Strategic Farming: Adjuvants For Maximizing Herbicide Efficacy

Published online: Feb 26, 2024 Articles Angie Peltier, UMN Extension crops educator, Joseph Ikley, North Dakota State University Extension weed management specialist, Greg Dahl, retired Winfield product development manager
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On Feb. 14, 2024, Joseph Ikley, NDSU Extension weed management specialist and Greg Dahl, retired Winfield adjuvant development advisor joined UMN Extension crops educator Dave Nicolai for a wide-ranging discussion of herbicide adjuvants and how best to use them effectively.

This was the fifth weekly episode of the 2024 Strategic Farming: Let’s talk crops! webinars. The series runs through March.

To watch this and other episodes, visit z.umn.edu/SFrecordings 

A Long History And Current Abundance Of Adjuvants

Tank-mix adjuvants are products that can be added to the tank that make a pesticide active ingredient work better. Formulators may add adjuvants directly into the herbicide formulation if there is enough room and the adjuvant does not cause storage or compatibility issues. Often there is not room for the adjuvant to be added to the herbicide so tank-mix adjuvants are used instead. 

Adjuvants have been in regular use since the 1700s when molasses, sugar and resins were used to help sulfur and copper fungicides stick to grapes and kerosene emulsions and whale-oil-derived soaps were used to improve efficacy of insecticides made from arsenic. It wasn’t until the 1940s however, that adjuvants began to be studied and routinely used with herbicides. It was then that non-ionic surfactants (NIS) were identified and ammonium sulfate (AMS) was found to increase herbicide uptake. Methylated seed oils (MSO) began to be used in the 1980s.
Currently, there are hundreds of different adjuvant active ingredients that can be placed into several functional classes, including activators, spray modifiers and utility modifiers. Spray modifiers modify the spray solution in the tank on its way to the target or on the target itself. Utility modifiers widen the range of conditions under which a herbicide can be used. Activator adjuvants such as non-ionic surfactants and crop oils increase the biological activity of the pesticide. The discussion during the webinar focused on activator adjuvants.

Surface Active Adjuvants

Surfactants - or surface active adjuvants - work at surfaces to help solve various issues. 

Non-Ionic Surfactants (NIS)

The outside of plant leaves are often coated in a waxy, hydrophobic (water-repelling) coating that makes water droplets bead up and away from the surface. NISs reduce the surface tension of water droplets which allows the same volume of water in the droplet to spread out over more of the leaf surface, providing more opportunity for the herbicide to absorb into the plant tissue. Some NISs have additional functions above and beyond reducing water’s surface tension including emulsifiers, wetters, stickers and dispersants.

Surfactants can also ensure that the spray droplet makes its way to the leaf surface and doesn’t just get hung up on leaf hairs, slowly shrinking in size as it evaporates, never having made its way to the leaf blade surface.

Crop Oil Concentrates (COC)

Because oil loving herbicides and oil adjuvants are hydrophobic like the waxes on a leaf surface, the primary function of petroleum-based COCs is to increase the movement of a herbicide active ingredient through a leaf’s waxy cuticle and reduce water evaporation from within the spray droplet. Similar to NISs, COCs can enhance spreading of spray droplets on the leaf surface. When herbicide carrier volume is relatively low (~15 gal/acre) as it often is in northwest Minnesota, it is important to use enough COC to facilitate the deposition onto and movement into the plant. To ensure that oil and water (which don’t naturally mix) do indeed mix, an emulsifier that will provide a consistent mixture  needs to be used in the tank. 
Crop oil concentrates are most commonly used with oil soluble herbicides such as ALS inhibitors (Group 2), triazines (Group 5), ACCase inhibitors (Group 1), HPPD Inhibitors (Group 27) and PPO inhibitors (Group 14).
Methylated seed oil (MSO), derived from soybean, sunflower and canola oil, is made using crop-based fatty acids mixed with methanol or ethanol. An emulsifier type surfactant is also added to blend the MSO with water. MSO can be more effective as an adjuvant but can sometimes make a mix that is ‘too hot’ and harms the crop. However, from a weed management strategy, an injured crop, particularly one like soybean that has an innate capacity to compensate for some injury, also means that weed management is likely better in that field. MSOs provide better control than petroleum oils in the drier west (including the first couple of counties into MN from the west). In lusher, wetter environments, MSOs can cause crop phytotoxicity.

COC Weather Considerations And Antagonism

Rather than use an NIS in a drought year, more weed scientists recommend that folks consider using a COC. During drought conditions, the waxy cuticle on the outer layer of plant leaves tends to get thicker as the plant desperately tries to stop water loss. Oil-based droplets will have a slower evaporation rate than droplets made up primarily of water, which can provide a bit more time for the herbicide active ingredient to be absorbed by the plant.
Some emulsifiers that are used in the oil adjuvant can cause glyphosate antagonism. AMS and NIS are good adjuvants to use with glyphosate. COC or MSO adjuvants may cause glyphosate antagonism unless you use one that is ‘glyphosate friendly.’ If using one of these glyphosate friendly adjuvants, be sure to use a high rate of glyphosate to help overcome any antagonism.
A newer class of adjuvants are the HSOC or high surfactant oil adjuvants. These are a combination of petroleum or MSO type adjuvants with at least 25 percent emulsifier by weight. These combine the performance of oils and surfactants and can be used at lower rates than COC’s or MSO’s as long as they do not become oil limited. If these products are used at GPAs at or above 15 gallons per acre they should be fine.

If the GPA is below 12 gallons per acre, these products should be used at an oil per acre rate. Otten 1 pint/a would be fine. Be sure to check the label instructions.

Adjuvants That Modify Water Quality

Carrier pH is an important factor for solubility of some herbicide active ingredients. For example, the sulfonylurea herbicides (Group 2), the post-emergence HPPD inhibitors (Group 27) and the Group 14 herbicide Sharpen tend to be more soluble and work better in water with a pH above 7.
In some areas of the state, many of our sources of water have high concentrations of both positively charged ions such as calcium, magnesium, sodium, potassium and iron and negatively charged ions such as sulfate, chloride, bicarbonate and nitrate. Ammonium sulfate (AMS) can be added to the tank in sufficient quantity to be able to tie up the positively charged ions. By adding up the concentration of positively charged ions in your water obtained in a water quality report, one can determine how much AMS would need to be added to overcome a specific water source’s hardness. Both granular and liquid AMS formulations work well. While granular formulations tend to be much less expensive, they also take some time to move into solution and so which formulation one chooses to use will depend upon one’s budget and patience during a busy time of year.

Some weed species such as lambsquarters, volunteer sunflower and velvetleaf can have positively charged ions on their surface, requiring additional AMS in the tank in order to overcome this antagonism. AMS or water conditioning adjuvants should be used at rates above those needed to condition the water when soil and dust is present on plants, when weeds are hard to control, when the environment is dry and hot and when weeds populations are high.
Research has shown that AMS can often improve weed control with herbicides even when distilled water is used. The positively charged ammonium in AMS can bind to negatively charged weak acid herbicides and facilitate movement within a plant demonstrating its benefit in distilled water which does not need to be conditioned.

Research Results Tell The Story

After reviewing thousands of adjuvant research studies, the Council of Producers & Distributors of Agrotechnology (CPDA) found that when a herbicide is applied without its required adjuvant, herbicide efficacy can be reduced by 30 to 90 percent. For example, when Sharpen is used without an adjuvant, no injury symptoms will be observed on weeds, whereas Sharpen with the required adjuvant provides excellent control. Efficacy was reduced and there was between a 5 and 50 percent increased risk of crop injury when the wrong type of adjuvant was selected. Lastly, when a ‘good enough’ adjuvant was selected over a ‘premium’ adjuvant in the same class of adjuvants, there was between 5 to 25 percent less weed control with the good enough adjuvant than when using a premium adjuvant.

Tank Mixing Best Practices

As herbicide mixtures have become more complex over time, there is a great need to determine the proper mixing order to avoid a sludgy mess setting up in your spray tank. Know that when the water is cold, as it is when it is first drawn from the well, it will take longer for dry materials to solubilize. The best and least costly way to ensure that you have the proper mixing order is to do a jar test. Because cleaning out a jar is much preferred over cleaning out an entire sprayer.

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program.