Rethink Weed Control in Continuous Corn | The More You Grow | Corteva Agriscience
 12/14/2021

Rethink Weed Control in Continuous Corn

Weed control in continuous corn

Continuous cornfields may need extra weed control help in 2022 if glyphosate or glufosinate are the routine postemergence weed control tools, especially in no-till corn. Due to supply chain issues, these two herbicides might be in short supply and may carry a higher price tag. 

“Fortunately, there are many good broadleaf herbicides for corn, plus some of our worst herbicide-resistant weeds don’t manifest themselves all that much in corn,” says Bill Johnson, weed scientist with Purdue University. “It also may be a good year to add tillage or cover crops to the mix to reduce reliance on herbicide tools.” 

Most importantly, Johnson advises a greater focus on and use of full-rate, multiple site of action residual herbicides that match the weed species by field. In addition, he emphasizes the need to use different tools each year to keep weeds guessing by avoiding doing the same thing over and over.  

Examples include using atrazine as the sole broadleaf herbicide in continuous corn or the persistent use of glyphosate to control the same weeds. This can lead to herbicide resistance in weeds. 

Control Grasses Early 

For continuous corn, Johnson says to make sure your corn weed control program is fully-loaded with residual herbicides. "I would certainly make a plan for an additional herbicide in your residual program that can take some pressure off the importance of your postemergence application,” he says. “If you are using one of the older atrazine premixes with two active ingredients, I would certainly max out the atrazine rate for your area and consider putting in some isoxaflutole, or mesotrione to pick up waterhemp or clopyralid + flumetsulam to pick up giant ragweed. And if you’ve got grasses escaping later in the year, use some additional Group 15 herbicides like acetochlor, metolachor, dimethenamid, or pyroxasulfone.” 

Johnson believes that if you do a good job controlling grasses with a residual program, then your postemergence treatment can focus on controlling broadleaf weeds. “That's where dicamba or dicamba plus another broadleaf herbicide can be a pretty effective post-program. If you have grass come through and it’s not ALS-resistant, use glufosinate or nicosulfuron postemergence for grass control. That’s another way to take pressure off glyphosate use,” he adds. 

Switching up sites of action in your herbicide program from year to year is an intelligent system that most growers follow, depending on the bundled programs offered by retailers. “These premixes have from two to three sites of action, and you can add a fourth herbicide without making it overly complicated to help keep herbicide-resistant weeds from growing,” Johnson says. “We are starting to see more weeds breakthrough the bleacher (Group 27) herbicide group, like mesotrione, isoxaflutole, tembotrione and topramezone. And there’s more variability in the control of giant ragweed. That’s where dicamba or dicamba plus diflufenzopyr post-application does a good job cleaning those up.” 

Use Cover Crops to Reduce Weeds

Aside from using tillage to reduce weed populations, more farmers are turning to no-till and/or cover crops as part of their weed control program. 

In Indiana, Johnson says farmers are mainly using cover crops in three ways. “First, to help take care of fall emerging winter annual weeds, some growers seed an oat and radish mix either before or after harvest that winter kills so they can plant no-till corn into the decayed residue.  

"Second, suppose glyphosate and glufosinate are limited, and growers plan to use a light spring tillage. In that case, you can seed the same winter kill mix or use a straight legume that produces less biomass and is easier to terminate in the spring,” he adds.  

“The third system that more experienced cover croppers use is seeding cereal rye in the fall, maybe with some crimson clover, radishes or winter peas, then terminating it before it competes with corn,” Johnson says. “And they often use lower rates of cereal rye, like 20 to 30 pounds per acre, to reduce biomass for easier termination and less nitrogen tie up.” 

Finally, to overcome possible herbicide supply chain challenges, Johnson expects renewed interest in postemergence products like tembotrione, topramezone, and nicosulfuron among continuous corn growers, depending on the weed spectrum. “Who knows, we may even see bromoxynil make a comeback.” 

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