Several factors contributed to the first appearance of waterhemp resistance to the HPPD inhibitor herbicides reported in two fields in Iowa and Illinois in 2009. These two case studies show that managing herbicide resistance is not as simple as counting the number of herbicide groups used, but that the contribution of each herbicide in controlling individual weeds must be considered.
That's what Iowa State University Extension weed specialist Bob Hartzler concludes, after studying the history of weed management in these two fields.
Waterhemp caused significant economic losses for many Iowa corn and soybean growers during the mid-1990s, he explains. With the introduction of new herbicide-related trait technologies, primarily Roundup Ready soybeans, most farmers have minimized the economic impact of waterhemp in corn and soybean production. However, the selection of herbicide resistant waterhemp biotypes (Table 1) threatens to return us to an era where uncontrolled waterhemp causes significant yield losses across the Corn Belt.
The first appearance of resistance to the HPPD inhibitors was reported in 2009 in waterhemp found in these two fields in Iowa and Illinois. Several factors contributed to this appearance, says Harztler, who gives the following explanation and insight as to what happened.
The two fields where HPPD resistance evolved are approximately 140 miles apart, but they have several similarities in management practices. The Illinois field was maintained in continuous corn since at least 2003, whereas the Iowa field was a corn-soybean rotation. However, both fields were used for seed corn production during the period of HPPD use. The specific herbicide programs of the fields are provided in Tables 2 and 3.
A cursory evaluation of the weed management programs for the two fields suggests the growers took a reasonable approach to managing waterhemp. Preemergence herbicides were used in all years, a benefit since the specific herbicides have activity on waterhemp. In most years, atrazine was tank-mixed with Callisto, a combination with synergistic activity on susceptible weeds.
The herbicide programs resulted in waterhemp being exposed to three herbicide groups (groups 5, 15 and 27) each year of corn production. In soybeans, two herbicide groups (groups 4 and 9) with activity on waterhemp were used. Exposing weeds to multiple herbicide groups is the key to resistance management, so what went wrong in these fields?