Spraying weeds in less than optimum conditions

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Most areas of Eastern North Dakota and Minnesota have received excessive amounts of rainfall in June. The moist/wet soil conditions have created a perfect environment for weed seed germination and emergence. Growers planning postemergence sprays will first evaluate field conditions to determine if they can get spray equipment over the fields. A second consideration will be wind velocity. Some pesticide labels limit application to certain wind velocities. Growers need to abide by the label. However, there are methods for minimizing the effects of spray drift even when wind speeds are within ranges denoted on the label. Strategies for reducing spray drift include:

  • Selecting nozzles that produce larger spray droplets
  • Reducing boom height over target plant species
  • Reducing speed of ground application equipment

The benchmark nozzle standard for many years was extended range flat fan nozzles. These nozzles allowed the operator to reduce the spray pressure to as low at 15 psi and maintain the correct spray pattern. The idea was less spray pressure equated to less droplet fines susceptible to spray drift. However, extended range flat fan nozzles still produced small droplets. There are several brands and styles of ‘air induction’ nozzles available in the marketplace, today, which reduce small spray droplet size even more than extended range flat fan nozzles. Air induction nozzles operate by drawing air into the nozzle body to create large, air-filled droplets that break and provide coverage when they come into contact with the leaf surface. There are a number of different air-induction nozzles types; careful consideration needs to be given to proper operation of these nozzles including the correct spray pressure. One should also consider the pesticide to be applied when making a decision on nozzles. For example, contact sprays require good coverage of the leaf surface to provide excellent weed control. Spray coverage is not as critical for herbicides such as glyphosate, which translocate in the plant.

Lowering the boom height and using nozzles that produce a broader pattern is a second approach for reducing drift. Reducing the distance between droplet release point and the target means less time the droplet will be in the air and susceptible to drift. However, it is not as simple as reducing the boom height. Consideration must be given to ensure the spray pattern can be maintained as the operator travels across the field and there is appropriate overlap of spray solution between spray nozzles to deliver coverage.

Speed is a third variable to consider for minimizing drift. Reducing sprayer ground speed reduces drift since there is less bounce and less sprayer vortex effect, which means droplets are aloft in air for a shorter duration of time. However, speed impacts the amount of time needed to spray a field. Ultimately the operator will need to decide on the compromise between ground speed and the number of acres one can cover in an hour and potential for spray drift.

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