Spray droplet size plays a critical role in maximising herbicide contact on the weed surface. Some applicators have been concerned that the move to coarser nozzles to reduce spray drift could reduce the efficacy of non-selective and phenoxy herbicides used for summer weed control.
André Sabeeney, technical marketing lead at Nufarm, and field development officer, David Keetch, say the aim is to select nozzles that deliver the largest spray quality to reduce spray drift and implement measures that don’t compromise summer weed control.
“Droplets less than 100 microns in diameter only have enough inertia to travel approximately 34cm downwards after exiting the nozzle. As a result, even when the boom is set correctly at 50cm above the target, these droplets will not hit the mark and will often be suspended in the air with the potential to drift off-site,” Mr Sabeeney says.
“Conversely, droplets over 720 microns are likely to bounce off the leaf surface or run off vertical surfaces.”
“For an effective spray application, the product must hit the target and be held on the plant surface long enough to be absorbed. If this doesn’t happen due to poor application techniques, such as low water rates or excessive ground speed, the investment in product and application is potentially wasted.”
‘Spray well – correct nozzles, adjuvants and water rate’ is WeedSmart Wisdom to reduce the risk of low-dose applications that can contribute to herbicide resistance in weeds.
In brief: The correct height is 50cm above the target. This is deceptively low!
The details: The drift potential of spray droplets increases dramatically when the boom is higher than 50cm. At the height of 75cm, the drift potential increases four-fold; at the height of 1m, the drift potential increases ten-fold.
From an elevated position in the tractor cab, it is difficult to judge the height of the boom, which will appear closer to the ground than it is.
Another essential consideration is ground speed. At 15 km/hr, most droplets are likely to hit the target. If the spray rig travels at over 20 km/hr, a larger percentage of droplets will not hit the target. Higher ground speed can interrupt fan angles and produce turbulent effects behind the machine. Getting over the farm quickly loses its benefit if a considerable percentage of the product is not hitting the target.
In brief: Summer spraying conditions are rarely ideal. Decisions must consider the weed type and size, plant stress and environmental conditions at the time of spraying.
The details: Droplets at the larger end of the preferred range are more efficacious when the spray conditions are less than ideal. The herbicidal effect is better because the droplets are less susceptible to evaporation and can ‘survive’ for longer on the plant surface.
When choosing nozzles, ask about the volume mean diameter, or VMD, of different brands and nozzle types. Within each classification, there is a wide variety in droplet size distribution. Nozzles that can produce a Very Coarse (or coarser) spray quality are ideal for spraying weeds over summer.
Avoid compromising herbicide and water rates when using an Ultra Coarse spray quality. While the droplet size is ideal for herbicidal effect, the number of droplets per litre of water is significantly less than with a Very Coarse spray quality.
Droplet size influences various factors that can result in low dosing of herbicide. Low dosing is a real risk in summer weed control that can contribute to the evolution of herbicide resistance.
In brief: Spray formulation and adjuvants can significantly affect droplet size.
The details: Compared to water, surfactants can increase the percentage of driftable fines (droplets less than 150 microns) and polymer drift-reducing agents often increase the percentage of droplets larger than 720 microns.
Around the world, companies are working on several droplet-altering technologies. Nufarm has developed a new formulation technology for 2,4-D amine herbicides that ensures 96 per cent of the spray droplets are within the optimal range of 150 to 720 microns. This is a 16 per cent improvement on the droplet size distribution of current 2,4-D Amine herbicide formulations, enhancing herbicide efficacy and on-farm efficiency.
Ask your questions about spray application on the WeedSmart Innovations Facebook page WeedSmartAU or Twitter @WeedSmartAU
Dave Keetch and André Sabeeney, Nufarm.
