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Issue No. 8, Article 8/May 14, 2004

Tips for Keeping Herbicides on Target

The nice weather has led to a flush of weeds and an accompanying flurry of herbicide applications, with many more acres of weeds to be covered in the future. Here are some tips for reducing drift and controlling weeds.

Use coarse droplets. Select nozzles that produce coarse droplets (for example, venturi, turbo flood, or turbo flat-fan nozzles; see Table 4).

Use low pressure. Use pressures in the lower portion of a nozzle's recommended pressure range. Check a nozzle catalog to determine the size of spray droplets created by the type and size of your nozzle at various operating pressures, then select a pressure that generates coarse droplets. Contrary to belief, increasing spray pressure doesn't force the droplets into the canopy. It actually creates smaller droplets with less momentum. An exception to this rule is the use of venturi (air-induction) nozzles, which are designed for high pressure.

Lower boom height. Wind velocity increases with height. Keep the boom at the minimum distance from the target (weed) while maintaining uniform coverage. Maintain appropriate speed to avoid "boom bounce."

Use drift-reduction nozzles. Use nozzles designed to reduce drift (see Coarse droplets). Example: venturi nozzles mix air into the liquid stream and increase droplet size.

Use drift-reduction agents. Drift-reduction agents increase droplet size. Read instructions carefully, and use caution when combining drift-reduction agents with drift-reduction nozzles.

Spray when wind is less than 10 mph. Off-target movement increases with wind speed. However, spraying in completely calm conditions can result in off-target movement.

Increase spray volume. Most applications are made at volumes of 5 to 10 or 10 to 20 GPA. Increasing the GPA with a larger nozzle size produces larger droplets.

Avoid sensitive areas. Keep a buffer zone when spraying near sensitive plants, gardens, wetlands, and so on. If appropriate, return when conditions are favorable.

Read the label. Herbicide labels provide instructions that you must follow in order to make a safe and legal application. Label instructions do change, so read them carefully each time you purchase a pesticide.

Know your herbicide. All herbicides can drift as spray droplets, but some herbicides are sufficiently volatile to cause plant injury from drift of vapor (fumes). For example, 2,4-D and MCPA esters may produce damaging vapors, while 2,4-D and MCPA amines are essentially nonvolatile.

Pay attention to the weather. Low relative humidity and/or high temperature will cause more rapid evaporation of spray droplets between the spray nozzle and the target than will high relative humidity and/or low temperature. Evaporation reduces droplet size, which in turn increases the potential drift of spray droplets. Avoid spraying during an inversion when the air is very calm. Inversions typically occur in the early morning and late evening. During an inversion, small spray droplets remain suspended and concentrated. They can later be blown off target in unpredictable directions.

This leaves a lot to be considered before and when loading up the sprayer and heading out to the field. Always remember to check the nozzles on the boom for damage and assurance that they are producing the desired GPA. A few minutes calibrating can save a lot of time and money in the long run.

For every field, please follow this checklist to ensure proper application and reduced drift:

  • Ensure that weeds are present and within the range of control on the herbicide label.
  • Record the herbicides and adjuvants used and their rates.
  • Identify the crop stage.
  • Note the nozzle type and size used.
  • Measure your boom height.
  • Record spray pressure (PSI) and volume (GPA).
  • Record operating speed (MPH).
  • Record number of acres sprayed.
  • Be aware of sensitive areas.
  • Record wind direction and speed.
  • Note weather conditions such as relative humidity and temperature.
  • Record the time and location of the application.

There are many resources for information on herbicide drift. Here is a partial list:

Keep Your Herbicides on Target: free pocket-size card with drift reduction tips. Available from University of Illinois Weed Science Extension.

Selecting the Correct Nozzle to Reduce Spray Drift.

Virtual Spray Table.

Spray-Droplet Size Measurement and Classification.

Spray Drift Resources.

Kansas State University Drift Education Materials.

Compendium of Herbicide Adjuvants.

Spray Drift Task Force.

Drift Management and Nozzle Selection Resources.

Effect of Major Variables on Drift Distances of Spray Droplets.

University of Illinois Weed Science Extension; (217)333-4424

University of Illinois Pesticide Safety Education; (217)244-2363 or (800)644-2123

Illinois Department of Agriculture--Bureau of Environmental Programs; (217)782-2172 or (800)273-4763; Damaged property (800)641-3934; Complaint form (217)785-2427

--Dawn Nordby and Scott Bretthauer

Dawn Refsell
Scott Bretthauer

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