GroundCover™ Supplement Issue: 137 November - December 2018
Spray operators should use reliable weather forecast information to plan for summer weed spraying and then adjust variables they can control to reduce spray drift
Summer 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 risk.
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 and particulate drift. Droplet drift during inversion conditions is the major cause of off-target damage.
In summer, 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 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.
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: crops, bee hives, waterways and vegetation and to 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 GRDC’s GrowNotes™ Spray application for grain growers.
Spray Application Manual
Spray Inversion Fact Sheet
Maintaining efficacy with larger droplets Fact Sheet
New 2,4-D instructions: apvma.gov.au/node/15581