Spray drift is currently the big thing with adjuvants and formulations, says John Garr. Today, more people live in rural areas and often put pressure on government regulators to make sure pesticide drift never happens. Nobody wants spray drift. One of the main applicator goals is always no unwanted effects or contamination of non-target organisms (NTOs).

The most common way that drift occurs is when the wind blows small droplets away from the target, says the President at GarrCo Products. Pesticide labels often forbid application when winds are above a certain speed, such as 16 or 20 kph (10 to 12 mph), but a strong wind is not always the culprit. Pesticide labels also prohibit application when wind speeds are less than 4.8 kph (5 mph). If there is no wind to mix the air, a temperature inversion can occur that suspends spray droplets and moves them unpredictably.

Historically, spray drift has been a problem for herbicides, where a thimble full of certain herbicides diluted in a spray load can injure a non-target crop. Today, there is a greater focus on preventing drift onto any endangered species, protected habitats, and organic farms. Unacceptable drift can be any unwanted exposure. Just because no one sees an effect, does not mean that drift did not happen. Today, chemical detection methods are often more sensitive than biological symptomology, and any measured pesticide residue can be unacceptable on some crops.

Droplet size

Commercial spray nozzles produce a range of droplet sizes. Since droplets are spherical or nearly spherical, their diameter is used to measure their size. The Volume Median Diameter (VMD) is the most common droplet measurement and defines the droplet diameter in microns where half of the spray volume contains smaller droplets, and half contains larger ones.

A more important measurement for the smaller driftable droplets is the VD(0.1), which defines the droplet diameter where 10% of the spray volume contains smaller droplets, and 90% contains larger ones. The higher the VD(0.1), the lower the chance for drift.

The perpetual question when applying pesticides is what is the ideal droplet size. Of course, the question really should be the droplet spectrum and not a single droplet size. Experts cannot simply answer because the ideal spectrum depends on many factors, including the pesticide mode-of-action, spray mixture, targeted pests, crop, risk of drift, environmental conditions, application method, as well as application parameters and goals. However, all agree that applying larger droplets will reduce drift.

One of the myths about drift is that nozzles alone are the best way to manage off-target movement. That myth would be more valid if nozzles performed the same way in commercial field applications as they do with water sprays in wind tunnels. The industry needs to look at what happens after the droplet leaves the nozzle.

The title of a recent article was “Think outside the tip.” Nozzles, nozzle orientation, and spray pressure cannot do everything. One limitation of nozzles is that they cannot control the properties of the spray water that often changes with concentrated spray mixtures. For example, commonly used surfactants, oils, formulation ingredients, and even fertilizers added to spray mixtures can change the properties of spray water and decrease droplet size. Adding an effective DRA can help control the properties of spray water.

Drift reduction adjuvants

A drift mitigation or drift reduction adjuvant (DRA) is a material used in liquid spray mixtures to reduce spray drift. The driftable fraction of the spray is the small droplets, often characterized as 150 μm in diameter or less. The primary function of a DRA is to reduce the driftable fraction, but most have other benefits, such as increased retention and reduced evaporation. The general rule is that the larger the droplet, the less drift and more deposition on the target.

The first widely used DRAs were polyacrylamide polymers, sometimes referred to as polyvinyl polymers. Polyacrylamide DRAs are commonly sold in dilute 1 to 2% formulations or as emulsions in 20-fold higher concentrations. The most common alternative to polyacrylamides is guar gum and its derivatives. Both of these polymer types are effective and mix well when formulated properly.

Polymers increase the viscosity of the spray water from breaking into smaller droplets as it exits the nozzle, and counteract the negative effect that some tank-mixtures have on droplet size. The higher viscosity increases retention by reducing droplet rebound off the leaf surface. Polymers also act as humectants and keep the spray deposit moist longer.

Commercial DRAs often use the terms drift reduction and deposition interchangeably. A deposition aid is a material that improves the ability of agrichemical sprays to deposit on targeted surfaces, essentially the more on the target, the less drift off-target.

Recently, experts have started to group DRAs into two broad categories: rheology modifiers that delay spray fan break-up and increase droplet resistance to shattering, and water-immiscible droplet technologies such as emulsified oils and specialized surfactants that accelerate spray fan break-up. Paradoxically, breaking the spray fan more slowly and more rapidly can result in less driftable fine droplets. 

Formulation solutions

The most important new weed management technologies are the new herbicide-tolerant crops that allow for new uses of auxin herbicides with long histories of drift problems. To address this issue, BASF and Monsanto registered new formulations of dicamba, with claims of lower volatility and less spray drift when applicators follow label directions.

The Monsanto formulations use the diglycolamine salt of dicamba and acetic acid, or its salt, as a proton scavenger to reduce volatility. Monsanto uses the monoethanolamine salt of glyphosate in combination with dicamba.

BASF formulations use the BAPMA (N, N-Bis-(3-aminopropyl) methylamine) salt of dicamba. BASF has sold dicamba for over 50 years, and according to our count, this is their sixth new dicamba formulation.

Dow registered a unique formulation that contains the choline salt of 2,4-D alone and the dimethylamine salt of glyphosate. These new salts and formulations have greatly increased the value of these old auxin herbicides over existing formulations and generic alternatives.

The new product formulations also rely on an array of extensive and somewhat complicated application directions that include nozzle type and orientation, spray pressure, boom height, droplet size, wind speed, ground speed, air temperature, buffer zones, and approved DRAs and other tank-mixtures posted on an associated website. 

Fine droplets are not always fine

The bottom line is that fine spray droplets are not fine. The applicator should always have the spray system set up correctly, use best application practices, follow label directions, and use low drift formulations, if available. Using an effective DRA is also very important and will reduce the production of driftable fine spray droplets and drift complaints.