Waterhemp continues to get more difficult to manage as scientists this year confirmed the first case of waterhemp with six-way resistance, according to the Weed Science Society of America (WSSA).

A waterhemp population in northern Missouri was treated with eight herbicides, and scientists found 16 percent of the plants in the studied weed population contained genes stacked for six-way herbicide resistance — 2,4-D, atrazine, chlorimuron (ALS inhibitor), fomesafen (PPO inhibitor), glyphosate and mesotrione (HPPD inhibitor). Only dicamba and glufosinate provided accepted control.

In Iowa, waterhemp has demonstrated resistance to five different groups of herbicides, said Bob Hartzler, Iowa State University weed scientist. The primary waterhemp resistance in Iowa is to Group 9 (glyphosate) and Group 14 (PPO inhibitor). In 2016, 75 percent of waterhemp samples analyzed by the University of Illinois Plant Clinic were resistant to both herbicide groups. It’s important to note that the sample was skewed because weeds sent for analysis were suspected to be resistant, but the numbers still reflect a troubling trend, said Hartzler.

“The good news is that the presence of Group 9 and 14 resistant waterhemp in Iowa is much lower than reported with the samples...