Corn and soybean growers apparently aren't doing enough to prevent weeds from thumbing their noses at herbicides.

Weed scientists say weed resistance to herbicides is a rapidly expanding problem across the Corn-Soybean Belt. The resistance is primarily to triazine and ALS-inhibitor products. In some cases, weeds are resistant to both herbicide classes. It's a serious problem, says University of Illinois weed scientist Aaron Hager.

"Weed species with biotypes resistant to triazine herbicides in Illinois include smooth pigweed, common lambsquarters and kochia," Hager reports. "Species with biotypes confirmed resistant to ALS-inhibiting herbicides include common and tall waterhemp, smooth pigweed, common cocklebur and kochia. And we are suspicious of several other species as well."

Hager points out that most of the newer resistance problems involve ALS-inhibitors because many herbicides use that mode of action.

"The overuse of ALS-inhibitor herbicides appears to have backfired on Ohio and Indiana farmers," says Ohio State University weed scientist Jeff Stachler. "Ohio tests have confirmed that giant and common ragweed are resistant to ALS-inhibitor herbicides. Also, horseweed/marestail, Powell amaranth and common cocklebur show some resistance."

Growers in Kansas, Oklahoma and the Texas Panhandle are encountering Palmer amaranth (pigweed) and kochia with resistance to both triazines and ALS-inhibitors, reports Ron O'Hanlon, president of Crop Quest Agronomic Services, Dodge City, KS. They're also seeing shattercane and waterhemp resistance to ALS-inhibitors, he says.

Hager lists two key steps growers can take to combat herbicide resistance: 1) Scout fields to learn which weeds are there, and also to determine whether a resistance problem is present. "If I have used the same chemistry for six or seven years and never had a problem controlling a particular weed, and then that weed starts to thrive, it's a red flag that resistance could be the cause," Hager points out.

2) Use herbicides with different modes of action from one year to the next. That can solve a current resistance problem and help prevent a future one.

O'Hanlon's Crop Quest agronomists scout each field to determine the weed spectrum. Then, to prevent herbicide resistance, they normally recommend a herbicide program that includes more than one mode of action.

"We may use three or four products, utilizing different modes of action, in a given field," he explains. "We generally suggest a preplant or pre-emergence treatment followed by a postemergence treatment."

Company agronomists also generally recommend different modes of action from one year to the next in a given field. "This is true whether we have continuous corn or there is crop rotation," says O'Hanlon. "By making certain we have multiple modes of action in our herbicide recommendations, we are able to reduce the resistance problem."

Although there has been no known incidence of weed resistance to glyphosate (Roundup) in the Corn-Soybean Belt, weed scientists caution against an over-reliance on that chemical.

"Two weed species have been confirmed resistant to Roundup," reports Ohio State's Stachler. "They are Italian ryegrass, first found in Australia and now in California, and goosegrass in Malaysia. That shows us it can happen."