When applying crop protection chemicals, spray drift is a term used for those droplets containing the chemicals which are not deposited on the target area. The droplets most prone are usually small in size, less than 200 µm micron in diameter and easily moved off the target area by wind or other climatic conditions. Drift can cause crop protection chemicals to be deposited in undesirable areas with serious undesirable areas with serious consequences, such as:
  • Damage to sensitive adjoining crops.
  • Water contamination.
  • Health risks for animals and people.
  • Possible contamination to the target area and adjacent areas or possible over-application within the target area.

 

Causes of Drift

A number of variables contribute to spray drift; these are predominantly due to the spray equipment system and meteorological factors.

Droplet Size
Within the spray equipment system, drop size is the most influential factor related to drift.
When a solution is sprayed under pressure it is atomized into droplets of varying sizes: The smaller the nozzle size and the greater the spray pres-sure, the smaller the droplets and therefore the greater the proportion of driftable droplets.

Spray Tip Height
 As the distance between the spray tip and the target area increases, the greater impact wind velocity can have on drift. The influence of wind can increase the proportion of smaller drops being carried off target and considered drift.
Do not spray at greater heights than those recommended by the spray tip manufacturer, while taking care not to spray below the minimum recommended heights.

Operating Speed
 Increased operating speeds can cause the spray to be diverted back into upward wind currents and vortexes behind the sprayer which trap small droplets and can contribute to drift.
Apply crop protection chemicals at maximum operating speeds of 10 to 13 mph. As wind velocities increase reduce operating speed*.

 *Fertilizer applications using the TeeJetT tips with very coarse droplets can be performed at higher operating speeds.


Wind Velocity
 Among the meteorological factors affecting drift, wind velocity has the greatest impact. Increased wind speeds cause increased spray drift. It is common knowledge that in most parts of the world the wind velocity is variable throughout the day (see Figure 2). Therefore, it is important for spraying to take place during the relatively calm hours of the day. The early morning and early evening are usually the most calm. 
Refer to chemical label for velocity recommendations.
When spraying with traditional techniques the following rules-of-thumb apply:

  • In low wind velocity situations, spraying can be performed at recommended nozzle pressures.
  • As wind velocities increase, spray pressure should be reduced and nozzle size increased to obtain larger droplets which are less prone to drift. Wind measurements should be taken throughout the spraying operation with a wind meter or anemometer. Based on current experiments the following guidelines can be followed with respect to the minimum ten percent volume diameter (D V0.1 ) allowed with respect to the corresponding wind velocity:
  • up to 4.5 mph: D V0.1 ³ 130 µm
  • up to 7 mph: D V0.1 ³ 140 µm
  • up to 9 mph: D V0.1 ³ 160 µm

  • up to 11 mph: D V0.1 ³ 200 µm

  

Air Temperature and Humidity
 In ambient temperatures over 77°F/25°C with low relative humidity, small droplets are especially prone to drift due to the effects of evaporation. High temperature during the spraying application may necessitate system changes or suspending spraying.

Crop Protection Chemicals and Carrier Volumes
 Before applying crop protection chemicals, the applicator should read and follow all instructions provided by the manufacturer. Since extremely low carrier volume usually necessitates the use of small nozzle sizes, the drift potential is increased. As high a carrier volume as practical is recommended.

 

Drift Reducing Nozzles 1st and 2nd Generation
Drift potential can be minimized even when it is necessary to use small size nozzles by selecting the appropriate style. Nozzles such as the Turbo TeeJetT (TT), Air Induction TeeJet (AI) and the Drift Guard TeeJetT (DG) produce medium to coarse sprays even in the smaller sizes. Large size droplets are much less susceptible to drift, but in some cases target coverage may be reduced due to a reduction in the number of drops. This needs to be taken into account especially when using contact crop protection chemicals.

Wide angle flat spray nozzles with pre-orifice technology can achieve a larger drop size range at equal pressures without a reduction in flow rate. The DG, AI and TT incorporate pre-orifice technology which performs the primary flow metering function. The larger exit orifice provides secondary metering and pattern formation.

Venturi type nozzles, such as the AI use a pre-orifice to create a high velocity liquid stream and then draw air into the stream through a side opening. This mixture of air and liquid is then discharged at a low exit velocity thus creating very coarse droplets with air inclusion. However, air-filled droplets only occur with chemicals containing a sufficient concentration of surfactants.

The difference in droplet sizes between the TeeJet XR,DG and TT nozzles on the basis of VMD.

  • The DG nozzle in comparison to the XR achieves 30% higher VMD values. However, as the pressure increases the percentage difference decreases.
  • The TT nozzle achieves about 10 to 20% higher VMD values than the DG at equal pressures.
  • The VMD values for the TT nozzle at 15 PSI pressure is about 70% higher than the XR.

Summary

Drift can be managed successfully with the right knowledge of the equipment the right knowledge of the equipment application must be balanced between managing drift and maintaining effective crop protection. Below is a list of factors that must be considered to insure a safe, accurate spray application.

  • Spray Pressure

  • Nozzle Size

  • Application Rate

  • Spray Nozzle Height

  • Operating Speed

  • Wind Velocity

  • Air Temperature and Relative Humidity

  • Buffer Zones (safe distances from sensitive areas)

  • Instructions from the crop protection chemical manufacturer

Having taken into account all the variables that can have an impact on the drift potential, it may still be necessary to consider the use of drift control nozzles, such as the DG, AI or TT.

 
    

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