Herbicide Layers Reduce Tough Weeds in Corn

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A planter tune-up to improve consistent corn emergence is important, but a weed management strategy that properly layers the best available herbicides can put more bushels in the bin this fall. 

Uncertain supply chain issues and higher input prices are the biggest challenges to achieving cost-effective weed control in 2022, says Bryan Young, Purdue University weed scientist. “Along with short supply and higher costs of glyphosate and glufosinate, we also hear issues with atrazine supply. As a result, growers may need to change their weed control plan by using premixes that contain atrazine and finding more economical glyphosate options due to a three- or four-fold price hike.” 

Young says options include auxin herbicides like dicamba and 2,4-D that can be combined with bleaching herbicides (HPPD inhibitors) to optimize weed management in corn. “The challenge might be grass control. Old ALS-inhibiting herbicides like nicosulfuron can help, but you must be mindful of ALS-resistant grass species and which specific grass species are in the field.” 

Avoid Reducing Rates 

Courtesy: Iowa State University

Given higher input costs, Young recommends avoiding cutting herbicide rates to save money. “Once you move away from glyphosate, the opportunity to cut your rate and see the same level of weed control just isn't there,” he says. “No other herbicide has the same flexibility for rate structure. As soon as you cut rates, there's a fairly high risk of compromising weed control.” 

The need for layered herbicides is due to weeds like waterhemp, giant ragweed, Palmer amaranth and marestail. “Current weed strategy is not just controlling lambsquarters or giant ragweed in April or May. Instead, we must control weeds like morningglory, waterhemp, Palmer Amaranth and maybe some grass species that germinate from April through August, if we let them. We've selected for these weeds because we apply herbicides early in the season of crop growth and development,” Young says.  

Corn Senses Seed Competitors

Start clean and stay clean is critical in corn to save bushels, maintain strong plant growth and stop adding weed seed to the soil that will cause future problems. 

That is important because corn plants can sense through light reflectance the presence of weeds from an early stage, which changes the crop's growth and development for the rest of the growing season. “With so much invested per acre, the risk of one early postemergence application for weed management isn’t worth the negative corn growth and expanded weed problems,” points out Young. 

“With so much invested per acre, the risk of one early postemergence application for weed management isn’t worth the negative corn growth and expanded weed problems.”

He says some growers start with fall herbicide applications to start clean. Others gain a good start in the spring with an early preplant application. And some growers wait and apply within 14 days of planting corn. “All those are fine as long as you are free of green vegetation when you plant corn,” stresses Young. 

Pay attention to the timing of the first herbicide application and the length of residual control with different products given the soil and weather variables. “A fall application layer may not last into spring, just like a preemerge application layer two to three weeks before planting may not last long if spring rains push the herbicide below the germination zone of weeds,” he adds.  

Time Atrazine Applications

To stifle season-long germinating weeds like waterhemp and giant ragweed in corn, save some atrazine. “We’ve had success in Indiana using 1.5 pounds atrazine per acre preemergence, then come back with 0.5 to 1 pound of atrazine postemergence on 10- to 12-inch corn,” he says. “That provides extensive weed control of late-germinating weeds.” 

One concern to watch for is atrazine degradation. “It’s been documented across the Corn Belt that some growers see reduced residual control due to microbial activity causing accelerated degradation. It could be weed resistance, or the herbicide degrades faster. That's why using a mixture of different herbicides is critical,” he adds. 

Young says Group 15 herbicides like metolachlor, pyroxasulfone and others improve control consistency of grasses and small-seeded broadleaf weeds in the postemergence layer. “The more residual herbicide modes of action you use, the greater the consistency in the length of residual,” he says. 

Before you expend too much shop time tinkering with the planter, take Young’s advice to spend energy on a layered weed control plan that will save more bushels than a picket-fence stand of corn. 

Content provided by DTN/Progressive Farmer.