Which factors reduce pesticide drift?

Dr. Jason S.T. Deveau, Application Technology Specialist

By now, hopefully, everyone knows there are two different kinds of pesticide drift.

• Vapour drift is the movement of pesticide vapours outside the area being treated.
• Particle drift is the movement of pesticide droplets or solid particles outside the area being treated.

Vapours are created when spray droplets evaporate both at the time of application and for some time after the spray has dried on plant or soil surfaces. The potential for vapour drift is more a product of the volatility of the active ingredient, the formulation (e.g. esters) and environmental conditions (e.g. hot and dry) than the equipment used.

To reduce vapour drift: Spray in cooler, humid temperatures with low wind speeds and use products that have less likelihood of volatilizing. If the label says not to spray in hot temperatures, it’s likely that product will become a pesticide vapour. In certain conditions, vapour has the potential to travel for kilometers.

So, how far can a particle travel downwind? The following graph (figure 1) was developed using USDA Driftsim software. It shows the distance a 100 µm (fine) or 200 µm (medium) droplet released 0.5 and 1.0 m above the ground can travel downwind at 40% RH, 20^◦C and sprayed at 40 psi. Droplets smaller than 100 µm evaporate into tiny, concentrated particles (like dust) before they hit the ground and travel great distances like a vapour.

Of course, spray is comprised of more than one size droplet. Even if the nozzle is rated “medium”, a certain fraction of the spray volume will made up of “fine” and “coarse” droplets. The following graph (see figure 2) represents reference curves based on data developed through the US Environmental Protection Agency and used by the Australian Pesticides and Veterinary Medicines Authority (AVPMA) to develop buffer zones. While the validity of this data has been questioned, it serves as a good approximation.

The graph shows how much of a given spray volume can travel downwind. Zero on the horizontal axis represents the downwind edge of the application area. The wind is blowing from left to right (speed not indicated, but higher speeds would result in greater distances). The numbers along the horizontal axis represent the distance in metres downwind from the application area.

The airblast scenarios are based on standard radial airblast applications where the machine was set up optimally for each situation. For the ground boom situations, ‘high boom’ refers to a boom set 1.27 metres above the ground and ‘low boom’ refers to one placed 0.5 metres above the ground. Coarse, Medium and Fine refer to the standard ASABE droplet size categories.

So what factors have the most influence on how far a spray droplet can drift? According to models and experiments described in “Modelling spray drift from Boom Sprayers”, the three biggest impacts on droplet drift are:

• Cross-wind speed
• Boom height (i.e. release height) and
• Nozzle size (i.e. mean droplet size)

Perhaps surprisingly, the following factors only have a limited effect on droplet drift:

• Operating pressure
• Driving speed (although if driving into the wind, apparent wind speed will increase drift)

Therefore, to reduce particle drift:

• plant a windbreak,
• use drift-reducing (large droplet) nozzles,
• install drift shields on the boom,
• keep the boom height at the lowest, practical distance from the target,
• do not spray when wind is unreasonably high or changeable, and
• do not spray when prevailing wind is blowing towards sensitive areas, such as residential homes

The importance of good relations between rural and urban neighbours cannot be overstated – they often prevent situations like this from escalating. Think of your neighbours before you spray.

To find out more about pesticide drift, and what to do if you suspect drift damage, consult OMAFRA factsheet 11-001 “Pesticide Drift from Ground Applications”

http://www.omafra.gov.on.ca/english/crops/facts/11-001.htm

Download DriftSim at:

http://www.ars.usda.gov/services/software/download.htm?softwareid=252

Check out the APVMA drift site at:

http://www.apvma.gov.au/use_safely/spray_drift/scenarios.php

“Modeling Spray Drift from Boom Sprayers” – H.J Holterman, J.C. van de Zande, H.A.J. Porskamp, J.F.M. Huijsmans. Computers and electronics in agriculture 19 (1997) pp.1-22