Combine tools to fight herbicide-resistant weeds in corn

Sweet-corn growers are gaining new tools to supplement triazine herbicides as herbicide-resistant weeds become more common.

New this year is the post-emergent herbicide Impact (topramazone) from Amvac Chemical Corp. of Newport Beach, Calif. It is not registered in California.

Two other herbicides are nearing registration, says Chris Boerboom, Extension weed scientist at the University of Wisconsin in Madison.

Triazine resistance in such broadleaf weeds as lambsquarter, kochia and waterhemp has been an ongoing concern for the past 20 years, says Aaron Hager, Extension weed control specialist at the University of Illinois, Urbana-Champaign in Urbana.

While not as widespread as resistance to ALS inhibitors, triazine resistance is more of a challenge to sweet-corn growers because their herbicide options are limited, he says.

“We’re faced with that every year,” says Dan Hinkle, owner of Hinkle Produce in Cissna Park, Ill.

Resistant weeds in his 1,000 acres of sweet corn reduce the efficacy of Bicep Magnum (atrazine and s-metolachlor), a versatile herbicide from Syngenta Crop Protection of Greensboro, N.C., that doesn’t damage the crop.

Weeds hit the bottom line

Weeds are a major problem in sweet corn, and most fields are treated with pre-emergent herbicides containing atrazine, says Marty Williams, a weed ecologist with the U.S. Department of Agriculture’s Agricultural Research Service in Urbana.

Weed competition affects the number of ears produced per acre as well as kernel fill and other quality measures, says John Masiunas, University of Illinois, Urbana-Champaign associate professor of vegetable crop weed management.

Weeds compete for water and nutrients, concerns not limited to dry years as growers fight to cut input costs, Hinkle says.

Weeds also shelter pests and restrict pesticide application coverage, Masiunas says.

Overcoming crop injury from post-emergent herbicides is a priority, Williams says.

Hybrid sweet corn selection affects how well a crop tolerates weeds, he says. GH2547, for example, produces a canopy that shades out weeds more efficiently than the shorter Spirit hybrid.

Williams’ research includes identifying specific traits that make some hybrids more weed-tolerant.

Timing is everything

Adjusting planting schedules, when possible, may overcome some weed problems, Williams says.

Yield losses from lambsquarter and foxtail are more significant in early plantings; early corn often needs more intensive management over a longer period to keep weeds in check.

A wide planting window that spreads out harvest and extends the marketing season makes early plantings necessary. But knowing the extra challenge presented by early-season weeds will help, Williams says.

Not all weed species produce seed during sweet corn’s relatively short growing season, he says. Post-harvest cleanup prevents greater problems later.

Growers can schedule planting accordingly if they determine each field’s dominant weeds, Masiunas says. A field with continuing lambsquarter problems might be planted later in the season than one with waterhemp.

“There’s no one tactic that gives you complete control over weeds,” Williams says. “But having competitive hybrids may be important.”

An ounce of prevention

Preventing weeds altogether may be a better tactic, Hager says.

“The easiest way to deal with resistant weeds is to avoid them from the get-go,” he says.

Alternate herbicides with different sites of action to ward off resistance, and control weeds through such nonchemical methods as tilling and cultivating rows—but also stop weeds from spreading in the first place, he says.

Equipment can pick up and carry weeds and weed seeds from an isolated patch and distribute them through the entire field, as well as transport them from one field to another.

Cleaning equipment by pressure washing or other methods before moving to the next field helps minimize transport of weed seeds.

Masiunas suggests tankmixing herbicides. Depending on the target weed mix, growers might combine atrazine with Dual Magnum (s-metolachlor) from Syngenta or Outlook (dimethenamid-p) from BASF Corp. of Research Triangle Park, N.C., or combine Accent (nicosulfuron) from DuPont Crop Protection of Wilmington, Del., with Callisto (mesotrione) from Syngenta.

Growers also should select crop rotations that use different herbicide chemistries or other weed-control options, Masiunas says. A vegetable rotation may also wind up relying on ALS inhibitors and create additional resistant populations.

Masiunas is studying cover crops to determine which ones outcompete weeds and prevent their emergence.

Pre-emergent herbicides form the core of an effective weed management program, Boerboom says. To get the most value from these sprays, apply them as close to planting as possible.

Timing also is key to prevent crop injuries when using post-emergent herbicides, he says. Young weeds are easier to control with sprays, and attacking small patches prevents larger stands later.

Along with herbicides, Hinkle relies on rotary hoeing and multiple passes through fields with cultivators. It’s a difficult decision, given rising fuel and labor costs, but necessary in the face of herbicide resistance, he says.

Resistance is “already a problem, and I guarantee in the future it will be, if anything, a worse problem,” Masiunas says. “Farmers shouldn’t be complacent. No matter what tool we develop to manage pests—in this case weeds—they evolve to match.”