by Deborah Jeanne Sergeant

With a shortage of glyphosate, it seems like farmers should expect their cornfields to suffer yield losses this year; however, Dr. Bill Johnson, professor of weed science at Purdue University, offered farmers a few options in his presentation “Corn Weed Control in the Herbicide Shortage Era” as part of Cornell’s virtual Corn Congress.

“I don’t remember a time we’ve ever had these kinds of shortages,” Johnson said. “There’s occasionally a shortage of an ingredient here and there, but not a main ingredient. We’re working on trying to deal with this.”

He began by reviewing a few principles on weed competitiveness with corn. “What we typically find with corn is it’s not typically able to recover from early weed interference,” he said. “Corn is not as hardy as soybean when it comes to overcoming weed pressure. Weeds influence the quantity of light that hits that corn plant. It won’t grow like it would’ve in the absence of weeds.”

With a glyphosate or glufosinate treatment, plus additional residual herbicide, the yield remains unaffected. With no treatment, the potential yield begins decreasing as early as week two of the weedy period after planting. Weed competition reduces the macronutrients to corn plants as well as water and light.

“A field of corn never sees much in the way of weed competition,” Johnson said. “What there’s been a lot more interest in the past 15 years when commodity prices were low and glyphosate was low is moving away from residual herbicide and Roundup-ready or Liberty-ready and trying to accomplish our weed control with just post-emergent treatments.”

Numerous studies indicate crop yield suffers with any weed competition whatsoever. Interestingly, typically high yielding fields are more responsive to weed competition. “Many things can impact that,” Johnson said.

The toughest weeds to control include giant ragweed, burcucumber, fall panicum, shattercane, johnsongrass and water hemp. “We desire simple weed control with one pass,” Johnson noted.

He referenced a study which looked at what amount of pre-herbicide is needed in pre- and post-programs for Liberty Link, glyphosate-resistant corn. “We did see that the ragweed control did tail off as we did those evaluations later in the summer,” Johnson said.

The research showed that weed-free, high-yielding corn is the key to maximizing returns on herbicide investments. The return on investment of a strong pre-herbicide is less when the post-treatment is delayed. Johnson also said that a pre-herbicide with a group 15 and atrazine was always better than just atrazine, especially as post-emergence weed control timings are delayed.

“When corn prices are where they are now, you want to design a weed management program that give you weed-free fields,” Johnson said.

He believes the program should include residuals to maximize farmers’ investments. Farmers should also use a broad-spectrum pre-herbicide and apply post-herbicides when weeds are still relatively short.

“Make sure your post-emerged timing is optimized,” Johnson said. “You want your weeds small and relatively early, V2 or V3, and four-inch-tall weeds.”

Especially in light of the glyphosate shortage, he said he encourages growers to not get sloppy with weed control in corn.

Properly identifying weeds can help farmers know the weed and crop growth stage. Traditional weed ID sources and apps such as iNaturalist or PictureThis can aid them.

“The whole Roundup-ready thing has in my opinion dumbed down our ability to be weed managers,” Johnson said. “Our biggest decision was ‘Where can I get the cheapest glyphosate and how much do I apply?’ We have a whole generation of people who need reeducation on weed identification. Hopefully, we don’t need to use those on them very often.”

He encouraged farmers to peruse weed control guides offered by state and industry organizations. “[Purdue] breaks ours out by crop, talks about strategies and has tables about how to choose an herbicide. We talk about the directions for using the different herbicides and things to be careful about with rotational strategies,” he said.

Grasses are best treated by Impact Z, nicosulfuron, Revulin Q and Steadfast Q. For broadleaf weeds, look into 2, 4-D, atrazine, bromoxynil + atrazine, dicamba + atrazine, Impact Z, Revulin Q, Status and Steadfast Q. Both types are impacted by Impact Z and Revulin Q.

Johnson said farmers should prioritize which weeds are the worst to narrow down the herbicide list. “This gets down to how good of a manager you are,” Johnson said. “Which weeds are worse? Grass or broadleaves? Is one weed in higher density or something that appears every year that you need to get on top of?”

Farmers should also check the tank mixing compatibility of any foliar fertilizers, fungicides and insecticides before purchasing. Johnson added that products with three or more active ingredients are farmers’ best options.