NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
May 20, 1999
Producers Alerted to Herbicide Drift Potential
Herbicide volatility is not a precise indicator of whether spray drift and damage to
nontargeted crops will occur. Nonvolatile herbicides such as Roundup, Liberty and
Gramoxone, along with the micro-rate for sugarbeets, all have the potential to damage any
emerged crops in adjacent fields, says a weed scientist at North Dakota State University.
Paying attention to droplet size and droplet movement will help minimize herbicide drift.
"People tend to think that if a herbicide drifts a lot, then it's volatile. That's
not exactly true," says Alan Dexter, extension sugarbeet weed specialist for North
Dakota State University and the University of Minnesota. "Controlling droplet
movement of nonvolatile herbicides can control damage from drift, but under certain
environmental conditions, small spray droplets can move long distances and cause
damage."
Dexter and Vern Hofman, an extension agricultural engineer at NDSU, offer the following
recommendations for controlling herbicide spray drift from aerial and ground applications.
The recommendations are based on research conducted at NDSU and other land-grant
universities:
- The primary method of reducing spray drift involves controlling droplet size. Small
droplets can move farther off target than large droplets. Both ground and aerial
applicators can increase droplet size by reducing spray pressure, increasing spray nozzle
size, using drift-reduction nozzles and configuring nozzles in a rearward orientation.
- Ground applicators can reduce the distance between the nozzle and the spray target.
However, a minimum boom height is essential to ensure a proper overlapping spray pattern.
Using a wider-angle nozzle allows a lower boom height while maintaining proper overlap.
- Ground applicators can use lower pressures with extended-range nozzles. Also, switching
to drift-reduction nozzles, such as Turbo Teejet flat-fan nozzles or air-induction
nozzles, can reduce drift significantly.
- Regardless of application method, avoid spraying during temperature inversions, which
create vertically stable air. Under normal conditions, air is warmest near the soil
surface, but an inversion occurs when the air near the soil surface is either cooler or at
a similar temperature to higher air. In this situation, the air does not mix vertically
but instead creates a "ceiling" that forces small spray droplets to move
horizontally and contact nontargeted crops, some long distances away. Road dust or smoke
that hangs low along the ground signals a temperature inversion. Likewise, a fog can occur
only during a temperature inversion, so spraying during a fog or shortly after it lifts is
especially risky.
- Avoid spraying when winds are high. The ideal time to spray is in the absence of a
temperature inversion when wind velocities are low. However, the combination of a low wind
speed and a temperature inversion allows small spray droplets to move long distances, so
spraying when wind speeds are low is not always safe.
- Apply herbicide, either by ground or by air, when the wind is blowing away from a
susceptible crop, if possible.
- Leave an unsprayed buffer on the downwind side of the targeted field if a susceptible
crop lies adjacent to it.
- Use a ground sprayer with a spray shield, which can reduce spray drift anywhere from 50
to 90 percent.
- Air-assist sprayers can reduce drift if set so that the air dissipates when it reaches
the ground. Air from such a sprayer that curls up off the ground instead of dissipating
will be carrying spray droplets with it, and these suspended droplets will be small and
quite susceptible to drift.
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Sources: Alan Dexter (701) 231-8131 and Vern Hofman (701) 231-7240
Editor: Dean Hulse (701) 231-6136