Large droplet size can still get the job done

Combining technology may improve spray performance, and dispel a few myths.

Posted: May 26, 2005

Research shows while coarser droplets means less coverage, herbicides are still effective.

Even though air-induction venturi nozzles, for example, deliver less coverage on the plant surface, herbicide efficacy doesn't appear compromised, at least not in post-emergent chemical treatments.

In most of the dozens of treatments involving both contact and systemic herbicides, such as Roundup Transorb, Odyssey, Liberty Link, Everest/Buctril M, Puma Super/Refine Extra/MCPA and Reglone, the products were equally effective regardless of nozzle type.

Although coverage/droplet size varies depending on the nozzle size and operating pressure range produced by different manufacturers, the research shows, in relative terms, that venturi nozzles, which deliver about 35 droplets of chemical per square centimeter (cm²), appeared to be as effective as nozzles that deliver much finer sprays. That contradicts conventional thinking, but the research results speak for themselves.

Extended range nozzles, used in this research, produced fine to medium sprays, with coverage of about 125 to 150 droplets per cm².

Turbo TeeJet nozzles, on research equipment, produced medium to coarse sprays with a droplet density of about 75 to 100 droplets per cm².

Venturi nozzles produced coarse to extremely coarse sprays with a droplet density of about 20 to 50 droplets per cm².

"With the coarser droplets produced by venturi nozzles, drift is reduced," says Storozynsky. "Standard thinking is that coarser droplets would reduce efficacy, but that wasn't the case. Control with venturi nozzles was just as good as with nozzles producing finer droplets. That result surprises many people."

Fit for shrouds

Still a role for shrouds in helping to reduce spray drift under adverse conditions.

Another contradiction is that sprayers equipped with low-pressure venturi nozzles can produce more spray drift than high-pressure venturi nozzles, says Storozynsky. "In the past, high pressure has been associated with high drift, but not with the high-pressure venturi," he says. "When operated between 40 and 120 psi, drift is reduced."

To minimize drift with ground sprayers in higher wind speeds, shrouds should be considered, says Storozynsky. Producers trended away from using shrouds with the development of low drift nozzles, but shrouds may help increase the operating range of low pressure venturi nozzles.

"Shrouds were popular up until the mid-90s, but dropped off with the introduction of newer nozzle technology," says Storozynsky. "Most farmers found them a nuisance. They couldn't see the spray, so they didn't know if the nozzles were plugging. Dripping and sweating of chemicals was also a concern. Chemicals clinging to the shroud could damage the next crop. They also had to wash the shroud when switching chemicals. With lower drift nozzles, many stopped using shrouds."

While it's an area of ongoing research, he suspects low drift levels can be maintained even under slightly higher wind conditions, if shrouds are used in conjunction with low pressure venturi nozzles.

Role for air assist

Expanding research treatments to include pre-emergent, pre-harvest herbicide and insecticide applications may better demonstrate the potential benefit of sprayers equipped with air assist and electrostatic technology, says Storozynsky.

The sprayer accessories, intended to boost the performance of spray penetration on high clearance sprayers, increase overall sprayer costs and need more horsepower to operate. However, some manufacturers claim improved spray deposition and penetration increases efficiency, which means both herbicide and water rates can be reduced.

Storozynsky's research, under field conditions, showed air assist and electrostatic sprayers performed as well as standard pull type and high clearance sprayers for post-emergent applications at 5 and 10 gpa. Results may be different in a pre-seeding burn-down where weeds have no crop cover, or in a pre-harvest herbicide treatment, which generally means higher and heavier crop canopy.