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Accurate weather forecasting data underpins best-practice weed spraying

Off-target spray drift poses significant risks and attention to nozzle set-up is important to help minimise adverse outcomes.
Photo: GRDC

It pays to check weather conditions, labels and machinery set-up to prevent spray drift.

Spray operators are advised to use reliable weather forecast information to plan for weed spraying, and then adjust variables that can be controlled to reduce spray drift.

Summer (and autumn) weed spraying is an important part of grain production.

But off-target spray drift poses significant risk, particularly to horticultural and cotton crops, and it needs to be actively managed to minimise the potential risks.

Spray drift is the movement of pesticide away from the target area in the atmosphere.

The three main forms of drift are:

  • Droplet drift
  • Vapour drift
  • Particulate drift.

In summer (and autumn), growers often avoid spraying during the heat of the day.

But spraying at night will increase the risk of spray drift because the air movement is often different at night.

As the ground cools on summer (and autumn) evenings, it can cause the low-level air in contact with the ground to cool, which reduces turbulence or mixing of the air.

This is known as an inversion because it is the opposite of the more usual conditions where the air temperature is warmer closer to the ground.

In cases where inversions occur at night, airborne droplets tend to stay in the air for longer and can be carried for more than 40 kilometres.

If the weeds are not at the point of stress during the day, with the correct risk assessment and control measures growers could possibly spray longer during the day.

Tips to reduce spray drift

  • Always expect that surface temperature inversions will form later in the day, as sunset approaches. They are likely to persist overnight and beyond sunrise on many occasions.
  • Use weather forecasting information to plan the application.
  • Only start spraying after the sun has risen more than 20 degrees above the horizon and the wind speed has been above four to five kilometres per hour for more than 20 to 30 minutes, with a clear direction that is away from adjacent sensitive areas.
  • Choose all products in the tank mix carefully. This includes the choice of active ingredient, the formulation type and the adjuvant used.
  • Read the crop protection product label and understand the products mode of action and coverage requirements on the target.
  • Select the coarsest spray quality that will provide an acceptable level of control. New instructions for use of products containing 2,4-D require very coarse (or larger) spray qualities (check through the Australian Pesticides and Veterinary Medicines Authority website).
  • Set boom height to the lowest level required to achieve a double overlap of the spray pattern.
  • Avoid high spraying speeds, particularly when ground cover is minimal.
  • Be prepared to leave unsprayed buffers when the label requires, or when the wind direction is towards sensitive areas.
  • Continually monitor the weather at the site of application during period of application.

Managing risk

There are variables that spray operators can directly control to minimise the risk of off-target movement of spray.

Spray quality, boom height, spraying speed and tank mix can all have an impact on drift potential.

Nozzle selection will have the biggest effect on drift, but the choice of adjuvant to increase droplet size can also assist.

Where factors cannot be controlled, such as the weather, use reliable forecast information to plan when to apply products at times of lowest risk.

Bureau of Meteorology meteograms and forecasting websites provide information on likely wind speed and direction for five to seven days in advance of the intended day of spraying to assist in determining the likely development of a surface inversion.

It helps to be aware of sensitive areas around spray zones, including:

  • Crops
  • Bee hives
  • Waterways
  • Vegetation
  • Also, talk to neighbours and local advisers about areas that may be sensitive to spray drift.

For detailed guidance to reduce spray drift when spraying summer weeds refer to GRDCs GrowNotes™ 'Spray application for grain growers'.

GRDC Project Code: BGC00003

More Information: Bill Gordon,

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