Spray Drift: A Serious Problem You Can Manage

By Erdal Ozkan

Spray drift not only result in wasting expensive pesticides and pollution of the environment, it may damage non-target crops nearby, and poses a serious health risk to people living in areas where drift is occurring. Drift happens! It accounts for about half of all non-compliance cases investigated by the Ohio Department of Agriculture. As you know, we are experiencing an unusual weather situation in Ohio and several other corn-belt states this year. Wet fields have made planting of corn and soybeans delayed or in many cases forced farmers to abandon it altogether looking for alternatives such as planting cover crops. Either situation presents added caution when applying herbicides in terms of spray drift which is defined as movement of pesticides by wind from the application site to an off-target site during or soon after application is done. When the same types of crops, such as Genetically Modified GM) beans, or non-GM beans are planted in neighboring fields, herbicide drifting from one field to another may not show injury symptoms. However, drift must be one of your most serious concerns when spraying herbicides in fields where the adjoining fields have been planted with some other crops or with cover crops. Even a small amount of drift may create significant damage on such crops under these conditions.

Although complete elimination of spray drift is impossible, problems can be reduced significantly if you are aware of major factors which influence drift, and take precautions to minimize their influence on off-target movement of spray droplets. The factors that play a role in either the creation, or reduction of spray drift are: a) Spray characteristics, such as volatility and viscosity of pesticide formulation; b) Equipment and application techniques used for spraying pesticides; c) Weather conditions at the time of application (wind speed and direction, temperature, relative humidity and stability of air around the application site); and most importantly, d) Operator care, attitude, and skill. Extensive information related to factors influencing spray drift, is in OSU Extension publication FABE-525. “Effect of Major Variables on Drift Distances of Spray Droplets.” (http://ohioline.osu.edu/factsheet/fabe-525). Here are some cost-effective things you can do to minimize spray drift.

1. Pay attention to wind speed. The risk of spray drift will increase with increases in wind speed. There is no magic wind speed number below which drift will not be a concern. There are many other factors some of which I already mentioned and some others mentioned below will influence the wind speed that may be considered dangerously high from the drift point of view. Generally, wind speeds below 5 mph, coupled with other good management practices, will significantly reduce the injury caused by drift. The best investment you can make is to buy a wind meter that tells you how high the wind velocity is at any given time. Having a wind meter handy will help you avoid a costly problem associated with spray drift.
2. Pay attention to wind direction. If the wind is blowing in the direction of some sensitive crops downwind, stop spraying. Don’t take the risk. Come back later in the day or the next day when the direction of the wind has shifted away from the sensitive crops.
3. Keep nozzles as close to the target as possible while still producing a uniform distribution of spray on the target. This doesn’t cost any money as long as it is practical to make it happen.
4. When you’re ready to change nozzles, consider selecting nozzles that produce much fewer of the extremely small droplets that are most likely to drift away. Low-drift nozzles are in the market and do a tremendous job of eliminating extremely small, drift-prone droplets from the droplet spectrum. This is especially important when spraying systemic chemicals like Glyphosate. Since the active ingredients in these types of chemicals are translocated, not requiring a thorough coverage on the target weeds, there is no need to use small droplets that increases the risk of drift.
5. There are chemicals that are designed to increase the droplet size, and reduce the number of very small droplets when added into the spray mixture. They are usually referred to as “drift retardants”. Most of them are some sort of polymer that tends to increase the viscosity and density of the spray mixture which leads to larger droplets. This, however, should be the last defense against drift. First consider the other options such as better targeting of the spray and switching to low-drift nozzles.
6. If you are using nozzles that produce relatively smaller droplets, avoid spraying under extremely hot and dry weather conditions. Under these conditions, evaporation of liquid from a droplet decreases its mass rapidly, which increases the drift distance of droplets.
7. Pay attention to conditions that may be conducive to formation of a phenomenon called thermal inversion. Normally, warm air rises up. So, during late morning to early evening, the surface temperature is usually warmer than the air temperature near the ground. So, the small droplets discharged from a nozzle may follow this normal air movement from ground up, and eventually evaporate during this process. However, during very early morning (before sunrise) or sometime after the sunset, the air temperature at some distance above the ground may be warmer than the ground temperature. Under these conditions, the cold air above the ground is trapped between the ground and the inversion layer. Under these conditions, as shown in the picture below, the small droplets suspended in the air simply follow the horizontal air movement miles away from the application site. So, avoid spraying during very early in the morning or very late in the evening, if the weather is extremely calm.

Practicing the recommendations I mentioned in this article will help you reduce the risk of spray drift significantly. At the end, you will be the one making spraying decisions. If there is any doubts about a spraying job that might result in drift, wait until there is no longer that element of doubt. Review the forecast and schedule spraying accordingly. Use APPs that provide current, local wind conditions and estimate periods of time to spray or not to spray