Waterhemp Becoming a Superweed

Studying the first known case of waterhemp with resistance to HPPD-inhibiting herbicides such as Callisto, weed science researchers at the University of Illinois have identified two unique mechanisms in the plant that have allowed the weed to “get around” these herbicides.

“Waterhemp is very diverse, which you can see in the field. There are red plants, green plants, tall, short, bushy—basically a germplasm pool. If you  keep spraying the same herbicide over and over, eventually you’re going to find that rare plant that can resist it,” said Dean Riechers, a U of I Professor of weed physiology,

What the U of I researchers find alarming is that waterhemp resisted the herbicide in much the same way that corn naturally resists HPPD-inhibiting herbicides.

“It mimics corn but also mimics the super bacteria that are resistant to all the antibiotics out there. Weeds are kind of like bacteria in that respect; at least this population is. Whatever active herbicide we throw on it, with the exception of glyphosate, it doesn’t work anymore,” Riechers said.

The study was prompted in 2009 when a continuous seed corn grower from central Illinois realized the HPPD-inhibiting herbicides he was using were no longer killing waterhemp plants, which by then had grown into a mat of weeds across the field.

“It became obvious to the grower that something was wrong, but it probably started years before that,” Riechers said, adding that the grower had been planting continuous seed corn every year, using HPPD-inhibiting herbicides for at least eight years in a row.

“Mesotrione and atrazine are normally two very good herbicides that are safe on corn but still kill waterhemp,” Riechers said.

Although the 2009 incident was the first to document this type of resistance, Riechers said four or five other locations in the Midwest have since reported similar occurrences.

“It doesn’t appear to be isolated because it looks like there are other resistant populations coming up,” Riechers said. “The concerning thing is that some of these fields actually did have corn and soybean rotations. They weren’t just growing corn, they were rotating, which is what you’re supposed to do. But it still became HPPD resistant, and we’re not sure how that happened.”

Rong Ma, one of the researchers on the study, said growers should consider not using the same herbicide mode of action repeatedly. “For example, don’t use HPPD-inhibiting herbicides alone for several years in a row because it is then easier for weeds to develop resistance," she said

“Growers could also use tillage because there’s no resistance to tillage,” Riechers pointed out. “Farmers use no-till systems, often plant in narrow rows, and for the most part have gotten away from tillage for weed management. We have aided waterhemp in becoming a problem by not using tillage, using the same chemical over and over, and by not rotating crops.”

Source: Dean Riechers University of Illlinois, tel:+1 217-333-9655

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