Herbicide resistant weeds -- not just those resistant to glyphosate -- are continuing to wreak weed-management havoc.

“There are biotypes no longer controlled by previously effective herbicides,” says Aaron Hager, University of Illinois (U of I) Extension weed specialist. In one Illinois case, a waterhemp biotype is resistant to not only glyphosate, but also an additional three herbicide action modes.

The good news? Glyphosate (used on over 95% of soybean acres and 70% of corn acres) continues to be the cornerstone of most weed-control systems.

An online survey by Successful Farming magazine and Agriculture.com found that 92% of respondents are either satisfied or very satisfied with their glyphosate-tolerant cropping systems.

“In most Midwestern fields, Roundup Ready technology and glyphosate are still doing the job,” says Mike Owen, Iowa State University (ISU) Extension weed specialist.

Still, there’s apprehension about glyphosate’s performance. Of those responding to the survey, 55% felt it is as effective as it used to be; 45% noticed a decrease in its effectiveness. Meanwhile, 57% noticed an increase in hard-to-control weeds, while 44% noticed an increase in glyphosate-resistant weeds. 

“We are at the edge of a slippery slope over a deep valley,” says Owen. “It will get to be a serious problem very quickly if the system continues as it has without consideration toward stewarding the Roundup Ready technology.”

A disconcerting fact is that herbicide-resistant biotypes may be spreading via pollen. Research by Pat Tranel, a U of I weed scientist, indicates weed resistance to glyphosate can be transferred by both pollen and seed.

Pollen may move over a larger geography than seed does. Therefore, a herbicide-resistant biotype could be spread over a large area, just as herbicide-resistant weed seed can by hitching a ride on harvesting equipment.


How It Spreads

Doug Doolittle, Story City, Iowa, adopted Roundup Ready soybean technology early on. For years, two postemergence applications of glyphosate provided excellent weed control.

In a couple fields, however, he’s wrestled with waterhemp that’s become increasingly tougher to kill each year with glyphosate. He’s cooperating with ISU weed scientists with plots on his farm to find ways to manage it.

“You can brown it, but it keeps coming back,” he says.

Fortunately, the resistant waterhemp hasn’t been a problem in corn, as he applies Lumax preemergence. On soybeans, though, hard-to-kill waterhemp (which likely is glyphosate-resistant) prompted him last year to apply a rescue treatment of Flexstar when soybeans were beginning to bloom. This was after two glyphosate applications failed to kill the waterhemp.

The good news is the Flexstar worked. Those soybeans went on to be Doolittle’s top yielders on his farm.

However, the application dinged the beans and temporarily turned them brown. “Crop safety is important to me,” he says. That’s why he’s continuing to search for additional solutions.


What’s coming up

There are new tools coming later this decade that provide ways to manage glyphosate-resistant weeds. They include: