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Let droplet size guide sprayer nozzle choice

On a diversified farm, the sprayer is called in for many different types of pesticide applications, and these applications often require different spray quality. How to choose?

Dr. Tom Wolf, a researcher with Agriculture and Agri-Food Canada’s Saskatoon Research Station, says nozzle choice is a balance between coverage and drift, and is ultimately governed by droplet size. Larger droplets drift less but provide lower coverage of the target pest or weed. Smaller droplets have better coverage but drift more. Since the early 2000s, nozzle development has progressed to the point where conventional flat fan nozzles, which produced spray droplets more susceptible to drift, are no longer recommended for most applications. “The low-drift nozzles, particularly the air-induced nozzles, are providing farmers with a better choice. Research shows we can reduce the spray that drifts away from its target to less than 0.5 percent of the applied amount. That’s a decrease of more than 80 percent compared to conventional sprays,” says Wolf. “There is no evidence that we need Fine sprays except maybe with some of the insects.”

Plant target size and shape determines droplet size

Some plant targets are easier to hit than others, so droplet size is an important criteria when selecting nozzles. The more leaf area to be covered with a herbicide or a fungicide, the more droplets will be required. Wolf has found that vertical grassy weed herbicides require more droplets compared to horizontal broadleaf weed targets.

To increase droplet numbers to provide better coverage, Wolf advises using higher water volumes with low-drift nozzles. Using higher water volumes to produce better coverage has the advantage of allowing larger average droplet sizes to be maintained, minimizing evaporation and drift. With some pesticide active ingredients, Wolf also explains, larger droplets provide better pesticide performance than small droplets, such as with the Group 2 and 4 herbicides and glyphosate.

The other way to increase coverage is to use a finer droplet. Of course, smaller droplets drift more and may evaporate before they hit the target.
Put into practice, Wolf says a low-drift, air induction nozzle covers many western Canadian cropping circumstances. He says droplet size can be adjusted with a low-drift nozzle by changing pressure or water volume.

These nozzles, among them the Air Bubble Jet, the Greenleaf AirMix, the Hypro ULD, the Hypro Guardian Air, the TeeJet AIXR, the Hardi MiniDrift, the Lechler IDK, or the ComboJet SR and MR, operating at 60 psi and seven to 10 US gallons per acre (gpa), provide a very good compromise of coverage and drift control for herbicide applications.

Wolf says lower water volumes can be used for systemic early season herbicides. Higher water volumes should be considered for larger canopies or certain (contact) modes of action. Larger volumes to 12 or 15 gpa should be considered for late-season fungicides and desiccation.

Maintaining good spray patterns and overlaps is important to guarantee good pest control; use a wide enough fan angle and high enough pressure and boom height to ensure 100 percent overlap of the spray pattern of each nozzle.

Choosing the nozzle
Wolf explains all nozzles produce a wide variety of droplet sizes ranging from 5 µm (micron) to 1000 µm in diameter. The main difference between nozzles is the proportion of the spray volume across the range of droplets produced. Low-drift sprays have a greater proportion of their volume in Medium and Coarse droplets and less in Fine droplets that would be prone to drift, under 200 µm. Yet, low-drift nozzles have enough Fine to Medium droplets to provide sufficient coverage when needed in circumstances where coverage is more important, such as in grassy weed herbicide applications.

The American Society of Agricultural and Biological Engineers (ASABE) developed the ASABE S572.1 standard to help measure and interpret spray quality from tips. Some herbicide manufacturers are providing droplet size recommendations on their label to help guide farmers in application to maximize pesticide effectiveness. For example, the Liberty 150 SN herbicide label recommends: “DO NOT apply with spray droplets smaller than the American Society of Agricultural Engineers (ASAE) Medium classification.”

Furthermore, the Bayer CropScience website provides the following application tips for Liberty 150 SN herbicide:

  • “Use nozzles designed to achieve a medium to slightly coarse droplet size (approximately 200 to 350 microns in size). Avoid larger, higher output nozzles that increase droplet size (greater than 350 microns in size) as they can potentially reduce control from inadequate weed coverage.
  • If possible, avoid very fine droplet sizes (less than 150 microns in size) as the potential for drift increases dramatically.
  • Use application speeds that, in conjunction with spray pressures, will optimize the droplet size into the medium to slightly coarse range. Excessive speeds should be avoided as they can sacrifice weed coverage and increase the potential for drift from wind shear.”

Buffer zone distances are also being tied to spray quality by the Pest Management Regulatory Agency (PMRA). When using coarser sprays, distances can be reduced significantly. This will be an additional incentive to use low-drift sprays.

Nozzle manufacturers provide information on droplet size in their product literature. These charts help guide nozzle selection. A range of nozzle sizes shows spray qualities produced at various pressure and travel speeds. Utilizing these charts helps guide nozzle selection to produce a Medium to Coarse spray that is acceptable in most herbicide and fungicide application scenarios. It is worth noting that nozzle colours do not refer to droplet classification, so do not assume that a red nozzle means Very Fine droplet size.

Always check a nozzle manufacturer’s information to learn what spray quality is produced by the nozzle being used; this will vary with nozzle type, flow rate and spray pressure.

Wolf says balancing water volume, pressure and travel speed can be done with the correct nozzle selection to produce a Medium to Coarse spray quality that will perform in most cropping situations. “The standard in nozzle selection has become the low-pressure air induced nozzles,” says Wolf. He says most custom applicators have adopted this nozzle and many larger farm operators are using them as well.

Variable rate nozzle selection uses same principles
Variable rate nozzles use some sort of mechanism in each nozzle to control flow rate in response to sprayer speed changes. At the same time, droplet size remains constant. Nozzle selection is based on spray quality; a nozzle can be selected based on its ability to produce a Very Coarse, Coarse, Medium or Fine spray quality. In most cases, farmers and applicators would select a nozzle that produces either the Medium or Coarse spray quality.

Variable rate nozzles include Delavan Agspray’s VariTarget nozzle, the AIM Command Spray System from CaseIH (also as an after-market retrofit called the Capstan Sharpshooter), and Greenleaf Technologies’ TurboDrop Variable Rate nozzle.