Those who cannot remember the past...

Encyclopedia Article

The evolution and rapid spread of glyphosate resistant weeds has increased people’s awareness of the threat of herbicide resistance. In late 2012 Iowa State University Extension hosted a series of meetings across the state where the issue of herbicide resistance was discussed with farmers and industry representatives. A pre-meeting survey was conducted at these meetings where participants provided information on their weed management practices and perception of herbicide resistance. One of the questions attendees were asked was the prevalence of resistance to five herbicide groups (Table 1) in fields they managed or in their territories.

Table 1. Herbicide Group Numbers (WSSA system)




2 ALS inhibitors Classic, Pursuit
5 triazines atrazine, metribuzin
9 glyphosate Roundup
14 PPO inhibitors Reflex, Valor
27 HPPD inhibitors Callisto, Laudis

The most surprising result of the survey was people’s perspective on the prevalence of resistance to the Group 2 herbicides (ALS inhibitors). In the survey, 60% of farmers reported that Group 2 resistant waterhemp was not present in fields they managed or they were unsure of its presence (Table 2). Industry representatives were somewhat more aware of Group 2 resistance, with 38% saying resistance was widespread and 42% reporting it was isolated in their territories. Both groups reported that glyphosate resistance was more common than Group 2 resistance.

Surveys in Illinois and Iowa have found that over 90% of waterhemp populations are resistant to Group 2 herbicides. We don’t have a good handle on how prevalent glyphosate resistance is across the state, but evidence suggests that less than 20% of the fields have glyphosate resistant waterhemp populations at this time. We do know glyphosate resistance is increasing rapidly.

And now for the history lesson. The Group 2 herbicides were introduced in the mid 1980’s and were widely used in both corn and soybeans. In the mid-90’s, Pursuit (imazethapyr) was used on more than 75% of the soybeans in Iowa. Their popularity was due to broad-spectrum weed control and systemic property that provided greater flexibility in application timing compared to the alternatives available at the time.

The widespread use of Group 2 herbicides in the late 80’s and early 90’s resulted in the rapid selection of ALS resistant waterhemp. By the mid-90’s resistance within the waterhemp population was so widespread that the industry essentially stopped recommending Group 2 herbicides for this weed. The difficulty in managing Group 2 resistance waterhemp with the alternative herbicides available at the time was one of the reasons for the rapid adoption of Roundup Ready soybeans.

The survey indicates that the majority of participants at these meetings either have forgotten that Group 2 herbicides once were effective against waterhemp, or were not involved in weed management prior to the introduction of Roundup Ready crops. The loss of this Herbicide Group to manage a major weed problem shows us that similar things could happen with other herbicides if we do not manage weeds appropriately. As George Santayana said: “Those who cannot remember the past are condemned to repeat it”.

Table 2. Perception of the occurrence of resistance to five classes of herbicides, Dec. 2012











  HG2 HG5 HG9 HG27 HG2 HG5 HG9 HG14 HG27

Percent of Respondents









Widespread 11 11 27 2 38 9 44 2 2
Isolated 19 18 40 13 42 60 46 35 20
Not present 26 27 11 38 7 11 4 33 52
Unsure 44 44 22 47 13 20 6 30 26
Iowa State Weed Science Online