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Nozzle Selection and Water Rates for Post-Emergent Spraying

Recent industry focus on spray drift, offsite damage and new label requirements has resulted in most growers setting up spray booms with nozzles that produce a coarse to very coarse spray droplet. While this is satisfactory for most spraying that is done during the year, it is not an ideal set up for early post-emergent spraying. In addition, the drive to increase efficiency has resulted in water volumes of 50 L/ha becoming more common.

Group A herbicides, which are used for post emergent grass weed control, are slow to be taken in and translocated within the plant. These products require good coverage and the correct adjuvant to maximise results. Increasing water volume to 75 L/ha or greater and decreasing droplet size to medium rather than coarse, will increase the evenness of application and coverage of the target grass weeds.

When applying early post-emergent products such as Status, Topik or Axial to crops, a medium droplet should be used with a water volume of at least 75 L/ha. Where stubble cover or a crop canopy is likely to intercept spray droplets, a water rate of greater than 75 L/ha should be used.

Late post-emergent spraying using coarse droplets and low water volumes (50-70 L/ha), will provide a satisfactory result where minimal stubble cover exists, the target is large, or when products that are translocated rapidly through the plant are used, such as group B, I & M herbicides.