Proper adjuvant use boosts herbicide effectiveness

Tight supplies, inflation, and tough weeds nearly guarantee higher 2023 herbicide costs. One way to boost herbicide performance is through properly mixed adjuvants. 

Numerous products fall under the adjuvant banner, but most can be grouped into surfactants, crop oil concentrates, and ammonia fertilizers that accompany herbicides into a spray tank. Adjuvants also include drift reduction agents (DRAs) that curb herbicide off-target movement and pH modifiers that reduce tank mix acidity.  

Adjuvants work by enabling herbicides to get into, onto, and through a weed. “We must gain every advantage we can to control the target weeds. That’s where the optimization with adjuvants is critical,” says Bryan Young, a Purdue University weed scientist. 

That’s because herbicide costs are climbing. “A typical Indiana corn herbicide program cost $35 to $45 per acre in 2022, not including application costs,” Young says. “Cost for a soybean herbicide program was $40 to $50 per acre. Some soybean farmers were spending $50 per acre just on postemergence weed control. That’s with no preemerge or soil residual program of Enlist One, Roundup, and Liberty.” 

In comparison, quality adjuvants can cost $3 to $6 per acre. “Return on investment for adjuvants can be one to four times the cost,” he says. 

However, adjuvant management is key. If coverage rates lag because of poor adjuvant management, Young says a “rescue” post application will cost at least $20 per acre.

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Matching Adjuvants to Herbicide

Public and company crop protectant consultants can help growers select the right adjuvant for the job. “They look at how adjuvants vary by product, herbicide site-of-action, weed species and usually have lots of research to test and validate their recommendations,” says Gail Stratman, FMC regional technical service manager. 

Having the right adjuvant in the tank “will ensure every droplet sprayed is as effective as possible,” says Connor Ferguson, Wilbur-Ellis branded technologies data manager. “With contact herbicides, adjuvants help droplets adhere to the leaf surface, increasing contact area for better absorption and efficacy. With systemic herbicides, which move within the plant, it’s important to have the right adjuvant to get spray onto the plant leaves so the herbicide can work effectively. This can be difficult with a heavy weed infestation, waxy-type leaf surfaces, or when targeting an otherwise difficult weed like common lambsquarter. In these cases, oil-based adjuvants can help droplets penetrate tough plant surfaces.” 

Herbicide advisers do a good job of identifying and approving adjuvants that can help mitigate off-target movement, including synthetic auxin herbicides that include dicamba and 2,4-D, says James Reiss, Precision Laboratories senior vice president of product development.

“But the real opportunity to maximize overall performance comes from understanding the adjuvant needs of the herbicide tank mix partner that is being added to that auxin application,” he adds. 

High surfactant methylated seed oils (HSMSO) or even high surfactant oil concentrates (HSOC) can drive the performance of the tank mix partner and even improve the auxin herbicide’s performance, he says.

Mixing Mistakes

“Probably the biggest misstep by farmers is beginning the mixing process before the sprayer is at least half full of water,” says Kyle Gustafson, WinField United crop protection product manager. “Many crop protection products are formulated as a concentrate, so they need water to dilute. When you start putting too many products in the tank too early in the mixing process, they ‘fight’ for the free water and can react with each other, causing compatibility problems.”

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Gustafson says that dry products must be fully soluble in water before adding more products.

“You don’t want to add a dry product into the mixture, then immediately add another liquid product that contains oil that may cause the dry product not to suspend properly,” he says. “Always reference proper mixing order before you begin mixing up a load of products.”

Stratman says most product labels specify herbicide should be added to the tank first to ensure it is properly mixed or dissolved, especially in the case of drift function products.

“Many times, growers want to have their adjuvants already premixed in their spray solution for convenience or to ‘condition’ the water, particularly with solutions like UAN or AMS fertilizers,” he says. “That may work sometimes, but many times it causes problems getting suspension or dry-flowable formulations to disperse properly. This can result in a variety of problems that can range from clogging nozzles to uneven application, which in turn result in crop injury or weed control issues.”

Young adds that botched mixing may occur when growers or other applicators fail to consult with retailers or others.

“Not all product combinations and mixing scenarios can be covered by a herbicide or adjuvant label,” he says. “Retailers and custom applicators have more experience with products that are popular in your local area and have likely learned from previous mistakes.”

Off-Target Drift Reduction Potential

Besides new nozzle configurations and application techniques, adjuvants can help manage the off-target potential of synthetic auxin herbicides such as dicamba and 2,4-D. Adjuvants can reduce off-target potential by reducing the relative amount of driftable fine spray droplets being produced during the application, says Young. 

“Some adjuvants include retention aids that can reduce the bounce and shatter of these large spray droplets at impact and reduce droplet runoff on more vertical leaf surfaces like grass weeds,” he says. 

Young notes that while adjuvants can reduce the risk of spray drift, they “will not eliminate” the chance for drift, especially when poor application decisions are made regarding: 

• Nozzle design 
• Tip size 
• Spray pressure 
• Proximity to sensitive plants
• Wind direction 
• Excessive wind speeds 

Stratman points out that getting the most out of adjuvant with drift reduction agents also involves using proper boom height and humidity levels. The adjuvant is one piece in the toolbox that can help, but all the other aspects of spray drift management are equally as important, he says. 

Gustafson reminds growers that nozzle selection depends on the chemistries being used. “Low-volatility dicamba and 2,4-D formulations have specific nozzle specifications as required by the label,” he says. “Glufosinate requires fine, small droplets, while other postemergence corn herbicides accommodate larger droplet sizes. Be sure you have appropriate nozzles stocked for each herbicide you plan to spray to optimize application efficacy. 

“For contact-type chemistries, coverage is essential, which means higher carrier volumes and smaller droplet sizes are needed, he adds. “For systemic chemistries, coverage is less critical, so nozzles with lower carrier volumes and larger droplets can be used.”

Don’t Cut Rates

Applying an adjuvant at a reduced rate to cut costs can impact its efficacy on weeds. Reiss advises growers to ask themselves these questions about their drift-reduction adjuvant: 

1. Is the drift agent in the formulation vulnerable to pump-shear and will its performance decrease as the sprayer goes across the field?
2. Is the use-rate based on the labeled rate, or on how much the farmer wants to spend?

Generally, he says oil-based adjuvants should be used on a rate per acre basis: MSOs at 20 to 24 ounces per acre and HSOC or HSMSO at 10 to 12 ounces per acre. “The problem comes when oil-based adjuvants are used at a percentage by volume [% v/v] and lower spray volumes are per acre are used. 

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“For example, at a 1% v/v use rate of MSO [1 gallon in 100 gallons of spray solution] at 15 gpa [gallons per acre] provides 19.2 ounces of MSO,” Reiss adds. “But by cutting that water volume down to 10 gpa, the oil concentration falls down to under 13 ounces per acre, well below the 20 ounce per acre threshold for good performance.” 

Reducing adjuvant rates is a recipe for failure, adds Stratman. “Just like using the right rate of herbicide for the size or species of weed you’re after, the adjuvant rate is also important. Adjuvant rates can change based on the crop and environmental conditions, so it’s important to understand each situation. Inadequate adjuvant rates can impact the droplet dynamics and surface tension of the droplet on the leaf, which can reduce uptake and, ultimately, performance.” 

Gustafson says skimping on adjuvant rates — or quality — could force the need for repeat herbicide applications. “This goes back to the fact that we also don’t have an unlimited supply of herbicide,” he explains. “A few years ago, a farmer would have just said, ‘If it doesn’t work this time, I’ll just go get more and do it again.’ ” 

“Now, we’re at a point with some herbicides that there’s not a second time because there’s simply not enough. That’s one of the risks you run into with lower-quality adjuvants,” he adds. “Plus, there’s often less research on them. Some of them can have mixability issues where they may tie other herbicides up or form crud in your tank.”

Resistance Reduction

Because adjuvants increase efficacy potential, the reduction in weed survivors lowers the odds for building a resistant population. “Everything is a numbers game, and adjuvants help put the numbers in the grower’s favor,” says Stratman. 

“Ideal adjuvants can help reduce the frequency of herbicide failure,” says Young. “They can be important tools to mitigate herbicide resistance when combined with using multiple, effective herbicide mode of action groups and nonchemical weed control practices. Before selecting an adjuvant, Young suggests requesting local data on its performance. 

“Inquire with local crop consultants and determine if any research or recommendations are available from Extension specialists,” he adds. “Since newer herbicides are less forgiving if ideal weed sizes and weather conditions are not present at the time of application, applicators have even greater incentive to select the best adjuvant system.”