In an effort to minimise costly off-target crop damage, the Grains Research and Development Corporation is asking growers to be ‘community minded’ and ‘spray aware’ as rain and warm temperatures create a weed bonanza on many properties.
GRDC grower relations manager Richard Holzknecht said while it was important to control fallow weeds early, it was equally important spraying is only undertaken when weather conditions are just right.
“Growers need to talk with their neighbours to determine the location of any sensitive crops, such as cotton, and ensure they understand label recommendations and permit regulations, particularly those governing the use of 2,4-D labelled products,” Mr Holzknecht said.
He warned there were two types of spray drift — physical and inversion — which pose significant risks during summer as day and night temperature fluctuations create inversions overnight and early in the morning.
Physical drift is when spray droplets drift directly from the spraying application, carried by winds, and inversion drift is when spray droplets are preserved and scattered at a later time when warm air is held above cool air, creating a trapped atmosphere.
In an inversion, chemical droplets which remain suspended in concentrated form can be carried off your weeds and transported significant distances, destroying any hard work done.
“It is important growers understand the weather conditions that indicate an inversion is present and avoid spraying during these times,” Mr Holzknecht said.
Signs of an inversion include calm, low winds, dust remaining suspended, fog or mist forming in low areas and sounds travelling long distances.
Wind speed should be closely monitored while spraying at least every 15 to 20 minutes, and if the wind drops, spraying should stop.
Steps to reduce spray drift risk include:
● Only spray when winds are 3 to 20 km/h when using coarse to extremely coarse nozzles, and 5 to 15 km/h when using finer spray nozzles.
● Only spray when wind direction is away from sensitive crop areas.
● Monitor weed stress and target small and actively growing weeds for maximum product efficiency.
● Maintain a consistent boom height of 50 cm or lower from the spray target.
● Always use nozzles that produce a droplet size stated on the product label.
● Always choose the coarsest spray quality that will provide an effective level of control.
● Understand droplet survival. When evaporation is low (low temperatures and high humidity) droplets stay heavier and fall quickly to the ground, but they also survive longer. When evaporation is high droplets become lighter and liable to physical spray drift. Use larger sized spray nozzles and higher water volume in high evaporation conditions to increase droplet size and spray most efficiently.
● Operate nozzles at the pressure they are designed to be deliver at. Changing nozzle pressure can result in smaller droplets and less product hitting the weeds.
● Keep operating speed less than 18 km/h with trailer rigs and less than 22 km/h with self-propelled sprayers.
● Known what Delta T is. Spray conditions are optimal when Delta T is between 2 and 8. You can spray up to Delta T 10 provided you increase droplet size and water volumes. Delta T is a weather value calculated by subtracting the wet bulb temperature from the dry bulb temperature.
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