Articles about mixing and pesticide in horizontal boom sprayers
If you have a limited amount of clean water to rinse your sprayer, this can help you decide how to make the best use of it. Simple enter two values – the amount of pesticide remaining in your tank sump (including the suction line to the pump and the return line to the tank), and the amount of clean water you want to add. The units (gal, L) are not important as long as they are the same for both entries.
The app allows you to calculate the dilution power of up to 5 sequential rinses.
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:
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.
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.
We all know the importance of cleaning out a sprayer. It protects a sensitive crop. It protects people working with the sprayer. It protects the sprayer and its components. But cleaning the sprayer is a pain. Here are some tips to make it easier.
Some herbicide label instructions are cumbersome, requiring many flushes with full tanks of water. Many applicators look for shortcuts and hope they get away with it. It doesn’t have to be guesswork. The following is a checklist that may help.
A few supplies can help ensure a clean sprayer tank.
The products most frequently implicated in sprayer contamination are two members of the Group 2 modes of action: the sulfonyl ureas (e.g., thifensulfuron (Refine) and tribenuron (Express)), and the triazolopyrimidines (e.g., florasulam (Frontline, PrePass) and pyroxsulam (Simplicity)). Since these herbicides dissolve better at higher pH, proper cleanout usually requires ammonia, a weak base that raises the solution pH. The third member of Group 2 products, the imidazolinones (imazethapyr (Pursuit), imazamox (Solo, Odyssey), imazamethabenz (Assert), imazamox (Ares, Adrenalin, Altitude or Viper)), tend not be implicated in as many residue issues, and don’t require ammonia for cleanout.
Remove pesticide from mixing and spray equipment immediately after spraying – it makes the job easier. The main areas of concern are the tank wall, sump, plumbing (including boom ends), and filters. First, spray the tank completely empty while still in the field. It’s sometimes OK to cover previously sprayed areas – all herbicides must be crop-safe at twice the label rate to be registered by the PMRA. Take care with residual products that may create problems down the road. Reduce the rate or choose a fallow field to be certain. Second, add 10 x the sump’s remnant of clean water, circulate, ensuring agitation is on, and spray it out in the field as well. Repeat. These two rinsing steps will take care of the majority of the cleaning and won’t take very long. The less remaining volume there is in your tank after the pump draws air, the less water is needed to dilute this remainder to an acceptable concentration. Having a clean water tank on the sprayer and a wash-down nozzle makes this job easier.
Herbicide residue may precipitate out of solution in some parts of the sprayer or plumbing. A thorough visual inspection can identify these problem areas and ensure that they are cleaned properly.
Removal of residues from tank walls is best accomplished with a direct, pressurized spray. Make sure all parts of the wall have been in contact with clean water. Use a wash-down nozzle if it provides complete and vigorous coverage of the interior tank surface.
Empty the sump as completely as possible by spraying it out. Any spray liquid or herbicide concentrate remaining in the sump area will be re-circulated in the sprayer. The only way to remove any remaining herbicide is through dilution by repeatedly adding water, and leaving as small a remainder as possible.
Plumbing can be a significant reservoir of herbicide residue. Removal from plumbing can be achieved by pumping clean water through the boom while ensuring that all return and agitation lines also receive clean water and all residue is flushed out. This may require opening and closing various valves several times, and repeating the process with new batches of clean water. Boom ends can extend up to 6” beyond the last nozzle at each end of each boom section. These ends must be flushed to removed trapped residue. A useful product that does this automatically is the Pentair Hypro Express Nozzle Body End Cap, or better yet, consider recirculating booms.
The most effective use of a volume of rinse water is to divide it equally across several repeat washes. Assuming a 10 gallon sump remainder, three washes with 30 gal each are as effective as two washes with 70 gallons each, and equal a single 600 gal wash.
It’s even more efficient to use a separate clean water pump, introducing clean water as the rinsate is sprayed out. This saves water and time, and results in even more dilution.
Nozzle screens and in-line filters can be a significant reservoir for undiluted or undissolved herbicide and are one of the most overlooked parts of sprayer decontamination. Remove all filters and nozzle screens and thoroughly clean these with fresh water. Run clean water through plumbing leading to the screens.
Nozzle bodies can harbour herbicide mixture. When cleaning a spray boom, rotate through all nozzles in a multiple body to ensure clean water reaches all parts of these assemblies. Remove screens that may have been used with herbicide.
Adjuvants such as ammonia can assist the tank decontamination process, especially with sulfonyl urea and triazolopyrimidine-containing products. Ammonia does not neutralize herbicides, but it does raise the pH of the cleaning solution which helps sulfonyl urea herbicides dissolve. When decontaminating after an oily (EC) formulation, the use of a wetting agent such as AgSurf will assist in removing oily residue that may trap SU herbicide on tank and hose material. Commercial tank cleaning products that contain ingredients for removing persistent deposits are available.
Both plastic and stainless steel are common tank and wet boom materials, and both can be cleaned using the above procedures. However, stainless steel is easier to clean, and this means that less time may be required. Consider the choice of materials a productivity factor in your next purchase or upgrade decision.
Always spray out the tank in the field. Do not drain the tank while stationary unless you are certain it is free of pesticide and that you are away from sensitive areas and waterways. Consider a continuous rinse system. Consider building a biobed for safe disposal of dilute pesticide waste.
Sprayer cleanout will probably never be the easiest job on the farm. But looking at it in a smarter way can prevent frustration and save time.