numbers mean, have a look here.
If the water is hard, a generally accepted solution is to add ammonium sulphate (AMS) fertilizer at rates between 1 – 3% w/v of 21-0-0-24 to the spray tank, preferably before adding the herbicide. Spray grade liquid concentrate AMS product is available from Bayer CropScience, Winfield United, and some other suppliers. The sulphate anions tie up the hard water cations, preventing them from antagonizing the herbicide. Liquid urea-ammonium nitrate (UAN, 28-0-0) has also been shown to improve herbicide activity for some products, but because it does not contain the sulphate ion, it is not as effective as AMS.
Certain weak organic acids can also function as water conditioners. For example, citric acid can chelate hard water ions so long as the pH is not too low, that is, the necessary dissociable groups are ionic. If the pH is very low, these groups will be protonated and the chelating action is suppressed.
Be careful when lowering pH. It does affect the solubility of many herbicides and possibly the function of some formulations. The outcome may be an unusable tank mix.
Caution is also advised when adding foliar fertilizer specialty products. Adding a blend of fertilizer salts, combined with associated changes in pH, can result in unpredictable interactions with pesticides and water, resulting in sticky precipitates that may be very difficult to clean out of tanks and plumbing. Ask for compatibility data, and always conduct a jar test to be sure that the planned mixture mixes as expected. A recent study shows the effects of adding herbicides to UAN and ammonium thio-sulphate (ATS) plus nitrate stabilizers, where mixing order is critical.
Turbidity is a problem with surface waters, especially in areas of clay soils and after surface runoff. If spray water is taken from a pond, its turbidity can be reduced by adding aluminum sulphate at rates between 10 to 60 mg/L of pond water. Thorough agitation is required, and 80 to 95% removal of turbidity is achieved within 24 to 48 h (technical information here).
Pesticide manufacturers are usually aware of potential problems when their products are used in poor quality water. Consult with your local rep to learn of know issues and solutions before spraying.
Check out this 2022 Real Agriculture interview with Tom and Greg Dahl of Winfield United. Pan ahead to the 16 minute mark for a discussion on water quality.
May 27, 2015
How to Use a Nozzle Flow Chart, With a Surprising Twist
Undoubtedly, the number one question we get from operators is: “Which nozzle should I get”? Luckily there’s no simple answer, or we wouldn’t have jobs! The reason it’s not simple is because selecting the “right” nozzle for a sprayer is a process. It can be broken down into two steps:
- identifying the right flow rate (aka nozzle size)
- choosing a specific nozzle model (i.e. brand, spray pattern type, spray quality, etc.)
It’s a big question, so let’s tackle just the first bullet: identifying the right flow rate.
All sprayer nozzles come in standardized (ISO) sizes, and these sizes are usually identified by numbers stamped on the nozzle as well as the colour of the nozzle itself. The nozzle’s key characteristics (i.e. the fan angle and nominal flow rate), are identified in a format that looks like some version of this (Fig. 1):
Fig. 1: Typical information printed on modern nozzles.
The 110 refers to the fan angle (110°) and the 04 refers to the flow rate. 04 means 0.4 US gallons of water per minute (gpm) at 40 psi. Each nozzle brand has a slightly different convention, but no matter how the information is presented it ought to be on the nozzle somewhere.
Nozzle colour has an ISO standard across fan-style nozzles, and we have this table to match the nozzle colour to the flow rate:
Fig 3: ISO nozzle colours and flow rates
You’ll note that the nozzle we pictured earlier was “flame red”, matching the 0.4 gpm on the table. So how do we use the table to pick the right size nozzle?
Application rate (i.e. gallons per acre or L/ha) is a function of travel speed, nozzle spacing along the boom, and nozzle flow rate. Traditionally, this has been expressed as the following formula in US units:
This formula is famously represented in nozzle charts found in all sprayer catalogues (Fig 4). Along the left side are nozzle sizes and pressures. Along the top is sprayer speed. The body of the table contains application volume. Pick your speed, and look for your application volume in the columns. If you want to apply five gpa, you need to look for the number 5 (or as close as you can get to it), among these numbers.
Fig 4: Typical nozzle flow rate chart, with speed at top and volumes in body. Ugh.
The format of the chart can be confusing because it doesn’t follow a modern sprayer operator’s priorities. Usually, an operator decides on an application volume first, and this decision is not very flexible. Travel speed, decided second, has more flexibility.
We’ve therefore re-worked the table to make more sense (Fig. 5). Along the top are common water volumes. The body of the table are travel speeds. Pick a water volume at the top and follow the column underneath this value to find a speed range you’re comfortable with. To the left, the nozzle size and corresponding operating pressures are now visible.
Fig. 5: Nozzle flow rate chart with volumes at top makes it user friendly.
Try to operate at a spray pressure that’s in the middle of the nozzle’s operating range. For an air-induced nozzle, the range is usually from 30 to 90 psi, so the middle is 60 to 70 psi. That should be the target pressure. Look for a nozzle size that delivers this pressure at your expected travel speed.
These columns can be used to work out a nozzle’s travel speed range. If a nozzle can be operated between 30 and 90 psi, for example, the corresponding speeds are listed in the same rows in the volume column.
For example, say you want to apply seven gpa and think that 13 mph would be a good average travel speed.
Fig 6: Five solutions for the question, “which nozzle to apply 7 gpa at 13 mph?”
Move down the seven gpa column, and you’ll encounter a value close to 13 mph five times – the yellow nozzle at 90 psi, the lilac nozzle at 60 psi, the blue nozzle at 40 psi, the dark red at 30 psi, and the red at about 25 psi. Now use the columns to see which of these three best matches your expected travel speed range.
The yellow nozzle would allow between seven and 12.5 mph from 30 and 90 psi, the lilac nozzle nine to 16 mph, the blue nozzle 11 to 19 mph, the dark red 13 to 22 mph, and the red 15 to 26 mph.
The best choice for a typical air-induced tip would be the lilac 025 size, since it would meet the target speed of 13 mph at a perfect 60 psi, about right for nozzles of that size, and allowing some travel speed flex on the slower side.
Some operators try to extend that range, but dropping below 30 psi will likely result in too narrow a pattern, or too coarse a spray quality, so it’s not advised.
Note that the three-fold change in pressure from 30 to 90 psi translates to only a 1.73-fold change in travel speed. That’s due to the square-root nature of the relationship, as illustrated by this formula:
This exercise applies to sprayers with rate controllers that adjust pressure to regulate flow rates. However, if you use pulse-width modulation (e.g. Case AIM Command, Capstan Sharpshooter, Raven Hawkeye, or TeeJet DynaJet) check out this article describing these systems.
There are a number of apps and websites, usually developed by nozzle manufacturers, which provide similar answers. These are also very useful, and all of them rely on the same formulas used in our new, simplified table. You can go here to download a high resolution version, suitable for framing, in both US and metric units.
May 22, 2015
An Easier Way to Clean Your Sprayer
Farming can be divided into pleasant and unpleasant tasks. Seeding is pleasant. Hauling oats or barley not so much. Sprayer cleaning is…not. And yet it’s the unpleasant tasks that are often the most important. How can we make them better?
We all know the need for a properly cleaned sprayer. Herbicide residue can harm a sprayed crop, and the damage might not show up for over a week. When it does, it usually takes a while to identify the symptoms and damage patterns to be sure. And then we wait for the inevitable yield loss.
It wasn’t long after the introduction of the Group 2 Mode of Action that producers started noticing how even small residues of these products in sprayer tanks could damage crop yields, most noticeably canola, but also other broadleaf species.
Thirty years later, the problems persist. Let’s look at ways of preventing them.
Cleaning a sprayer is a lot like doing the dishes. Using the right detergent, soaking the hard stuff, being thorough, and rinsing properly – they all matter.
It all starts, though, with preventing the problem.
The main culprits that cause sprayer contamination have the following properties:
- They are typically dry formulations,
- they typically have poor water-solubility, and
- they are potent in low doses.
Many products in herbicide Group 2 MOA fit that bill. The ones that rise to the top of the list have an additional characteristic:
- poor solubility at low pH.
As a subgroup within the Group 2 MOA, the sulfonyl ureas are a top concern, with products like Refine, Express, and Ally on the most-wanted list. Another problem subgroup is the triazolopyrimidines, containing products like Frontline and Simplicity, which, like the SUs, have very pH-dependent solubilities (better solubility at higher pH). Other Group 2s have fewer issues. Everest and Varro have good overall water solubility, for example. The solubility of imidazolinones like Odyssey, Pursuit, Raptor increases with low pH. We tend to see fewer problems with these products.
Instances that add to the problem involve tank mixing with weak acid herbicides, including glyphosate, but especially those that are formulated as emulsifiable concentrates (oils, EC), appearing milky when mixed with water. Most of our herbicides are weak acids.
Two problems occur with these:
- The weak-acid herbicide lowers the pH of the spray mix, possibly reducing the solubility of the problematic Group 2s.
- Then, oily formulation can adhere the herbicide to plastic and rubber sprayer parts such as tanks, connectors, and hoses.
The best advice on preventing a cleanout problem, is, therefore, to make sure the product is fully dissolved or suspended. Proper mixing technique and time are the key components. Some products, like Simplicity, can benefit from a pH increase (adding ammonia) prior to mixing the product.
Once properly mixed, we can still have problems at the screens. Dry formulations require a screen mesh of 50 or coarser according to their labels. But many sprayers contain 80 mesh screens, some even have 100 mesh. All screens should be inspected both before, during, and after spraying these products. Screen residues cause longer-term contamination, and their cleaning is an important part of this whole process.
After spraying, the cleaning process relies on three main things:
1. We need to remove as much of the problem mixture as possible.
2. We need to dilute the remainder as much as possible and use it to clean the boom plumbing.
3. Ensure anything that came in contact with spray mix has been cleaned.
Removing the mixture
The best way to remove the remainder is to spray it out in the field you’ve just treated. You can overspray some products again, or if you have any land set aside it can be used for this purpose. It’s never a good idea to drain the tank on any land. Obviously, some experience and math is helpful to make sure the last tank empties nicely on the field.
Diluting the remainder
The next step is to dilute the remainder, using tank cleaning adjuvants like ammonia (this raises the pH and helps remove those products whose solubility benefits from this) and detergent (this removes the oily layer formed by EC formulations). Commercial cleaners like All Clear or Cleanout combine these properties in one jug.
Diluting is most effective when done in multiple smaller batches, as long as we can ensure the tank walls are reached. Wash-down nozzles installed in the tank can do this for us.
Let’s assume the sprayer has a 150 gallon clean water reservoir. It’s tempting to empty the whole thing into the tank. We can calculate the diluting power of this: if we had a 10 gallon remainder in the tank and added 150 gallons water, the remainder would be diluted by a factor of 16. After spraying this out, we’d then have to re-fill the rinse tank if we wanted to do more.
If we rinsed in two 75 gallon batches (add 75 gallons, agitate via wash-down nozzle, spray out, repeat), we would dilute by a factor of 72. If we did three rinses of 50 gallons each, our final dilution factor would be 216. That’s the same dilution as adding about 2150 gallons to the first 10 gal spray tank remainder, and is about 14 times better than dumping the whole 150 gallons in at the beginning!
An improvement in diluting power can be achieved by adding a separate clean water pump. Introducing clean water to the tank as rinsate is sprayed out the boom reduces water use even further.
Cleaning all spray mix contact points
The last step is to pay attention to the things you can’t see: Screens, boom lines and boom ends. The total interior surface area of black rubber boom hoses on a 100 ft sprayer with 7 sections can be as much as several square metres, and this surface can bind residues. Seven sections, each with boom ends, can hold several gallons, as well as accumulated debris. Scrubbing screens, soaking boom lines, and flushing boom ends is the necessary detail that this job requires.
A few final pointers:
- Adding a surfactant or a commercial cleaner can generate a lot of foam. Have de-foamer handy, it will save a lot of frustration.
- A bucket helps collect and clean screens and nozzles.
- Consider upgrading to more steel components on your next sprayer – tanks and booms. Stainless steel cleans faster than plastic.
- Install a way to flush your boom ends. Traditional ball valves do the job, but Hypro’s Express Nozzle Body End Caps do it automatically. These inexpensive units eliminate the dead space in boom ends and as a bonus, bleed air from the lines on the go.
Hypro’s Express Nozzle Body End Caps on a short length of stainless boom.
Done well, sprayer cleaning doesn’t have to be unpleasant. And it certainly results in a better night’s sleep.
May 21, 2015
Rate Controllers and Spray Pressure
Automatic rate controllers are standard equipment on almost all new sprayers. They ensure consistent application volumes, but they don’t do all the thinking for you. We explore how to make them work properly.
A rate controller needs to know the boom width (entered by the user), the total spray liquid flow rate (from a flow meter), and the sprayer speed (gps, radar). It controls the spray liquid pressure by opening or closing a bypass valve. More pressure equals more flow to the boom.
The rate controller allows the applicator to enter a desired application volume and the controller sets the spray pressure that gives the necessary flow for the application volume and sprayer travel speed being used. In practice, this means that higher travel speeds result in higher spray pressure, and vice versa.
But it’s not that simple. Rate controllers aren’t smart enough to know how pressure affects nozzle performance. Some nozzles don’t work well at low pressures. Others do a poor job at high pressures. Some sprayer pumps may even have a problem generating some of the higher pressures a rate controller calls for. What does that mean for the available travel speed range that’s possible with any given nozzle? To answer that question, we first have to have a closer look at how pressure affects nozzle performance.
Spray Pressure and Nozzle Performance
Nozzle performance depends on a number of factors. Of these, the most critical is spray pressure. Pressure affects the flow rate of the nozzle, the spray pattern (fan angle) and the spray quality (droplet size range). The last two of these affect coverage, overlap, and spray drift, so it’s important to get them right. Each nozzle model has a unique spray pressure range and unique spray qualities within that range, so one must obtain information that is specific to the nozzles on the spray boom from the nozzle manufacturer.
ASABE spray quality for the TeeJet AIXR nozzle.
Catalogues Contain Important Information
Nozzle manufacturer catalogues identify the pressure range over which the nozzle should be operated. At low pressures, engineers look for a uniform pattern that meets the advertised fan angle. The upper pressure limits are kept low enough to prevent the formation of excessively fine sprays. Manufacturers now publish tables containing “Spray Quality”, a broad categorization of droplet size, for their various nozzles and spray pressures in their product line. Common spray qualities for agricultural nozzles are Fine (orange), Medium (yellow), Coarse (blue), Very Coarse (green), and Extremely Coarse (white). An example table from a catalogue is shown in Figure 1. Note that for any given nozzle flow rate (left column), the spray quality changes with spray pressure. For example, the TT110025 nozzle can produce a Very Coarse or a Fine spray, depending on the pressure. Also note that for any given pressure, higher flow rate nozzles produce coarser sprays. At 40 psi, the TT nozzle can produce a Medium, Coarse, or Very Coarse spray, depending on its nominal flow. Both of these relationships depend on the nozzle model and manufacturer.
Speed-Pressure-Spray Quality Relationship
As we increase spray pressure, flow rate increases with a square-root relationship.
The square root relationship between travel speed (or flow rate) and spray pressure for hydraulic nozzles
This means that in order to double the flow rate, we need to increase spray pressure by a factor of four. Figure 2 shows three different flow rate tips, each applying 10 US gpa at a range of travel speeds. Assume the operator uses a AIXR11004 to apply 10 US gpa at 12 mph. The nozzle would operate at about 40 psi, producing an Extremely Coarse spray quality. If the sprayer slows down to 7 mph to initiate a turn, spray pressure will drop to 15 psi, producing an Ultra Coarse spray. The spray pattern would likely become noticeably narrower, and poor pest control performance is likely in this situation due to the coarseness of the spray.
Relationship between travel speed and spray pressure for three nozzles applying 10 US gpa
It would have been better to use the AIXR11003 nozzle. At 12 mph, this nozzle would have operated at about 70 psi, producing a Coarse spray. Slowing down to 7 mph would drop the pressure to 25 psi, producing an Extremely Coarse spray. If the pesticide being used is sensitive to spray quality, then perhaps such slow speeds should be avoided in order to maintain a higher pressure and finer spray.
The lesson from this exercise is three-fold: (a) size the nozzle to operate at a higher pressure at your target speed to avoid dropping the pressure too low when you slow down, (b) avoid going as slow as 7 mph to prevent the pressure from dropping too low (c) compromise by setting a minimum spray pressure on the rate controller, in which case you’d over-apply product somewhat when their speed dropped too low.
Spray Pattern Overlap
Flat fan nozzle patterns need the correct overlap in order to achieve a uniform spray pattern under the boom. Research has shown that the amount of overlap for low-drift nozzles needs to be at least 100% to achieve optimum nozzle performance. In other words, the edge of a fan should reach into the centre of the adjacent fan (Figure 3), with each fan covering twice the nozzle spacing at target height. This amount of overlap assures that not only the spray volume is uniformly distributed, but that the droplet density is equally uniform. Less overlap may result in fewer droplets depositing in the overlap region, resulting in poor coverage and reduced pesticide performance.
100% overlap means that all areas under the boom receive spray from two adjacent nozzles.
Adjust the boom height so that at the lowest expected spray pressure (slowest planned travel speed), the nozzles still achieve 100% overlap. There is no disadvantage with greater than 100% overlap, but higher booms will lead to greater drift. When a choice exists, choose 110º fan angle nozzles. Most air-induced nozzles are produced at one (usually wide) fan angle only, but actual angles often differ from those advertised. It is important to visually check the overlap before spraying.
Recommendations
What does this mean in practice? Spray operators need to know the right spray quality for the job, and should consult with the pesticide product manufacturer. They also need to use nozzle manufacturers’ charts to identify the spray quality their nozzle will likely produce at their expected application volume and travel speed. If it’s a poor match, a different nozzle may need to be found. Here are some rules of thumb:
1. Choose a nozzle that produces a Coarse spray over most of the operating pressures you expect to use. Although Very Coarse sprays can work in most situations, avoid them when using lower water volumes, controlling grassy weeds, or using contact modes of action.
2. Minimize spray drift by avoiding nozzles or pressures that produce Medium or Fine spray qualities.
3. Make your pressure gauge your speedometer. First, choose a pressure that is in the middle of the nozzle’s recommended operating range. If the range is 15 to 90 psi, select 50 psi. If it’s 40 to 100 psi, select 70 psi. This allows you slow down or speed up somewhat without breaching the nozzle’s capabilities.
4. Identify the travel speeds that are possible without creating spray qualities that could compromise your application goals.
5. Visually inspect the spray pattern at the pressure extremes you expect to spray at. At the lowest pressure, your nozzle should still produce 100% overlap (the edge of the spray fan should come to the middle of the next nozzle at target height). If it doesn’t, choose a wider fan angle nozzle, increase spray pressure or elevate the boom.
6. Make sure your pump can produce the higher spray pressures you expect to need. Pressure limitations are greatest at high flow rates (fast travel speeds applying large water volumes).
7. Be prepared to compromise. It’s rarely possible to travel at the exact speed, obtain the perfect pressure, and apply the desired water volume that’s been worked out in the office or using manufacturer’s charts. If in doubt, choose slower speeds or higher water volumes to make things work out.
Nozzle manufacturers are getting much better at producing information that helps applicators produce good spraying outcomes. Learning how to use this information is the first step.
April 22, 2015
Selecting the Right Water Volume
Low water volumes can mean less effort to apply pesticides. But there is a limit to how low water volumes can go before problems appear. To understand the reasons why, and help applicators use the right volume for a given situation, we briefly outline what happens to a spray cloud as it reaches the crop canopy.
Basic Principles
To choose the right water volume, we have to remember three criteria for sprays to be effective.
- First, the spray must reach the target.
- Second, there must be enough droplets to sufficiently cover the target.
- Third, the droplets have to be in a form (size and pesticide concentration) that allows the pesticide to be efficiently taken up by the target.
Reaching the target
Let’s start with the first criteria, reaching the target. Droplet size is important for minimizing both spray drift and droplet evaporation. Small droplets move off-target easily, they also evaporate to dryness very quickly and may not have the expected performance as a result. Larger droplets clearly reduce drift, but may bounce off the target and offer less coverage per water volume.
Droplets of various sizes are actually important to cover all parts of a target, so we shouldn’t eliminate all the small ones. For example, penetration of dense broadleaf canopies, or coverage of small targets like stems is best achieved with smaller droplets, while larger droplets are useful for penetrating grassy canopies or targeting the top of a broadleaf canopy.
Target coverage
We need to get the right number of droplets to the target. The more leaf area to be covered (i.e., the taller or denser the crop canopy), the more droplets will be required. Leaf Area Index (LAI), defined as the total leaf area per unit ground area, is a good indicator of canopy density.
To put this in perspective, consider a pre-seed burnoff or an early post-emergent herbicide spray vs. a late season fungicide. In the first case, the canopy can be described as being in a single plane near ground level, with leaf areas of target plants fully exposed and with an LAI of <1. High droplet density on the leaves will be achievable with relatively low volumes.
In the second case, the canopy will have more depth, and will contain large leaf areas in each of the lower, mid, and upper canopy regions, with LAI >>1. Providing the same droplet number to each of the regions in the second case will require more droplets, and therefore more volume.
Taken as a whole, the exclusive use of finer droplets can be counterproductive due to evaporation and drift. Higher water volumes have the advantage of allowing larger average droplet sizes to be used, minimizing evaporation, drift, and enhancing deposition.
Deposit efficacy
The third criteria, maximizing the performance of specific pesticides with droplet size, is more complicated. Typically, contact modes of action and grassy or difficult-to-wet targets require somewhat finer sprays and higher water volumes (Table 1). With tank mixes, such as glyphosate and Heat or AIM, the higher water volume and finer spray criteria should be used. For any specific herbicide, use the higher volume with coarser sprays.
Table 1. Herbicide modes of action, minimum water volumes with low-drift nozzles, and maximum spray quality
In practice, an applicator rarely encounters just one type of targeting situation. Most herbicides are either broad-spectrum, or are tank mixed to target both grass and broadleaf weeds. As a result, the same spray operation has to be effective on grass weeds and broadleaf weeds, some of which may be near the top of the canopy, or be more mature, whereas others may be just emerging. In these cases, a number of different droplet sizes will be required.
Low-drift nozzles
A low-drift nozzle can be used for most applications, as long as small adjustments are made for specific conditions. Increases in pressure above 60 psi (for finer droplets, Medium to Coarse spray quality) and volume to at least 7 to 10 US gpa (for better penetration) with this nozzle optimizes performance for grassy weeds. Lower pressures (down to 40 psi, Coarse to Very Coarse spray quality) are sufficient for systemic broadleaf products or when additional drift control is necessary. Higher volumes (12 – 15 US gpa) may be needed to obtain coverage in dense canopies. Always check with nozzle manufacturer information to learn what spray quality is produced by the nozzle you’re using – this will vary with nozzle type, flow rate, and spray pressure.
Droplet sizes in sprays
All nozzles produce a wide variety of droplet sizes ranging from 5 µm to 1000 µm in diameter. The main difference between sprays is the proportion of their volume in any given size fraction, with low-drift sprays having less of their volume in the drift-prone sizes.
Size distribution (by volume) of two spray qualities. Not that both of these sprays contain small and large droplets. The difference is the volume (=dosage) in each of these size fractions. Shaded areas highlight drift-prone droplets (left) and bounce-prone droplets (right).
But even low-drift nozzles produce small droplets, and these provide sufficient coverage in most cases. Low-drift sprays do create more larger droplets, and these do not contribute to coverage due to their relatively low number and poor retention.
Our main tools for droplet size selection are spray pressure (higher pressure reduces droplet size) or nozzle choice.
Spray Pressure
Higher pressures are sometimes thought to increase canopy penetration because they force the spray into the canopy. This is not true. While higher pressures create faster moving droplets, this speed quickly diminishes. By the time the spray enters the canopy, the faster velocity is lost, especially for the smaller droplets, and the only effect that remains is the finer spray. Finer droplets will penetrate many canopies further, but only if they are protected from wind. On a windy day, the finer sprays are more likely to blow downstream, or perhaps evaporate. The main benefit of higher pressure is better operation of the nozzle, especially air-induced nozzles, leading to more uniform patterns and better overall results.
Large Droplet Advantages
Although coarser sprays are often thought to work less well, they offer certain advantages.
- One advantage is that a coarser spray tends to provide the air assist mentioned above (dragging air into the canopy, and giving smaller droplets a greater chance of moving where they’re needed).
- Larger droplets also take longer to evaporate, increasing opportunities for uptake and translocation within the plant.
- Larger droplets are more efficient at targeting the exposed, large leaves of plants requiring disease protection, leading to greater deposition and fungicide performance.
- Most importantly, coarser sprays produce less drift, enabling application under windier conditions and thus ensuring that the timing of the application with respect to the crop or disease stage can be optimized.
Water Volume
Higher water volumes are the single most effective way of increasing dense canopy penetration. Higher volumes will deliver a greater number of droplets to the lower canopy, leading to greater performance when lower canopy coverage is of importance. When used in combination with lower travel speeds, the downward air flow created by sprays can provide significant benefits in forcing the smaller droplets further down. Larger volumes also decrease sensitivity to droplet size, permitting coarser sprays that reduce spray drift.
Nozzle Angling
Research has shown that exposed (upper canopy) vertical targets such as heads or stems will benefit from an angled spray. Forward-pointed sprays offer a slight advantage over backward-pointed sprays. Since angled sprays must maintain this trajectory to be useful, it is recommended that coarser spray qualities be used to minimize fine droplet production. Angled fine droplets will quickly deflect from their initial angled path and move with prevailing winds. Low booms heights also help in maximizing the benefit of angled sprays. Canopy penetration has not been shown to be improved with forward angled sprays, but backward angled sprays can help place some spray deeper into grassy canopies.
Broadleaf vs Grassy Canopies
How can an applicator decide the most appropriate water volume and spray quality for a specific application scenario? The following guides should help.
First determine the canopy density and form (broadleaf or grassy), and the target site within it (upper, mid, or lower). If the canopy is dense, but fairly vertical (i.e., a cereal), and a significant portion of it needs to be protected, the best strategy is to apply a higher water volume using a reasonably slow ground speed to allow the spray’s built-in air assist to work. If, on the other hand, only the upper layer of leaves, or the heads, are to be targeted, slightly less water can be used. If the water volume is appropriately high for the canopy, larger droplet sizes do not significantly diminish coverage or pesticide performance.
If the canopy is dense but more horizontally oriented (broadleaf crops), similar rules apply for water volume and travel speed, but now the use of a somewhat finer spray may be of benefit. The smaller droplets will be better able to move around and through the leaves to reach deeper into the canopy. Ensuring a downward trajectory of the spray through travel speed and water volume selections will be important.
Nozzle suggestions
A very good starting point for a conventional rate-controlled sprayer is any one of the low-pressure air-induced tips that now form the majority of the market. These tips are similar enough in terms of pressure range (30 – 100 psi), spray quality (Medium-Coarse-Very Coarse, depending on pressure), and spray pattern fan angle (about 100 degrees) to have comparable performance with most pesticides. Such tips are best operated in the middle of their pressure range, which is about 50 – 70 psi, offering some room to move as travel speeds change.
For those with Pulse-Width Modulation (PWM), where most air-induced tips cannot be used, nozzle choice is more limited but growing
All these tips are described in more detail here.
April 21, 2015