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Opening the Window of Spray Opportunity

09 April 2012


Making the most of the short window of opportunity to control pests and diseases safely and efficiently in agricultural crops is often a challenge for grain growers. Graeme Tepper, MicroMeterological Research and Educational Services (MRES) says this opportunity may be shortened by unsuitable weather conditions.

Mr Tepper says growers and spray applicators must be able to identify good or bad weather conditions at a local scale and react almost immediately to start or stop applications.

Mr Tepper has produced a new Grains Research and Development Corporation (GRDC) guide to assessing weather conditions, Weather Essentials for Pesticide Application.

He says the booklet aims to help those applying pesticides to understand, observe and interpret local weather conditions and micro-climates.

Together these control the amount, concentration, transport and final destination of airborne pesticides and this knowledge will help them plan and effectively execute spray applications to avoid spray drift, he says.

“Micro-weather factors important to the application of pesticides over a specific target can be significantly and critically different to the broad-area conditions indicated by weather forecasts, weather maps and off-site weather observations – especially overnight and into mid-morning,” Mr Tepper said.

The micro-weather factors are driven by local topography and variations in heating and cooling of the underlying surface.

He says growers should consider all weather factors before and while applying pesticides:

  • atmospheric stability (including up and down air currents and surface inversions);
  • wind speed and direction;
  • temperature; and
  • humidity.

Research has found that spray particles are likely to drift further and in higher concentrations when there is a surface temperature inversion, Mr Tepper says.

“Field tests show the greatest drift deposits occur with relatively high wind speeds (up to about 10km/h), coupled with a temperature inversion and spray in the small droplet spectra (about 200µm).

“It is essential that all spray-rig operators can recognise, and respond to, these micro-weather factors to minimise the risk of spray drift and maximise the potential for the pesticide to reach the appropriate target.”

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