RANDOLPH COUNTY, Mo. — Soybean and corn farmers have a new concern on the horizon according to University of Missouri weed scientists. The researchers have confirmed a population of waterhemp in north central Missouri is resistant to six different herbicides.

Waterhemp grows faster than most weeds and can greatly reduce a producer’s soybean or corn yields.

“It’s just another illustration of multiple resistance in Missouri in waterhemp,”  explained MU Plant Sciences Professor Kevin Bradley. “Which we’ve had three-way resistance, four-way resistance, even five which is very rare. But this one is, obviously, six-way resistance is nothing we’ve ever seen before.”

Bradley who is also the state Extension weed scientist said when he was first contacted a few years ago regarding this issue, waterhemp was not known to be resistant to the herbicide 2, 4-D. So the fact that farmers in the state located a possibly resilient strain fueled a need to identify exactly what the variety was resistant to.

After thorough examination, Bradley and his team discovered this particular population of waterhemp was immune to the following herbicides:

  • 2,4-D (herbicide site of action Group 4)
  • Atrazine (Group 5)
  • Chlorimuron (Group 2, Classic)
  • Fomesafen (Group 14, Flexstar, Reflex)
  • Glyphosate (Group 13, Roundup)
  • Mesotrione (Group 27, Balance Flexx)

The team of weed scientists are not worried about this type of waterhemp spreading by seed, but rather they are concerned about it spreading by pollen.

“As long as there’s no seedhead ever produced, no plants that survive, then we can reduce this situation and control it,” Bradley said. “So that’s what we’re shooting for. You just have to have clean fields.”

At this point, Bradley said his team is encouraging producers to consider all available cultural and chemical practices. These options could go along way in preventing further six-way resistant waterhemp from spreading.

“We could use alternative herbicides to which this is not resistant,” Bradley said. “And we can use cultural practices like cover crops and things like that. Really where you get the best control is where those two things happen together.”

Bradley said two products that are not currently showing resistance include LibertyLink and Dicamba.