We often write about how valuable water sensitive paper can be to visualize and assess the coverage we’re getting from a specific application method. A handy reference is this matrix that combines both factors. Print it and use it in the field to compare what your application method is doing to these relative standards. On average, you will want to see deposits similar to those in the middle of the matrix.
Download US units Matrix here (pdf)
Download Metric Matrix here (pdf)
Reproduced from an article written by Angela Lovell for Grainews, 2014
“The fundamental challenge of spraying is that it’s a compromise game,” said Tom Wolf of Agrimetrix Research and Training. “As operators and advisors we need to always balance the opposite needs of coverage, efficacy and drift.”
Wolf, in a presentation at the recent Manitoba Agronomists Conference in Winnipeg, sees a trend towards more fungicide use on farms across western Canada and technology that purports to make application more efficient. These trends include wider booms, faster speed capability, complex monitors, auto boom heights and bigger tanks.
As much as technology is a great thing, it’s still the operator that is the single most important part of any spray operation, so it’s important to make sure that he or she isn’t going out to the field with any conventional beliefs that simply aren’t correct.
The challenge with spraying is to control pests without harming you neighbour’s crops or the environment and over the years Tom Wolf has developed some pretty good ideas about how to do that and has had to dispel more than one popular myth about spraying.
Myth # 1: More pressure forces the spray into the canopy.
“There’s an element of truth to this but it’s forcing spray downward is the least thing that pressure does,” says Wolf. Spray pressure is primarily used to change spray flow rate. If you increase the pressure you will need to travel faster to allow the carrier volume to stay constant, and faster travel speed actually works against canopy penetration. Another important change is that spray quality will become finer with higher pressure. Finally, droplet velocity will initially increase, but even at higher pressure, small droplets still move slowly by the time they reach the canopy. “If you want to force a fine spray into the canopy, the best way to do that is to lower your boom, slow down, and increase the carrier volume,” says Wolf.
Myth # 2: Higher water volumes lead to run off.
There are two things that govern run off; droplet size and surface morphology of the leaf surface. “Anyone who says that anything more than 3 gallons/acre runs off the leaf surface is not telling you the whole picture,” says Wolf. “We’ve been unable to induce runoff from up to 200 US gpa in our tests, even using hard-to-wet grasses like green foxtail. Don’t be afraid of water. It’s a very good way of covering the canopies. Water gives you flexibility to use coarser sprays and that allows you to spray when it’s windier.”
Myth # 3: Spray drift is no issue for fungicides and insecticides
Aquatic organisms are extremely sensitive to most fungicides and insecticides. We might not see this effect, but it has a definite impact on our environment. It’s important to observe the buffer zones shown on product labels, which can vary depending on the product, the application method and the specific environment.
Myth # 4: Faster travel speeds save time and boost productivity
Wolf suggests evaluating this on a field by field basis. At faster speeds you lose control of the spray cloud and the finest droplets will go wherever the wind goes. Other problems with higher speeds are canopy penetration, pattern uniformity and pressure management. If you have an 800 gallon tank with an 80 ft boom and you are going 12 mph at 10 gallons/ac and your fill rate is 50 gallons per minute you are going to do about 84 acres/hour not including turns. If you go faster – 18 mph – you can do 110 acres/hour. But if you increase your fill speed, thereby decreasing the time spent filling you can increase productivity just as much. If you also increase your boom width you also increase productivity. “All I am asking is you don’t just look at travel speed to improve your productivity,” says Wolf.
Myth # 5: Double nozzles produce more droplets and improve coverage
“It’s the droplet size and water volume that drives the droplet numbers produced. It doesn’t matter how many nozzles produce this size,” says Wolf. Although some double nozzles produce finer droplets and therefore improve coverage, others actually produce coarse sprays which may decrease coverage. Pay attention to droplet size first – nozzle manufacturers publish spray qualities from their products. You can increase coverage from a single nozzle simply by increasing the spray pressure so yo produce a finer spray.
Myth # 6: Calm early mornings have the lowest drift risk
This is one of the biggest myths out there, says Wolf, and it’s all because of a condition called an inversion, which usually occur during clear nights, and which linger into the early morning hours. Under normal sunny daytime conditions, air currents rise, fall and disperse spray clouds rapidly but under inversion conditions they don’t. This can lead to severe drift issues, even significant distances away from the treated field.
Under sunny daytime conditions, air temperature cools with height and that allows for thermal turbulence to disperse the spray cloud. On clear nights, the temperature increases with height (the opposite temperature profile, therefore called an “inversion”), and this prevents air from mixing. As a result, the spray cloud will not disperse.
Assume that the atmosphere is inverted on clear summer nights, extending into a few hours after sunrise. Producers should never spray when an inversion is present, and a good indication might be if fog or smoke hangs in the air and not dispersing.
Myth # 7: A rate controller calibrates the sprayer
“Even with a $400,000 sprayer, the rate controller still relies on a single flow meter that sits at the back of the sprayer and measures the total flow to the boom. The operator has no idea where that total flow is going,” says Wolf. As a result, there is still no substitute for individual nozzle calibration. There are various new tools on the market to assist with that but they still need to be done individually.
Myth # 8: If I mess up agronomic decisions, I can correct that with a good spray application
A spray application has to be on time to be truly effective, says Wolf. In efficacy studies where yeield was measured, spraying herbicides “on time” (=early) produced a yield advantage over spraying just one week later, even with a spray quality that was so coarse that it resulted in relatively poor weed control. “If it’s breezy, use a low drift nozzle. This allows you the opportunity to spray on time,” he adds.
Myth # 9: Ammonia is a good general purpose tank cleaner
Ammonia raises pH and some chemicals like sulfonylurea products dissolve better at a higher pH. But if you have an oily emulsifiable concentrate (EC) formulation, either as a product or adjuvant, a soapy cleanout product will be needed. “Liberty exposes poor tank cleanout because the adjuvant in Liberty is such an excellent cleaner,” says Wolf. After use of an oily product, the use of a wetting agent such as AgSurf will assist in removing oily residue and many soap-based commercial cleaners are available.
Myth # 10: There is an optimal nozzle that does it all
“Right now a sprayer costs approximately 100,000 times more than the nozzle and the nozzle is still the part that makes you happy or sad,” says Wolf. “If we inverted the investment trend and said ‘let’s build a better atomizer’ there would be an optimal nozzle right now. But although we’ve made progress with low-drift nozzles recently, the industry still looks for inexpensive, simple ways to atmozie sprays.”
Spray quality is the language that is used when selecting nozzles. All manufacturers publish spray quality charts for their nozzles that also give recommended pressures to produce different spray qualities using a particular nozzle type. Spray qualities are colour coded and generally speaking the hotter (redder) the colour code the more drift-prone (finer) the spray. There are many nozzle choices and designs and typically grassy targets and contact products require nozzles that will produce Medium to Coarse spray quality. For broadleaf targets and systemic products a Coarse to Very Coarse spray quality can be used successfully. Selecting the right nozzle to produce the quality of spray required is important, says Wolf who recommends Coarse as a general purpose spray quality.
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.
To choose the right water volume, we have to remember three criteria for sprays to be effective.
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.
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.
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.
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.
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.
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.
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.
Although coarser sprays are often thought to work less well, they offer certain advantages.
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.
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.
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.
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.
Nozzle choice can be overwhelming due to the large selection available from many suppliers. But nozzles are the most important part of the sprayer, being responsible for metering the liquid, atomizing it into droplets, and distributing it across the boom. Review this list for what’s available, then follow these general recommendations.
All sprayer manufacturers voluntarily publish the spray quality of their nozzles at various flow rates and operating pressures. This information is available here from their websites, catalogues, or apps. The most popular tips for field sprayers are the air-induced and pre-orifice low-drift style, and these are typically operated between 30 and 90 psi. As a starting point, look for tips that produce a Coarse spray quality at an intermediate pressure of 60 to 70 psi. This seemingly high pressure is normal for air-induction style tips and provides some necessary travel speed range for self-propelled sprayers.
Spray quality is a useful way to manage spray drift and coverage. The coarser your spray, the higher your water volume must be because you must have enough droplets per unit area to hit your target and provide enough droplet density on the leaves. This is most critical for pre-seed burnoff, where weeds are smallest, and where low-volume, very coarse sprays will likely miss weeds entirely. It is also important for contact herbicides (that require high droplet densities) and for grassy weeds, most of which have a hard time retaining very large droplets. Use at least 7 to 10 gpa for in-crop herbicides, 10 to 15 gpa for fungicides. The taller your crop canopy, the more leaves there are to cover and the more water is required.
Even a good nozzle won’t work well at the wrong pressure. Air-induced nozzles and some pre-orifice nozzles require higher pressures to operate properly. The most common reason for performance complaints is when the spray pressure of a low-drift nozzle is too low, resulting in poor spray distribution between nozzles (see next point). If your sprayer cannot produce sufficiently high pressures, you should not be using these nozzles. Use your spray pressure gauge as your speedometer, and aim for pressures in the middle of the nozzle’s recommended operating range. Higher pressures increase drift potential, but less so for pre-orifice and air-induced nozzles. These tables help size your nozzles for your travel speeds.
Whereas finer sprays from conventional nozzles can re-distribute themselves with wind or turbulence, covering up poor patterns, the coarser droplets produced by low-drift sprays will go where they’re pointed. Therefore, there is only one chance to get uniform coverage across the boom. Before spraying, set your sprayer to your lowest expected spray pressure (say 30 psi) and your lowest expected boom height and inspect the spray pattern overlap. For low-drift sprays, try to achieve a nozzle pattern width that is twice your nozzle spacing at the target height. If necessary, adjust your boom height, increase pressure, or select wider angle nozzles. This will ensure that the coarsest droplets at the pattern edge are mixed in with the more abundant, finer droplets found in the middle of a pattern.
