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Herbicide Spray Drift Management

California growers rely heavily on herbicides for cost-effective weed control. It’s important to remember  that the goal of spraying herbicides is to deliver the correct dose of carrier (water), herbicide(s), and spray additive(s) to the target area (the weeds) accurately, uniformly, and efficiently. When this goal is achieved and the correct herbicides are selected for the weeds targeted and all label recommendations are followed, one should expect effective results. A number of factors must be considered before spraying for weeds. Off-site herbicide movement (or spray drift) is one such factor that can influence whether or not weed control will be effective and the surrounding area and people protected.

Spray drift occurs mainly when spray droplets are carried from the application site downwind and deposited onto an area not intended to be treated. Vapor drift and particle drift can also result in herbicide movement, but are not common. Consequences of spray drift include damage to surrounding vegetation, waterways, other sensitive areas, people, pets, and livestock. Spray drift is influenced mainly by droplet size. Spray droplets <200 microns in diameter are light, can remain airborne a long time, and are the most susceptible to drift. These are referred to as “fine” or “very fine” droplets (table 1). “Medium” and larger-sized droplets are heavier and less likely to drift. As spray drift occurs, a portion of the spray mix does not reach the target site, so the herbicide dose applied will be less than intended, often leading to poor or erratic control, and a waste of herbicide, time, and money.


Selecting appropriate spray tips for a weed control job is every bit important as spraying under “ideal” conditions. Spray tip choice directly affects droplet size, spray uniformity, spray coverage, and drift potential, which directly impacts weed control, economics, and environmental quality. A comparison of different spray tips used for drift management and herbicide use are shown in table 2. As a rule, operate spray tips at as low a pressure as possible, that will produce large enough spray droplets (at least “medium”-sized) to resist drift, while providing proper coverage for the herbicide type (preemergent, contact, or systemic) selected.

Extended range (XR) flat fan spray tips have been the standard choice for ground sprayer weed control for years, but are only about 80% efficient at reducing spray drift. More advanced spray tip technologies are available to help reduce spray drift by 94 to 99%, including chamber design (like Turbo), venture design II (like Air Mix and Air Induction XR), and venture design I (like Air Induction and Turbo Drop). Money spent on selecting appropriate tips for a good spray job is cheap compared to the investment in herbicide, application equipment, labor, and problems associated with spray drift.

Some additional factors to consider when selecting and using spray tips include the following:
● Adjust the direction of spray forward to allow for a lower boom height.

● In most cases, the wider the spray tip angle, the smaller the droplet size and the greater the risk of drift. For example, an XR8003 (80°, size 03) produces medium-size droplets, compared to an XR11003 (110°, size 03) which produces fine-sized droplets operating at 30 psi each.
● Don’t make large changes in the spray pressure to get a larger desired flow rate. Instead, select a nozzle size to meet the flow rate requirement at a given spray pressure. Keep in mind; it takes a 4-fold increase or decrease in spray pressure to equal a 2-fold increase or decrease in flow rate.

● Check the flow rate of spray tips before each spray job and replace those that differ by 5 to 10% from that of new tips or if they are damaged.

● Use a spray tip size of >02 for larger droplets and to prevent plugging from sand or other debris. Never use knives, wire, or other metal items to clear plugged tips; use a stiff-bristled brush. Visually inspect each tip to make sure the spray pattern appears uniform.

The only sure-fire way to completely stop spray drift from occurring is not to spray. Obviously, this is not a practical solution. So when using herbicides (or other pesticides), regardless of where or what you are spraying, it’s important to spray under favorable conditions and use good application techniques, like choosing appropriate spray tips for the job. This will help you achieve cost-effective weed control, while protecting desired vegetation, people, pets, livestock, waterways, and other sensitive areas.




(Source – http://ucanr.edu/sites/Weed_Management/files/74280.pdf)

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