PDF Format (632kb)
Table of Contents
This Factsheet explains causes of spray drift, ways to reduce spray drift, key steps if pesticide drift is suspected and the legal responsibility of those who spray pesticides.
Pesticide drift is the aerial movement and unintentional deposit of pesticide outside the target area. There are two forms of pesticide drift.
The potential impact of pesticide drift includes:
There are also legal responsibilities and liabilities associated with all these issues.
Figure 1. 2,4-D drift damage on grape leaves.
Figure 2. Glyphosate drift damage on a tomato
All spray operators need to be concerned about pesticide drift and must make efforts to mitigate drift in all forms. While some level of drift either during or after an application will always occur, understanding the factors that affect it can minimize its effects.
The factors that cause drift are a combination of:
The key environmental conditions are: wind velocity and direction, temperature and relative humidity. The most important spray practices involve the spray quality produced by the sprayer, the distance between nozzle and target, and the travel speed of the sprayer. Each factor is described below.
|Wind Condition||Wind Speed||Description||Visible Signs||Spraying|
|Still||0-2 km/h (0-1.25 mph)||May lead to vapour drift where finer droplets remain suspended in the air, prone to evaporation and drift long after spraying is completed||Smoke rises vertically||Do not spray|
|Gusty||Not applicable||Wind direction is unpredictable and may indicate an inversion||Direction keeps changing||Do not spray|
|Light air||2-3.2 km/h (1.25-2 mph)||Suitable for spraying||Direction shown by smoke||Spray|
|Light to gentle breeze||3.2-9.6 km/h (2-6 mph)||Ideal for spraying||Leaves rustle, wind felt on face, twigs in motion||Spray|
|High wind||9.6-16 km/h (6-10 mph)||Higher wind speeds pose the most obvious risk of drift through, around or over the target||Small branches move, raises dust||Spray with caution or do not spray|
Do not spray during periods of dead calm. These periods may occur in early morning or late evening, when the temperature is usually cooler and the relative humidity is typically higher. This combination of factors can result in drift-sized droplets staying in the field. When the wind picks up, these spray droplets can move away from the target area, possibly causing injury to adjacent non-target areas. This is of particular concern with certain herbicides when they drift onto sensitive neighbouring crops.
Hot and dry conditions increase drift because droplets rapidly evaporate and become fine droplets, vapour or particles of concentrated pesticide. On hot days the land also warms, giving rise to upward convection currents that carry droplets above the crop canopy during spraying. Lateral air movements then transport these particles as far as several kilometres from the target area.
Hydraulic nozzle tips are classified by their spray patterns, flow rates and average droplet size; this is called "spray quality". When the operating pressure changes, the average droplet size also changes. For example, a nozzle that produces a medium spray at low pressure would produce a finer spray when pressure is increased.
Generally, finer droplets provide better coverage but are more likely to evaporate and drift. They slow down quickly once they leave the nozzle opening, take a much longer time to get from the nozzle to the target and do not penetrate dense canopies without air assistance. Coarser droplets resist evaporation and, because they have more momentum, are not easily deflected by wind. But they are more likely to bounce and run off the target and provide less coverage.
Consider these two options carefully when selecting nozzles, operating pressures and travel speeds.
The droplet size classification system (very fine to extremely coarse) uses "volume median diameter" (VMD), which is a manufacturer's term to describe the average droplet size produced by a nozzle (see Table 2). This system was developed for flat fan nozzles spraying into still air under specific conditions and should not be used to classify cone-pattern nozzles, such as those produced by disc-core or disc-whirl nozzles. Generally, cone-pattern nozzles produce finer particles than most flat fans used in horticulture because they operate at high pressure, encounter shear from air assistance and are sprayed over larger distances.
Once released, spray droplet size decreases rapidly through evaporation and/or volatilization. As the release height above the target or crop canopy increases, the potential for spray drift also increases. The same holds true for the distance between an airblast nozzle or vertical boom and the target.
See the impact droplet size and environmental conditions have on particle drift. Developed by researchers with the United States Department of Agriculture, "DRIFTSIM" is an easy-to-use computer program that predicts drift distances of spray droplets under a wide variety of conditions. It can be found at www.ars.usda.gov/services/software/download.htm?softwareid=252.
|Classification||Volume Median Diameter (microns)||Description|
|very fine||<100||highly susceptible to drift - not advised|
|very fine/fine||154||attaches to under-leaf surfaces, but tends to remain in the air|
|fine/medium||241||lands on stems and narrow leaves|
|medium/coarse||356||lands on large flat surfaces, like broadleaf weeds, but may bounce|
|coarse/very coarse||>451||wind tolerant, but will run off crop|
While drift-reducing adjuvants are available, check to ensure they are compatible with the pesticide in use. If not compatible, they can change the spray quality. This could damage the crop, make coverage uneven and/or reduce canopy penetration. Test a small area before adopting on a large scale. As well adjuvants can break down and become ineffective when used in tanks with paddle or hydraulic agitation.
It is preferable to use a product already formulated to reduce drift. See Additional Resources at the end of this Factsheet for a list of publications that provide more detail on chemical product recommendations.
For example, low volatility amine formulations of acidic herbicides are less prone to vapour drift while ester formulations have a higher likelihood. If a drift-prone product must be used the product label will include additional precautions. For example, avoid spraying dicamba dimethylamine (a highly volatile amine) during high temperatures, since vapour drift can occur at high temperatures (> 25ºC) within one to two days after application. Even the slightest drift from dicamba can cause major damage to nearby sensitive crops.
There are several ways to minimize the potential for spray drift. Some involve adjusting equipment and others relate to the application method. All require that the spray applicator be both aware of the conditions that promote drift and be willing to make changes while spraying.
While there are many types of sprayers used in agriculture for the ground application of pesticides, the two most often associated with drift are the airblast sprayer and the horizontal boom sprayer. Minimize spray drift by adjusting these sprayers to produce a coarser spray quality at a minimal effective distance from the target. How to make these sprayer adjustments are outlined below.
An airblast sprayer is a vehicle-mounted or vehicle-drawn device that uses a high-speed stream of air to carry the spray from air-shear or hydraulic nozzle tips into the target. The potential for drift can be reduced by:
A horizontal boom sprayer is a vehicle-mounted or vehicle-drawn device that sprays through a series of nozzles attached to a boom that extends outward on one or both sides. The potential for drift can be reduced by:
Figure 3. Airblast sprayer with deflectors.
Figure 4. Airblast sprayer with towers.
Figure 5. Tangential sprayer.
Figure 6. Recycling sprayer.
Figure 7. Laser-scanning sensor-controlled air assisted sprayer. (Photo courtesy of Dr. Heping Zhu, USDA Application Technology Research Unit, Ohio)
Figure 8. Drop-arms on a horizontal boom.
Figure 9. Air-assist sleeve on a horizontal boom.
Figure 10. Shrouds on a horizontal boom.
Some sprayers are not easily categorized and can use a combination of the previously described modifications. Examples of these other sprayer categories include:
Figure 11. Rope-style wiper/wick system.
There are many techniques to minimize drift when using the equipment previously described. Equipment modification on its own is not enough to reduce incidents of drift. Spray application is only as good as the spray applicator's methods. Review the best practices below to help control drift.
Be sure to confirm coverage using water-sensitive paper and be aware that erratic wind can make spray coverage uneven.
For example, notifying neighbouring greenhouse growers of an early morning pesticide application gives them the opportunity to close the vents, thus avoiding the possibility of drift into the greenhouse.
Generally, a buffer zone is the downwind distance separating the point of direct pesticide application from the nearest boundary of a sensitive habitat (see Figure 12). Look on the pesticide label for any buffer zone requirements.
Health Canada's Pest Management Regulatory Agency has an online spray drift calculator that allows applicators to modify the labelled buffer zones as required by the product label, based on weather conditions, the category of sprayer or droplet size. For more information, see the Buffer Zone Calculator at www.hc-sc.gc.ca/cps-spc/pest/agri-commerce/drift-derive/calculator-calculatrice-eng.php.
Anyone using pesticides is responsible for their safe application. Where drift damages adjacent crops, insurance adjustors generally ask the following questions:
The Pesticides Act requires that licensed spray applicators carry a specialized liability insurance policy that provides appropriate coverage for their business. Operators who work on a "for hire" basis (e.g. a licensed spray applicator) or away from their own farm operation will need additional coverage.
A normal farm insurance policy may not cover spraying done by licensed spray applicators or those done in share-cropping arrangements.
It is important to consult with an insurance broker or agent to ensure there is adequate coverage for any type of operation.
Section 29 of the act requires anyone who uses a pesticide that causes an adverse effect to notify the Ontario Ministry of the Environment (MOE). Should there be any impairment to the quality of life or environment in the use of a pesticide, charges could be laid against the applicator under Section 4.
Consult the Blue Pages in the local telephone directory or visit the Ministry of the Environment's website at www.ontario.ca/environment to locate the nearest district office. After hours, contact the Pollution Hotline at 1-866-663-8477.
Figure 12. Defining buffer zones for common application methods.
There are a series of steps to follow when damage caused by pesticide drift is suspected.
Diagnose the problem
Contact the appropriate people.
Document all details.
The methods and materials described in this Factsheet can reduce, but do not eliminate pesticide drift. In areas near sensitive crops, all reasonable precautions must be taken. Follow the pesticide label instructions to mitigate drift. Extremely low, and often invisible, amounts of spray drift can be very damaging to sensitive crops or habitats, so it is essential to know and follow both the pesticide label and the manufacturer's directions for proper use of the spray equipment.
This Factsheet was authored by Dr. Jason S.T. Deveau, Application Technology Specialist, OMAFRA, Simcoe, and Denise Beaton, Crop Protection Lead, OMAFRA, Guelph. Kristen Callow, Weed Management Lead, OMAFRA, Ridgetown, Leslie Huffman, Apple Specialist, OMAFRA, Harrow, Mr. John Purdy, private consultant, Mr. Robert DeBrabandere, farm claims specialist and staff of both the Ministry of the Environment and the Ontario Pesticide Education Program made valuable contributions.