How to Reduce Spray Drift

Spray drift not only wastes expensive pesticides and pollutes the environment, it can also damage nearby non-target crops and pose a serious health risk to people living in areas where drift occurs.

The drift is coming! It accounts for about half of all noncompliance cases reviewed by the Ohio Department of Agriculture. We are experiencing an unusual weather pattern this year in Ohio and several other Corn Belt states. Wet fields have delayed corn and soybean plantings or, in many cases, forced farmers to abandon them altogether in search of alternatives such as planting cover crops. Either situation presents increased caution when applying herbicides in terms of spray drift, which is defined as the movement of pesticides by wind from the site of application to a non-target site during or shortly after application.

When the same types of crops, such as genetically modified (GM) beans or non-GM beans, are planted in neighboring fields, the herbicide that drifts from one field to another may not show symptoms of injury. . However, drift should be one of your most serious concerns when spraying herbicides in fields where adjacent fields have been planted with other crops or with cover crops. Even a small amount of drift can create significant damage to these crops under these conditions.

Although it is impossible to completely eliminate spray drift, problems can be greatly reduced if you know the main factors that influence drift and take precautions to minimize their influence on the off-target movement of spray droplets. Factors that play a role in creating or reducing spray drift are:

a) Characteristics of the spray, such as volatility and viscosity of the pesticide formulation;

b) Equipment and application techniques used for spraying pesticides;

c) Meteorological conditions at the time of application (wind speed and direction, temperature, relative humidity and air stability around the site of application); and especially,

d) Operator attention, attitude and skill.

Detailed information on factors influencing spray drift can be found in OSU Publication FABE-525 Extension “Effect of Major Variables on Spray Droplet Drift Distances”. Here are some cost-effective measures you can take to minimize spray drift:

  1. Pay attention to the wind speed. The risk of spray drift will increase with increasing wind speed. There is no magic wind speed number below which drift will not be a problem. There are many other factors, some of which I have already mentioned and others mentioned below, influencing the wind speed which can be considered dangerously high from a drift point of view. Generally, wind speeds below 5 mph, combined with other best management practices, will significantly reduce injuries from drifting. The best investment you can make is buying an anemometer that tells you the wind speed at any given time. Having an anemometer handy will help you avoid a costly problem associated with spray drift.
  2. Pay attention to the direction of the wind. If the wind is blowing in the direction of some susceptible downwind crops, stop spraying. Don’t take the risk. Come back later in the day or the next day when the wind direction has moved away from sensitive crops.
  3. Hold the nozzles as close to the target as possible while producing an even distribution of spray on the target. It doesn’t cost money as long as it’s convenient to do it.
  4. When you’re ready to change nozzles, consider selecting nozzles that produce far fewer extremely small droplets that are most likely to drift. Low-drift nozzles are on the market and do a terrific job of removing extremely small, drift-prone droplets from the droplet spectrum. This is especially important when spraying systemic chemicals like glyphosate. Since the active ingredients of these types of chemicals are translocated, not requiring full coverage on the targeted weeds, there is no need to use small droplets which increase the risk of drift.
  5. Some chemicals are designed to increase droplet size and reduce the number of very small droplets when added to the spray mixture. They are generally called drift retarders. Most of them are some kind of polymer which tends to increase the viscosity and density of the spray mixture, leading to larger droplets. This, however, should be the last defense against drifting. Consider other options first, such as better spray targeting and switching to low-drift nozzles.
  6. If you are using nozzles that produce relatively smaller droplets, avoid spraying in extremely hot and dry weather conditions. Under these conditions, the evaporation of the liquid from a drop rapidly decreases its mass, which increases the drift distance of the drop.
  7. Pay attention to conditions that may be conducive to the formation of a phenomenon called thermal inversion. Normally, hot air rises. From late morning to early evening, the surface temperature is generally warmer than the air temperature near the ground, which means that the small droplets discharged from a nozzle can follow this normal air movement from the soil upwards and eventually evaporate during this process. However, very early in the morning (before sunrise) or sometime after sunset, the air temperature some distance above the ground may be warmer than the ground temperature. Under these conditions, cold air above the ground is trapped between the ground and the inversion layer. Under these conditions, as shown in the image below, the small airborne droplets simply follow the horizontal movement of air miles from the site of application. Avoid spraying very early in the morning or very late in the evening if the weather is extremely calm.

Thermal Inversion

Practicing the recommendations mentioned in this article will help you significantly reduce the risk of spray drift. In the end, you will be making the spraying decisions. If in doubt about a spray job that may cause drift, wait until there is no longer that element of doubt. Review forecasts and schedule spraying accordingly. Use apps that provide current local wind conditions and estimate time periods to spray or not to spray

A more detailed discussion of these tips and other drift reduction strategies is described in the following OSUE extension factsheets available online:

FABE-525 (http://ohioline.osu.edu/factsheet/fabe-525)

FABE-523 (http://ohioline.osu.edu/factsheet/fabe-523)

FABE 524 (http://ohioline.osu.edu/factsheet/fabe-524)


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