As harvest has reached an end at the SFS sites, attention now turns to maintaining and preparing the trial sites for the upcoming sowing period.

Spraying for weed management is an important part of this process, however every effort is made to ensure that herbicide applications are performed in the correct manner during the right conditions for highest efficacy and reduced risk of herbicide resistance.

Multiple contributing factors to this are controllable by the operator:

• choosing a suitable nozzle and spray quality (droplet size) can influence the level of spray control.
• Droplet sizes less than 100 microns are highly drift-prone and can be difficult to hit the target, while droplets larger than 200 microns generally land on the target and are less likely to evaporate early.
• Therefore selecting the coarsest spray quality whilst maintaining appropriate level of control is critical.
• Boom height and speed can be adjusted to reduce change in droplet size and improve efficacy, whilst recommended product rates are vital for efficacy and reducing resistance.
• Greater amounts of active ingredient used or released requires a greater distance downwind to allow for sufficient dilution to occur for deposition on the surface.
• Addition of adjuvants such as wetters that reduce surface tension can also reduce droplet size, whilst oils will often increase droplet size.

Wind speed and direction are the most important factors when deciding whether to spray or not. Spraying should not be conducted when wind speeds are above 20 km/h, and a rule of thumb is that the maximum wind speed gusts should not be more than about one-third of the average speed above the average wind speed.

Temperature and humidity can play a role in droplet size, evaporation and translocation of the product within the target plant. ‘Delta T’ is the indicator used to determine the evaporation rate of droplets and is calculated from the difference in temperature between a wet bulb thermometer and a dry bulb thermometer. The common guideline is to spray when the Delta T value is between 2 and 8. Spraying during surface temperature inversion conditions such as fogs or haze must be avoided, as droplets can be prevented from diluting, and it is unknown how far they may travel.

For further information on reducing spray drift and suitable spraying conditions, read the chapters ‘Drift management strategies’ and  ‘Weather monitoring for spraying operations’ in the GRDC’s Spray Application Manual for Grain Growers, by Bill Gordon (2017). https://grdc.com.au/resources-and-publications/grownotes/technical-manuals/spray-application-manual

By Cameron Barr, SFS Research & Extension Officer