Herbicide Spray Drift Information

With the onset of auxin herbicides being applied to auxin-resistant (AR) crops, and the need to exercise extra caution when applying these herbicides, a discussion about reasons for and mitigation of off-target movement from these applications is warranted.

With herbicides, drift can be a problem when the sprayed material moves to areas where the application of the herbicide was not intended and sensitive plants are located. The risk of herbicide drift doing damage is directly related to the sensitivity of the non-target plants located in the path of the direction of the drift.

The auxin or growth-regulator herbicides, even in minute concentrations, can cause significant off-site damage to sensitive plants. This has been shown in research that was conducted with support of MSPB funds. Click here and here to view videos of results from that research that show just how even the smallest quantities of these herbicides can damage sensitive soybean plants both at and away from the application site.

Off-target damage caused by the auxin herbicides results when either the liquid or gaseous form of the herbicide moves away from the intended target area. The following narrative describes drift processes and mitigating factors.

Drift is the physical movement of herbicide sprays away from the targeted object or area after leaving the sprayer at the site of application. Drift usually occurs in windy conditions and/or when improper spraying techniques are used. The goal with herbicide applications is to have spray particles that are small enough to result in adequate coverage with the herbicide being applied, but large enough to reduce their drift potential.

Herbicide drift can occur with any herbicide, but the risk of damage to non-target areas varies among herbicides and among non-target plants that are located in the drift path. With auxin herbicides, the potential damage from drift is especially significant because of the high sensitivity of many plant species that will be growing outside the target area.

Factors that can affect drift potential of applied herbicides follow.

Nozzle type. One of the more common improper spraying techniques is using spray nozzles that produce particles that are too small and that are subject to drift even in moderate wind conditions. Spray particle size, as shown in the following table, is a significant contributing factor to drift. Nozzle types vary in the size of droplets they produce at various spray pressures and volume output. Since spray drift can be reduced by increasing droplet size, and since larger-orifice nozzles will produce larger droplets than nozzles with smaller orifices, selecting the proper nozzle type for the herbicide being applied is critical to control drift.

Click here for an article that presents information about and links to spray nozzles and spray droplet size.

Boom height (distance between the nozzle and the soil or plant target). The lowest allowable boom height above the target will mean less time for the herbicide spray to be affected by wind, and will also mean the herbicide spray is less affected by commonly occurring increasing wind velocity as distance above the ground increases.

Herbicide volatility. Some herbicides (such as the auxin herbicides) may easily volatilize, or transform from their liquid application phase into a vapor or gaseous form after being deposited on the intended target. This process may occur with materials that are applied to both plants and soil. The gaseous form drifts away from the target in the form of vapors or fumes. Volatilization potential is increased by rising temperatures and drops in external pressure. The tendency of a substance to volatilize is indicated by its vapor pressure–the higher a compound’s vapor pressure, the more likely it is to volatilize. If a herbicide has a high vapor pressure that allows a high proportion of it to be present in air, then that herbicide is more readily transformed from its liquid phase to a gas and will have an increased propensity to move away from the targeted area with the prevailing wind.

Relative humidity and temperature. Low relative humidity and/or high temperatures will cause more rapid evaporation of spray droplets between the boom and the target. This process will reduce the droplet size, which in turn increases drift potential. Generally, drift potential of a herbicide away from the target area will be greater when it is applied in low relative humidity and high temperature conditions. Increasing temperature may also increase vapor formation from volatile herbicides.

Wind. Both wind direction and wind velocity affect drift for obvious reasons; i.e., herbicides will drift in the direction of the wind, and the amount of herbicide lost and the distance it travels from the targeted area will increase with increasing wind velocity.

Spray pressure. Spray particle size increases as spray pressure decreases; conversely, increasing nozzle pressure results in smaller spray droplets. As indicated above, the smaller the spray droplets, the greater their drift potential.

Nozzle spray angle. Nozzles with wider spray angles (the angle formed between the edges of the spray pattern from a single nozzle) will produce smaller spray droplets than a nozzle with the same delivery rate but a narrower spray angle. The nozzle angle and boom height must be coordinated to ensure proper overlap where the spray meets the target. A boom with wide angle nozzles can be placed closer to the target than a boom with narrow angle nozzles. Thus, it is likely that a boom with wide angle nozzles (smaller spray droplets) that can be operated closer to the target will provide no more drift potential than a boom with narrow angle nozzles (larger spray droplets) that must be operated at a greater height above the target to ensure proper overlap at the target point.

Temperature inversion. This is an inversion in the temperature profile of the atmosphere, where the normal situation is cooler air on top of warmer air. This “inverted” situation of having warm air on top of cooler air thus supports the “temperature inversion” phrase. During this condition, the atmosphere is very stable and vertical air mixing is restricted. This results in small suspended droplets remaining in a concentrated cloud that can move in unpredictable directions due to light variable winds that are common during inversions. This condition is common on evenings and nights with limited cloud cover and minimal wind. An inversion can form at sunset and continue into the morning.

A herbicide label that gives the specifics for drift management of an auxin herbicide (DGA salt of dicamba) is the ExtendiMax Label for soybeans. The label specifies the 1) nozzle type (only one) and maximum operating pressure to be used to control droplet size, 2) minimum spray volume, 3) maximum equipment ground speed, 4) maximum spray boom height, 5) application guidelines when relative humidity is low or air temperature is high, 6) application restrictions based on wind speed and direction, and 7) restrictions for spraying during a temperature inversion.

With the now-present use of auxin herbicides on auxin-tolerant crops, producers must become aware of the components of drift so that they can manage spray operation procedures in strict accordance with all label specifications.

References

North Dakota State Univ. Ext. Serv. EST A-657

Univ. of Nebraska–Lincoln CROPWATCH

Dow AgroSciences Vistas 

Clemson Cooperative Extension Service

WeatherQuestions.com

Jed Colquhoun, Oregon State Univ. Extension Service

Composed by Larry G. Heatherly, Dec. 2016, larryheatherly@bellsouth.net