How to fight spray drift
Low drift nozzles only part of the equation
Posted: June 20, 2007
With safety, liability and environmental concerns driving new developments, producers today have more options than ever in the ongoing battle against the airborne drift of chemical sprays. Perhaps the most recognizable symbol of this spray drift revolution is the venturi nozzle, which continues to cut drift substantially as the technology becomes more and more popular among producers.
Venturi nozzles, however, are not the be-all and end-all solution to spray drift, says Brian Storozynsky, sprayer technology specialist with the AgTech Centre in Lethbridge, Alberta. Although venturi nozzles can play an important role, considerations such as boom height, nozzle pressure, water volume, wind conditions and even the use of shrouds are still key to a well-rounded line of defence against drift.
"Although venturi nozzles have been known to cut spray drift by half, the fact is that you still can't spray into 25 to 30 mph winds just because you're using a venturi nozzle," he says. "There are still other factors to consider, and in today's farming environment, it pays to start thinking about them well in advance of spraying."
The 8002 standard flat fan nozzle set at 10 gallons per acre was the industry standard for sprayer nozzles for many years. Today it is just one of a host of nozzle types, designs and capacities, with options including standard, extended range, pre-orifice, combo jets, and single and twin air induction nozzles, with a range of models within each type.
Few nozzle types have been found to reduce spray drift as effectively, however, as the venturi nozzle. Venturi-type, or air induction, nozzles use a venturi in the nozzle body to induce air into a mixing chamber. Compression in the chamber results in air bubbles inside the liquid spray droplets, producing larger spray droplets which minimize drift.
An AgTech Centre study found that low pressure venturi nozzles reduced drift by 35 to 60 percent compared to conventional, pre-orifice low drift nozzles when operated between 25 and 60 pounds per square inch (psi). The same study found that high pressure venturi nozzles reduced drift by 60 to 90 percent compared to pre-orifice nozzles when operated at 40 to 120 psi.
Because they produce a coarser droplet size, which carry a lower risk of drift when higher spray booms are used, high pressure venturi nozzles are best suited for high clearance sprayers if drift is the primary concern. Although low pressure venturi nozzles are suitable for high clearance spraying, using them for this purpose warrants extra attention to wind conditions and boom height.
Storozynsky says many producers are leaning towards using low pressure venturis because they get better coverage while still reducing spray drift. However, he's not sure if that holds true in extremely windy conditions.
"There hasn't been a lot of research comparing the efficacy of low and high pressure venturis in windy conditions. As a general observation, however, we're finding that droplets from high pressure venturis often seem to hit the target better in windy conditions than droplets from low pressure venturis in the same conditions."
Although the ideal thing to do in windy conditions is to hold off spraying until the wind dies down, this isn't always an option, he says. "Using high pressure venturis or other coarse droplet producing nozzles in wind is an option if it means spraying the weed at the stage of the weed's life when spraying would be most effective."
Boom height key
When it comes to deciding on boom height, the answer is simple, says Storozynsky: the lower the height, the lower the drift. "Of course, you don't want to go too low or you won't get adequate coverage. That's why we generally recommend a minimum operating height of around 18-20 inches. That height has become the target for a lot of producers because it usually ensures adequate coverage with minimal drift."
Heights of much higher than 18-20 inches, however, can increase drift significantly. The good news is that automatic boom height control, although still a relatively new technology, continues to make the process of keeping boom height level easier for producers.
"Back when I was custom spraying several years ago, I remember the hardest part of spraying was constantly adjusting boom height to the conditions of the field, and over a 16-hour day, that could get extremely tiring," he says.
"Often, what happens is you stop bothering trying to keep the boom at 20 inches. Before you know it, it's up to 30 inches and you have a lot of drift. Automatic boom height control can take the pain out of that process."
Shrouds still relevant
Many producers have stopped using shrouds, once a popular method for reducing drift, since the advent of the venturi nozzle. However, Storozynsky says shrouds, when used in concert with ultra low-drift nozzles, can add an extra line of defence against spray drift without the nuisance they tend to create when used with some older nozzles.
"Shrouds were commonly used with pull-type sprayers in the late '80s and early '90s when most producers were still using extended range nozzles," he says. "But when nozzle technology started reducing the percentage of fine droplets, we saw producers starting to take their shrouds off because they tend to block the view of the spray stream. Also, shrouds on suspended spray booms, especially on high clearance sprayers, were difficult to use and ineffective if not operated within a certain height."
Shrouds can also make changing nozzles a dirty job because of their tendency to collect chemical residue. However, this may not be such a problem when using ultra low-drift nozzles such as the venturis.
"Although there hasn't been a lot of research done on the impact of shrouds on venturi nozzles, the fact that venturi nozzles create a lower percentage of fine droplets suggests there may be less chemical swirling within the shroud."
The basic rule with water volume is the higher the water volume, the lower the drift, says Storozynsky. However, this rule of thumb only holds true when comparing volume within nozzle type — it is not true when comparing the efficacy of different nozzle types to each other. "For example, if you were to compare an extended range nozzle set at 10 gal/ac to a venturi nozzle set at five gal/ac, you would find that the venturi nozzle would produce less drift."
Because of its long term commitment to developing sustainable commercial systems for producers, the AgTech Centre's study of spray drift reduction is an ongoing process. Results of AgTech's machinery evaluations, applied and scientific research, and information on its development of innovative agricultural technologies are available to producers to help them make management decisions. For more information, contact the AgTech Centre at (403) 329-1212.
Author: Jeff Melchior
Sponsored by: AgTech Centre