Effectively managing tough weeds in corn crops relies upon more than just applying glyphosate. That’s what Bryan Young, weed scientist with Purdue University, shared with attendees of the 2024 Corn Congress hosted by Cornell Cooperative Extension’s Northwest NY Dairy, Livestock & Field Crops Program.

Young presented “Herbicide Tank Mix and Adjuvant Decisions for More Robust Weed Management in Corn.”

“What happens when glyphosate stops working?” he asked rhetorically. “Do you add other herbicides? Stop using herbicides? There’s organic; I know some who’ve done that, and it’s worked out to some extent. Do you take out glyphosate? Or sell the farm? That’s not a popular option.”

The problem of resistant weeds has increased. They include (although not exhaustively) those resistant to dicamba, including Palmer amaranth and waterhemp, mostly in Tennessee, Kentucky, Illinois, Indiana and Missouri. There are also those resistant to glyphosate, including Italian ryegrass, goosegrass, Johnson grass, jungle rice, bluegrass and fall panicum.

Waterhemp is resistant to very long-chain fatty acids in Illinois and to 4-hydroxyphenylpyruvate dioxygenase (HPPD) in Indiana. Giant ragweed is starting to show resistance to protoporphyrinogen oxidase (PPO).

Because shifting among various herbicides can eventually lead to more herbicide resistance issues, Young recommended taking an approach that focuses on best management practices. Culturally, this includes preventing introduction of new weeds, increasing crop rotation diversity, reducing crop row spacing, increasing crop seeding rate and integrating cover crops.

Best management also involves mechanical steps, like use of tillage when appropriate, harvest weed seed control, weed electrocution and hand weeding.

Among the chemical approaches, he promotes integrating diverse, effective herbicide sites of action, using tank mixtures in foliar applications and remembering that reduced herbicide rates can lead to reduced effectiveness.

“To be effective at treating weeds, you need two rounds of action, not just Roundup and Liberty,” Young said. “We need to use herbicides more effectively. We need to optimize at the right rate and timing.”

Young listed as broad-spectrum herbicides for use in corn: Acuron, Lexa, Lumax, Cinch ATZ + Instigate, Reiscore + atrazine, Corvus + atrazine (or atrazine premix) and Verdict + atrazine. Nearly all of these effectively combat waterhemp.

The newer corn herbicides include TriVolt, Kyro, Residore XL, Empyros, Restrain, Storen and Maverick. Young also offered post-corn alternatives to glyphosate, as several Group 2 (ALS) and 27 (HPPD) herbicides are effective, such as ALS herbicides Steadfast, nicosulfuron, Capreno, Katagon and Revulin Q.

The weed size and corn growth state can limit the use, and these are not broad-spectrum grass activity herbicides like glyphosate. Farmers should identify the grass species they’re fighting.

Another option is broadleaf weed control with atrazine and Group 4 (auxin) and 27 herbicides.

Herbicide tank mix matters

Bryan Young, weed scientist with Purdue University, recently presented at the Corn Congress. Photo by Deborah J. Sergeant

Four of the corn post-HPPD herbicides (mesotrione, tembotrione, topramezone and tolpyralate) “are someone similar,” Young said. “It gets to how we use those in combination.” He touted the crop safety of the herbicides, with minimal unwanted effects.

“Once glyphosate doesn’t work, you have to pay attention to the weed you’re trying to kill,” Young said. “What’s the weed species? What’s the size? Hopefully, it’s less than three feet.”

He added that waterhemp activity for Groups 4 and 27 herbicides is variable and best on weeds less than three inches tall.

“Effective tank mixtures have always improved the consistency of control for these herbicides,” Young said. “Increase the selection pressure for Group 4 and Group 27 resistance across the weed spectrum.”

By prioritizing the weeds that are the worst, farmers can reduce the number of herbicides needed. For example, is one bigger or presenting in higher densities? Could it pose a potential perennial threat to the field? Young also said that farmers need to look at their corn hybrid for herbicide sensitivity.

“There’s very little opportunity to flex to another option so you have to plan ahead,” he said.

Farmers also need to check the tank mix compatibility of their herbicides and the adjuvants required.

“Read the label and in some cases, read it again,” Young said. “Be familiar with it and its challenges.”

Sprays may require adjuvants such as nonionic surfactant, crop oil concentrate, modified seed oil, nitrogen fertilizer, buffering agent, drift control agent or compatibility agent.

Surfactants reduce the surface tension of water, and most herbicides that don’t list it as a necessary additive to the tank already have it in the mix. Oil-based adjuvants improve wetting and spreading. They increase the movement through the waxy leaf cuticle and reduce water evaporation during the spray’s movement through the air toward its target. These include crop oil concentrates that are petroleum- or seed oil-based and modified seed oil-based options.

“High surfactant concentrates plus hybrids are the new thing,” Young said.

Herbicides enter leaves through the path of least resistance. Making it easier for the herbicide to enter leaves can increase its efficacy.

Young also said that ammonium fertilizers increase herbicide absorption, condition hard water and acidify the spray solution (although as a side effect, not as a direct goal). Using ammonium fertilizers also shows evidence of improved herbicide translocation.

Environmental conditions like temperature and moisture also matter. When spraying during dry conditions, “if you start with small droplets, you won’t have much left after it evaporates,” Young noted.

Using the right spray and adjuvant can ensure optimal efficacy. For example, applying glufosinate with the correct nozzle size at the rate of 15 gallons/acre resulted in a 40% reduction in Palmer amaranth control compared with poor application at a higher droplet size.

Young warned against surfactants and other products claiming to perform as a complete spray additive. “One product cannot do it all,” he said. “There are a lot of claims on adjuvants. They’re not regulated the same way as herbicides.”

He encouraged farmers to ask for data – not testimonials – since those offering testimonials may have different growing conditions.

“Use the application methods and adjuvant systems that optimize activity of the herbicide being used for control of the herbicide-resistant weed,” Young said.

He reiterated that herbicide alone won’t do. “We need effective tank mixes and two modes of action. In some cases, we have only one herbicide we know will work.”

That is why effective management and chemical application are vital for combatting weeds.

by Deborah Jeanne Sergeant