During my early assignment as a farm advisor, I received a complaint of sudangrass “burned down” by a grower. This problem was later discovered that it was caused as a result of herbicide drift from the nearby spray of a round-up ready sugar beet field sprayed under windy conditions. Regardless of the large gap between the two crop fields, the drift has damaged a large area of the sudangrass crop.

After having a tour of the fields with one of my colleagues and observing the effects, I decided to write this article believing that it may serve as warning; learning and taking necessary measures to avoid such problems. I believe that this may be an important piece of information for growers, pest control advisors and pesticide applicators in minimizing or avoiding herbicide drift.

An herbicide is simply defined as a chemical substance used to destroy or inhibit the growth of plants; especially weeds. Therefore, it can actually kill any plant unless there are selective control properties within the herbicide. It must also be known that the term weed is user specific, because someone’s weed could be the other person’s crop.

What is herbicide drift? Herbicide drift is movement of an herbicide away from the target area. There are three main forms of herbicide drift; droplet drift, vapor drift and particulate drift. Droplet drift is the most common cause of off-target damage, but the easiest to control because under good spraying conditions, droplets are carried down by air turbulence and gravity, to collect on plant surfaces. Particle drift occurs when herbicide carriers evaporate quickly from the droplet leaving tiny particles of concentrated herbicide and may damage susceptible crops up to 30 km from the source. Vapor drift is confined to volatile herbicides such as 2,4-D ester. It arises directly from spray or evaporation of herbicide from sprayed surfaces and may occur hours after the herbicide has been applied.

It should be noted that the main goal in herbicide use is to maximize the amount reaching the target and minimize amount reaching off-target areas. Reaching the target would maximize the effectiveness of the herbicide, while reducing damage or contamination of crops and/or areas. Since all pesticides are capable of drifting, one who sprays these materials has the moral and legal responsibility to prevent drift and hence, avoid contamination or damaging of crops and sensitive areas.

Signs of herbicide damage

Signs of herbicide damage

Herbicide damage is difficult to tell, because similar symptoms may be caused by several other things including; nutrient shortages or excesses, water excess or drought, wind driven sand or soil particles, high or low temperatures, mechanical damage, or other pesticides. However, general symptoms can be in the form of reduced leaf size and shortened internodes or that new leaves may turn yellow or have chlorotic spots, curling, cupping, and vein distortion of the leaves on some trees, plants may become more susceptible to disease and other pests, petiole and stem twisting and malformed leaves (such as effects by 2,4-D). An overall symptom from herbicides is the reduction in growth of the affected plant (from inhibition of cell division or other effects) and decline or loss of yield.

How to reduce Herbicide Drift

The first thing an herbicide applicator has to do is to read labels for droplet size requirements of the herbicide and then set up equipment to ensure drift minimization. A drift can be minimized by selecting appropriate nozzle type and pressure with due consideration for the weather conditions, most importantly the wind velocity and temperature inversions. Transport of herbicide away from the target area could be under the influence of gravity, buoyancy and wind. In this case, large droplets may fall faster than small droplets. In other words, the higher the droplet is released, the further it will move away from the target area because there is more time for the wind to move the droplet before it lands. Hence, an herbicide drift is closely related to boom or flying height. In drift prone conditions, boom should be operated at lowest height possible. Decreasing nozzle spacing will also allow boom to be operated at a lower height.

Temperature inversions would occur when warmer, less-dense air moves over cooler, denser air. The situation could create subtle horizontal air flows that can move concentrated amounts of spray long distances. Herbicide applicators should make clear understanding of the following aspects before and during herbicide applications;

(1) Before spraying

• Always check for susceptible crops in the area.

• Recognize all sensitive areas (wildlife and people).

• Know what is around the application site.

• Notify neighbors of your spraying intentions.

• When spraying, record weather and relevant spray details.

During spraying

(2) During spraying

• Always monitor weather conditions and understand their effects. Do not spray if unsuitable, and stop if conditions change.

• Minimize spray release height.

• Select herbicide type to minimize potential drift. Always use least-volatile formulation available. If sensitive crops in area, use least damaging herbicides. If possible, add drift retardant.

• Keep a supply of various nozzle types.

• Spray early morning when wind is still calm.

• Have ultimate control of drift management.

There is a legal aspect of herbicide drift from the Agricultural Practices (Disputes) point of view, registration of chemicals and the Environmental Protection Act. In addition to crop yield loss legal suite, the Department of Environmental Protection can prosecute a person that "causes or allows to be caused pollution”. Herbicide drifts can be simply prevented or reduced, if properly handled. In all cases, one should avoid spraying during certain weather conditions such as midday turbulence, high temperature, high humidity and still conditions (high wind).

Documenting chemical drift (required information)

• Date of application and herbicide/tank-mix information.

• Herbicide name and rate.

• Wind direction, speed and temperature.

• Type of applicator, boom height, nozzle type, spray pressure, nozzle orientation and spray volume.

• If crop damage occurred, record crop and herbicide history of the damaged field, map of the area, yield loss estimates to predict the extent of damage and of course take a large number of high-quality photographs.


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