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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