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Spray Drift Discussion: Factors That Affect Off-Target Chemical Movement

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Spraying agricultural chemicals needs to be carefully planned and properly executed to minimise the risks of off-target chemical movement. Farmers and other agricultural chemical users have an obligation to ensure that the herbicides or pesticides they spray stay within the target area. According to Agriculture Victoria, it is a legal offence to perform agricultural spraying that will injure any livestock, plants, crops and land outside of the target area.

One of the most common causes of off-target chemical movement is spray drift. When chemicals are sprayed, droplets are produced that can remain suspended in the air. Spray drift occurs when the airborne droplets move away from their intended target (for example, weeds or crops) and transfer to a neighbouring area. When this happens, the chemicals may injure or damage plants, animals, properties and the environment. They can even affect human health.

Below are three factors that contribute to spray drift and some handy tips to help chemical users minimise off-target movement.

Wind speed

Wind speed is the number one factor to influence spray drift. Whether farmers are using backpack or tank sprayers, spraying in extremely windy conditions puts nearby areas at risk. It is also a waste of time and money as it is highly unlikely that the chemicals will reach their target. However, this doesn’t mean that spraying should be done under calm or still weather conditions. Droplets are more likely to remain suspended in the air under calm conditions.

Business Queensland states that the ideal wind speeds for spraying are between 3 to 15km/h. During spraying, the wind should be blowing steadily. Light or gusty winds will make it difficult to predict where the sprayed chemicals will go. Users should also avoid spraying when humidity is low and in temperatures above or near 30 degrees Celsius. During these conditions, spray droplet size may reduce, increasing the possibility of spray drift.

Boom height

The height of the spray boom during an application plays a vital role in mitigating the risks of off-target drift. As boom height increases, the distance the droplets must travel before they reach their target also increases. This increase in distance lengthens the time in which the droplets are exposed to the wind, allowing the wind to blow them away to other areas.

To help reduce the possibility of drift, users should invest in wider nozzles that allow them to spray closer to the crops. For example, 80-degree nozzles with a 20-inch overlap enable you to spray 30 inches above the crops. On the other hand, 110-degree nozzles enable you to spray 20 inches above your target, decreasing the amount of time it takes for the droplets to reach the intended area.

Droplet or particle size

agricultural spraying on a field

Managing the size of spray particles or droplets is critical in mitigating spray drift. This is because spray droplets that are smaller than 150 microns are the ones most prone to drift. These particles fall more slowly than large ones and they are likely to float in wind currents as they do not have enough weight to overcome air resistance.

Even small changes in droplet size, diameter and weight can reduce the possibility of drift. For instance, an increase in droplet diameter from 150 microns to about 190 microns will double the particle’s weight. When the particle becomes heavier, it will fall more quickly and be less affected by air movement.

Limiting spray drift is vital for economic, safety and environmental reasons. Understanding the three factors that influence spray drift will help users manage their spraying practices and avoid the consequences of chemical misuse.

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