Iowa Weeds: Is 80% of Waterhemp Really Resistant to PPO Herbicides?

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The University of Illinois Plant Clinic recently published results of their 2016 herbicide resistance screening program.  For a $50 fee the Clinic analyzes waterhemp for resistance to both Group 9 (glyphosate) and Group 14 (PPO inhibitor) herbicides.  Unfortunately, samples from Iowa displayed a high frequency of resistance to both herbicides (Table 1).

Table 1.  Results of IL herbicide resistance screening program.

State No. of samples % resistant
to HG9
% resistant
to HG 14
% resistant to
both HG 9 and 14
IA 87 89 80 75
IL 378 74 65 48
MN 78 74 44 35
MO 11 91 91 82
NE 8 63 13 0

A colleague recently asked me if I trusted the molecular tests used by the Clinic, and therefore the results of the program.  My response was yes to both questions, but it is important to recognize the sampling method.  Selecting a random sample is a guiding principle of scientific research, and the samples submitted to the Plant Clinic are far from a random sample.  Samples are sent to the Clinic because people suspect the weeds are resistant to the herbicides, therefore creating a biased sample. 

Herbicide Resistance Info


I suspect most people are comfortable identifying glyphosate resistance, but not quite so sure with Group 14 herbicides due to their less consistent performance. A high percentage of the samples were probably submitted from fields with known glyphosate resistant waterhemp and where a Group 14 herbicide failed to provide effective control in 2016.  This bias results in a much higher frequency of resistance than if waterhemp was collected from randomly selected fields.

The good news is that the presence of Group 9 and 14 resistant waterhemp in Iowa is much lower than reported with the samples submitted to the IL Plant Clinic.  The bad news is that resistance to these, and other herbicide groups, is increasing rapidly across the state.