Give your weed control investment a better chance

Written by MFA STAFF on .

Proper choice and use of adjuvants helps crop protectants do their job

With weed challenges from the previous growing season and new herbicide technology on the market, paying attention to adjuvants takes on new importance in 2016.

The increase in the seed bank from an odd growing season and the proliferation of herbicide-resistant weeds means that weed management for the next growing season will be crucial. If there were major weed escapes, it may take years to return the seed bank to a previously lower level. In the upcoming growing season, getting adjuvants right can give your weed control program a beneficial edge.

Adjuvants improve weed control in various ways. As new herbicide technology comes to the field, one important way adjuvants contribute to weed control success is keeping spray on target. Drift control will be a priority as dicamba and 2,4-D herbicide platforms reach fields.

Adjuvants also play an important role in the efficacy of herbicides once they hit target plants. All leaves have a waxy covering (cuticle) surrounding the outside of the leaf. It’s a biological defense mechanism for the plant. The covering reduces fungal problems by allowing water to run off the leaf and helps protect the leaf under harsh environmental conditions. A herbicides has to pass through this waxy barrier to its job. Adjuvants play a role in aiding herbicide movement into the plant. There are three types of adjuvants that work directly with herbicide chemistry: surfactants, concentrated crop oils, and fertilizer salts. The herbicide a grower uses will determine which adjuvant is recommended.

Surfactants have many different roles depending on the chemistry used. Cationic and anionic (positive and negatively charged) surfactants are great wetting agents and some cationic surfactants are pre-formulated with herbicides.

Nonionic surfactants like Astute Extra, Astute and Astutue Lite do not have a charge and can be used with a wide variety of herbicides. These are not affected by ions in hard water. Nonionic surfactants are good dispersing agents. They are soluble and stable in cold water and have low toxicity to plants and animals.

Silicone-based surfactants work even better than nonionic surfactants. They are efficient at dispersing water droplets and are humectants (humidity creating). Combining these qualities increases the amount of herbicide entering the plant in addition to reducing the time for herbicides to become rainfast. However, silicon surfactants are not compatible with herbicides that need small concentrated deposits, such as glyphosate.

Regardless of surfactant, they all share a goal—to disperse water droplets. Surfactants aid in droplet creation by breaking the surface tension of water, which allows droplets to spread out rather than bead up. This is especially important when using contact herbicides such as Cobra. Contact herbicides do not move through the plant, so coverage area is extremely important.

Concentrated crop oils and methylated seed oils like Relay, Xpond and Soy Plus enhance uptake of herbicides by penetrating the cuticle. The waxy cuticle is made up of fatty organic compounds. Because oil-based adjuvants have similar chemical properties, the oils and cuticle are chemically attracted to one another. That provides better penetration of the herbicide through the cuticle. Oils also keep herbicides in a liquid state longer, further aiding herbicide uptake.

Methylated seed oils are smaller less complicated molecules, which makes them lighter and more effective at penetrating the cuticle. Crop oils and methylated seed oils must also contain an emulsifier and require agitation to keep the oil suspended in water, preventing buildup on the water surface.

AMS “conditions” hard water (AMS Advantage and Waypoint) high in magnesium and calcium ions for herbicides, especially glyphosate, which binds to these ions reducing its effect. Adding AMS before adding glyphosate to the mix can protect the herbicides active ingredient against binding.

MFA’s line of water conditioners, including AMS Advantage, Waypoint, Overide and Impetro are good options if hardwater is a obstacle.

Of course, the pesticide label is the primary source to consider when choosing an adjuvant. Here are some things to consider when the label provides you a choice.


• If both oil concentrate (crop or vegetable oil) and non-ionic surfactant are options, consider nonionic surfactant under normal weather conditions if weeds are small clearly within label guidelines. If weeds are stressed from dry weather or more mature, their cuticle material will likely be thicker. Use oil concentrate in these conditions.
• If labeled, try to include oil concentrate to control grass weeds.
• Use nitrogen fertilizer only if it is recommended on the herbicide label.
• If your herbicide has high potential for crop injury, consider nonionic surfactant instead of oil concentrate.
• To improve crop safety, do not include oil concentrates with plant growth regulator-type herbicides (i.e., dicamba, 2,4-D, etc.)
Herbicide manufacturers research the best surfactants to use with their products. Newer herbicides tend to have labels with very specific directions for amount and type of surfactant. However, some herbicides have more general recommendations in the label.
A word of caution: adjuvants are not regulated by the EPA and therefore you may hear exagerated claims about what the products can do. Make sure you buy adjuvants from a reputable source. Here are some red flags for products that might sound too good to be true:
• It is only available on the internet.
• Any claim that includes “resistant weeds” in its purpose
• An unknown company that is located outside MFA’s market area.

If you have questions about the best adjuvant for your weed control program, contact a local MFA agronomist HERE or contact This email address is being protected from spambots. You need JavaScript enabled to view it..

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