Get acquainted with adjuvants before spraying

Supply chain constraints in the U.S. over the past year have caused consumers to become acquainted with unfamiliar product brands that perform similarly to one they use regularly. Ultimately, product substitutions provided by a retailer should have little impact on the consumer, as long as the alternative performs the same function, has minimal or no quality difference when compared with the desired product choice, and the consumer’s goal is achieved.

Rest assured, this is not another article about supply chain issues. Rather, it highlights the importance of proper adjuvant selection—espe­cially when maximizing herbicide efficacy with products that may be unfamiliar to growers.

There are several categories of adjuvants, grouped based on function: activator adjuvants, spray modifiers, and utility adjuvants. I won’t cover utility adjuvants, which are formulated to change physical characteristics of the spray solution. They include buffering, antifoam and drift-control agents.

Activator adjuvants are designed to improve the “activity” of the pesticide, typically by increasing its absorption rate and reducing the surface tension on the leaf. Activa­tor adjuvants include surfactants, oils and nitrogen-based fertilizers.

Some glyphosate formulations, such as Roundup brands, come “pre-loaded” with an activator adjuvant already in the product to maximize herbicide performance and provide consistent weed control under normal, stress-free growing conditions. Credit 41 Extra is another example of pre-loaded glyphosate. Non-ionic surfactants (NIS) are the preferred choice for glyphosate herbicides because they reduce surface tension and provide more droplet coverage on the leaf surface. Without an NIS, the spray droplet beads up on the leaf, like an automotive window treatment causes water to shed from glass. The goal is to get a lethal dose of glyphosate through the leaf cuticle and other waxy barriers that protect the plant from the environment.

Several generic glyphosate for­mulas are available, and I anticipate more will be used in 2022 than previously. Products that need an NIS adjuvant include AgSaver 5.4 Glyphosate, Vanish Max, Alligare Glyphosate 5.4, and Glypex 5 Extra. MFA offers Astute Xtra, Astute and Astute Lite brands in its Crop Advantage NIS lineup.

Crop oil concentrates (COCs), also activator adjuvants, are effec­tive in softening and dissolving the leaf’s waxy cuticle. These adjuvants must be included with postemer­gence grass herbicides such as Volunteer or Assure II; the bleach­ing herbicides Armezon, Laudis and Explorer; and Atrazine. Herbicide performance is typically more con­sistent when a COC is added under dry conditions or cool weather.

More often, COCs cause in­creased crop response when com­pared with an NIS. Only under cer­tain circumstances and crop growth stages is a COC recommended with some labeled synthetic auxin herbicides in corn. It’s important to fully understand the product label to avoid crop response. MFA’s Xpo­nd, a high surfactant load COC, is applied at half the normal use rate of a standard COC.

Methylated seed oils (MSOs) work similarly to COCs but can cause greater crop response. Few herbicide manufacturers suggest utilizing MSOs in corn. Impact is one example in which MSO can be safely used to control weeds with­out injuring corn. It is more com­mon to include an MSO in burn­down programs using Gramoxone or Sharpen before crop emergence.

Soy Plus and Soy Plus HD are examples of MSO surfactants. Soy Plus HD has a high surfactant MSO load and is applied at a lower rate than conventional MSOs. MFA also carriers Hawker, a blend of NIS, MSO and organosilicone surfac­tants, which increase coverage on the leaf surface by greatly reducing surface tension and drying time.

Another important group of adju­vants, spray modifiers, influence the solution’s delivery and placement. The focus is to reduce the amount of fine droplets, those less than 150 microns in size or the thickness of sewing thread. These fine droplets are susceptible to moving off-target or evaporating before reaching the plant. Drift-reducing adjuvants create larger droplets, approxi­mately 500 microns or the thick­ness of very coarse sand, which are less likely to move off-target under higher wind speeds and low humidity.

Herbicide rates should not be reduced below manufacturer rec­ommendations when using an ad­juvant. Always review labels when mixing multiple pesticides. Emul­sifiable concentrate formulations contain solvents and can potentially reduce crop safety when additional adjuvants are used. Tank-mix order also matters. And remember, adju­vants will not control herbicide-re­sistant weeds.

Get to know your adjuvants and make the best of every pesticide application this spring.

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