As if corn and soybean farmers didn’t have enough to worry about, there’s this: University of Missouri (MU) weed scientists have discovered a waterhemp population that resists six herbicide sites of action.

It all started when farmers in the north-central Missouri county of Randolph reported a population of waterhemp that appeared to resist 2,4-D. MU researchers then conducted field experiments that confirmed 2,4-D (herbicide site of action Group 4) resistance. But they also found the same waterhemp population resisted:

• Atrazine (Group 5)
• Chlorimuron (Group 2, Classic)
• Fomesafen (Group 14, Flexstar, Reflex)
• Glyphosate (Group 13, Roundup)
• Mesotrione (Group 27, Balance Flexx)

Of the eight herbicides applied, only dicamba (Group 4) and glufosinate (Liberty, Group 10) provided acceptable control, say the MU scientists.

The MU researchers say six-way resistant waterhemp requires a diversified management approach. Rather than relying on glyphosate, 2,4-D, or any other single herbicide, they recommend a variety of appropriate cultural, mechanical, and biological control tactics. For example, rouging weed escapes this summer and hauling them away from the field can help forestall future waterhemp infestations.