As glyphosate herbicides continue to gain popularity among corn growers, so do questions about their use with adjuvants to improve weed control performance in glyphosate-tolerant corn.

Not all glyphosate products need an adjuvant for use in corn, points out Bryan Young, Southern Illinois University (SIU) weed control specialist. He advises farmers to read the herbicide label carefully to avoid making costly mistakes.

The four most common activator adjuvants used with postemergence corn herbicides are nonionic surfactants (NIS), crop oil concentrates (COC), methylated seed oil (MSO) and nitrogen fertilizers, says Young. He emphasizes that an activator agent such as these generally helps improve herbicide performance, but only if called for on the product label.

“Don't use methylated seed oil or crop oil concentrates with glyphosate products,” he cautions. “Those additives can cause crop injury or reduce weed control.”

A nonionic surfactant is generally the cheapest and most effective adjuvant to use with a glyphosate formulation that contains only a partial surfactant load, says Young. However, he still advises checking the label before using one.

“All formulations of glyphosate may benefit from ammonium sulfate (AMS),” he says. AMS is a common nitrogen fertilizer product.

Bob Hartzler, Iowa State University Extension weed scientist agrees. He says, “The best source of information for glyphosate is the herbicide label.”

Although some glyphosate products give the option of using an additive, an adjuvant might not always boost the bottom line. Hartzler says, “Most research shows that adjuvant use with most glyphosate products doesn't add much benefit, except to control large weeds.”

On the other hand, “hard water can be a real problem with glyphosate,” he adds. For those farmers who have hard water, Hartzler advises adding AMS in the tank with glyphosate to avoid herbicide performance problems.

Farmers should also be cautious about purchasing so-called “top-end” adjuvant blends, which may provide much more of the active ingredient than is really needed, says Young. For example, top-end blends may have ingredients that condition water, improve herbicide uptake or provide buffering or anti-foam agents.

While these may be legitimate ingredients that are needed on some farms, they may be unnecessary for your situation. Either way, you'll probably pay a lot more for them, he adds.

“Some of the top-end blends may cost $2/acre,” says Young. “But if you're using glyphosate and you're solely looking for improved weed control, you might do just as well by using AMS for about 25¢/acre.”

Field conditions can also play an important part in adjuvant and herbicide use, says Young. If conditions become too hot and dry, it may make sense to wait for more moisture or cooler temperatures before spraying postemergence herbicides.

“You need actively growing plants for herbicides to work,” he says, “and that remains true no matter what adjuvant you may be using with it.”

Young adds that SIU offers a compendium of herbicide adjuvants for $3. To order a copy of the booklet or to review the online version, visit www.herbicide-adju vants.com.

Three Tips For Using Adjuvants

Unlike herbicides, adjuvants undergo much less government oversight, says Bob Hartzler, Iowa State University Extension weed scientist. As a result, it's difficult for consumers to know which adjuvants work the best.

“If you buy an adjuvant from a company that also sells the herbicide, they will be less likely to sell a bad product,” he says. “That's because they have to deal with the farmer if the product doesn't perform.”

Hartzler gives three guidelines for using adjuvants with corn herbicides:

  1. Follow the label.

  2. Be cautious purchasing products from businesses not directly involved in crop protection.

  3. Seek advice from company representatives, particularly if tankmixing herbicides.

“Herbicide companies find out which additives don't work very quickly, because of performance complaints,” says Hartzler, who emphasizes that “the herbicide label indicates the most effective way to use that product.”