Managing spray drift in spring
Normally we consider spray drift to be a risk when spraying a post emergent herbicide alongside a susceptible crop, e.g. Applying 2,4-D or MCPA to a wheat crop next to a canola crop. However, spray drift can be an issue all year round. Pre-harvest applications of herbicides to crops can drift onto neighbouring paddocks and result in chemical residue issues. Prior to “crop topping”, “spray topping” or pre-harvest herbicide application is a good time to think about your sprayer set up and application process.
Spray Quality
Spray quality is defined by the ASAE standard S572 or ASABE S572.1. The purpose of these classifications is to provide users with droplet size information primarily to indicate off-site spray drift potential, and secondarily, for application efficacy. VMD is Volume Median Diameter, a droplet dimension which indicates that half of the spray volume is in droplets smaller than this number and half of the spray volume is in droplets larger than this size. It is important to note which of the standards is used by manufacturers since the ASAE S572 (old) standard is determined using only water, while the ASABE 572.1 is measured with the addition of an adjuvant.
While VMD can be useful for classifying spray quality it is also important to consider the very small droplets and the proportion of the spray volume that they make up. Driftable fines are considered to be all droplets under 150 microns. These droplets are not likely to hit the target and can be easily moved in the wind or prevailing weather conditions. A medium spray quality will have approximately 20% driftable fines. That’s 20% of product that is unlikely to reach the target. It’s also 20% of product that could be deposited onto neighbouring crops.
Spray quality required for the application of different products is often defined on the label and this is a legal requirement. Glyphosate products require a minimum COARSE droplet, while 2,4-D products require a VERY COARSE minimum. When setting up a boom spray, operators need to maximise efficacy of the products being applied whilst minimising spray drift and complying with the label.
Table 1*: “Driftable” proportion of total spray volume of each spray quality
| Spray Quality | Approximate microns |
|---|---|
| Fine | - 40-50% less than 150 microns |
| Medium | - 20% less than 150 microns |
| Coarse | -10% less than 150 microns |
| Very Coarse | - 5% less than 150 microns |
| Extremely Coarse | - <2-3% less than 150 microns |
| Ultra Coarse Spray | - <1% less than 150 microns |
*Table reproduced from Nufarm
How can I limit spray drift?
Select appropriate nozzles
Nozzles have the greatest impact on spray quality and the best way to manipulate spray quality is to choose suitable nozzles. Choose nozzles that provide the coarsest droplets that are acceptable for each application task. The GRDC has produced Nozzle Selection Guides for standard boom sprays and Pulse Width Modulation. These guides can be found here:
GRDC Standard Nozzle Selection Guide - GRDC
Pulse width modulation - standard nozzle selection guide - GRDC
Spraying speed
Speed should be limited to 20 km/hr and the lower the speed the lower the drift. The major impact of speed to standard sprayers (not Pulse Width Modulation machines) is the change in pressure at the nozzle, which will then affect droplet spectrum and coverage. Increasing speed can result in a greater number of driftable droplets. Air movement past the nozzle can have an impact on the spray pattern (shear). High speeds also effect the aerodynamics of the machine or boom that can reduce spray deposits.
Boom height
The most commonly used nozzles in broadacre spraying in WA have a fan angle of 110°. The optimal height for double overlap with these nozzles is 50 cm above the target. Raising the boom above this will dramatically increase the movement of the spray. Research has found that raising the boom from 50cm to just 70cm above the target can increase the amount of drift by 4 times while doubling boom height to 100cm can result in a tenfold increase.
Product choice and rate
Product choice can influence the droplet size produced by a nozzle. It is important to note that the spray quality defined by the ASAE standard S527 is determined using water ONLY. Addition of glyphosate, paraquat and adjuvants to the spray mix will generally reduce spray quality and create a greater number of fine droplets.
Spring weather conditions
Spring is widely regarded as the best time of year for fishing due to the favourable weather conditions, however this can have a different impact on spraying. Spring weather conditions can lead to low winds and temperature inversions, particularly around dusk and dawn. Crop or spray-topping should NOT be conducted at night. Ensure that wind speeds are above 5 km/hr and below 15 km/hr. Be aware of the crops that are downwind and be prepared to implement a buffer zone (Unsprayed area).
Paraquat use pre-harvest
Spray drift of paraquat being applied pre-harvest to pulses or pastures can cause significant residues in the grain of neighbouring crops. Cereal and canola crops are particularly susceptible to accumulating residues in the grain at this time of year when grain filling or ripening is occurring.
Some tips for applying paraquat in spring:
- Apply with a minimum COARSE spray quality. Water volumes should be increased to improve coverage (80-100 L/ha minimum).
- Observe weather conditions closely, especially wind direction and be aware of temperature inversions. Only begin spraying when wind has been above 5 km/hr for more than 30 minutes.
- Do NOT spray at night or after 3pm and only begin spraying once the sun has risen 20 degrees above the horizon. This is the time when temperature inversions are expected to form. Additionally, there are no proven benefits in weed seed control when paraquat is applied at night.
- A 360 g/L product with a drift neutral or drift reducing adjuvant will generally produce less driftable fines than a 250 formulation. 250 g/L have inbuilt surfactants that can increase spray drift.
- Maintain boom height at 50cm above the target (crop canopy or weeds).
- Limit speed to 20 km/hr or lower. The lower the speed the better.
- Spray when wind blows away from “sensitive” crops. Be prepared to leave unsprayed buffers where necessary.
- All cereals and canola have very low MRL’s for paraquat, avoid drift onto these crops
Further information on spray application can be found at:
https://grdc.com.au/resources-and-publications/resources/spray-drift
https://grdc.com.au/resources-and-publications/grownotes/technical-manuals/spray-application-manual
Other news
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Are your spray records up to scratch? 8/25/2021
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Observing harvest withholding periods in broadacre crops 8/12/2021
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Paraquat use on lupins 9/13/2021
