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Environmental conditions and windspeeds affecting spraying operations

The following environmental conditions at the time of spraying will influence the effectiveness of the spraying operation:

  • Weather: High temperatures, low humidity, high delta T and wind speeds less than 3km/h or greater than 15km/h during spraying may mean a loss of herbicide through drift or evaporation;
  • Plant stress: Lower levels of control often occur when weeds are under environmental stress when sprayed. For example, for glyphosate to work it must be translocated around the plant to its site of action and weeds need to be actively growing (not stressed) to maximise the uptake and translocation. Stress may be caused by environmental conditions such as drought, water-logging or frost;
  • Rainfall: Rainfall shortly after spraying may wash the herbicide off the plants before it has had time to act.

Wind speed is the main factor to be considered at the time of spraying for winter crops. The following points should be followed in windy conditions:

  • Read the herbicide label or seek advice. Take particular note of the toxicity of the herbicide and any adverse effects it will have on other crops, trees, wildlife or human health.
  • Assess possible damage downwind from the paddock to be sprayed. An advantage of spraying in high wind speeds is that you know the direction in which the spray will drift. Drift can also be a problem in very light winds, particularly if there is a temperature inversion.
  • Adopt methods to reduce drift. For example, using low pressure nozzles or low nozzle height. Remember that no current method eliminates drift.
  • Assess the risks and decide if they are worth taking. For example, if you are spraying a selective herbicide in wheat and down-wind there are other wheat crops, you are probably fairly safe to spray in a strong wind. On the other hand, if you are spraying knock-downs and down-wind there is a belt of new trees you have been trying to establish, wait for more favourable spraying conditions.
  • Do not spray if the maximum wind speed is greater than the speed at which you are spraying. If the wind speed is greater than your speed of travel, you risk coming into contact with the drift when spraying with the wind behind you.

Use a small hand-held anemometer (used to measure wind speed and is relatively cheap and a good investment when spraying). Wind speed is variable, so when deciding whether to spray, read the maximum wind speed during a wind gust. The average wind speed will be lower than this figure, so allow a margin of safety. Wind speed and direction should be recorded for all spraying.

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