Causes of Non-Target Injury

Herbicide drift is the movement of herbicides awayfrom the intended area of application. Drift most often occurs when fine spray particles move through the air at the time of application (particle drift) or form a gaseous vapor due to volatilization of the herbicide after application (vapor drift). These types of drift are most likely to occur when an herbicide is applied in high wind conditions (> 10 mph) or very high air temperature conditions. Some herbicides (e.g. dicamba) or specific formulations of an herbicide are more susceptible to volatilization than others, for example the ester formulation of 2,4-D compared to an amine formulation. Off-site movement occurs by means other than particle or vapor drift as well. For example, high winds or rain soon after herbicide application can move soil particles that contain the herbicide to another location where non-target plants are then exposed to the herbicide.

Herbicide carryover is another way non-target herbicide injury occurs. The active ingredient in the formulated product remains in the soil or other organic material (e.g. manure, compost, grass clippings) for a period that varies based on the persistence of the active ingredient. When susceptible plants are planted into soil with high enough residual levels of herbicide, or organic material with high levels of herbicide residues inadvertently comes in contact with plants, injury can occur.

 

 

Operator error or faulty application equipment can also lead to non-target injury. These factors may result in an herbicide being applied at a rate higher than what is recommended, and the high application rate can lead to injury to plants other than the intended weed.

Some examples of faulty application equipment or operator error include improper sprayer calibration, inconsistent application speed or pressure, poor agitation and settling of the herbicide in the bottom of the application equipment or overlapping of spray pattern leading to higher application rates in overlap areas.

Sometimes herbicides are mixed to broaden the spectrum of weed control, but the herbicides may not be compatible with each other. Synergism, which is when two active ingredients exert a greater effect together than the sum of the independent effect of each, can cause non-target injury. Different combinations of adjuvants or other pesticides (e.g. an insecticide or fungicide) can also result in non-target herbicide injury. Herbicide labels will describe any issues related to incompatibility, and it is therefore important to read the entire label.

Environmental and plant fars influence the risk of non-target injury. For example, non-target injury to crops can occur when an herbicide application coincides with a certain growth stage of the crop species. For example, young vegetation is generally more susceptible to injury from foliar applied herbicides than more mature vegetation. Consult the product label to determine the appropriate timing of an herbicide application, both to protect the crop species and to treat the target weed at its most susceptible growth stage. Examples of environmental factors that lead to herbicide injury include site characteristics and weather. For example, light textured soils can increase injury potential for certain soil-applied herbicides at the time of crop establishment. Plants stressed due to adverse growing conditions (e.g. lack of or too much moisture, cold temperatures or light frost, hot temperatures) may become susceptible to an active ingredient that they would otherwise tolerate during ideal growing conditions. Because weather can’t be controlled, safety margins are incorporated into herbicide application rates to minimize non-target injury.