Glufosinate is one of the few remaining effective herbicides for POST control of waterhemp in soybean. However, we have received an increasing number of reports of poor waterhemp control following glufosinate application during the past three growing seasons. Results from preliminary greenhouse-based efficacy trials have been inconsistent: multiple populations showed decreased glufosinate sensitivity (surviving >2x field dose) in some trials, but these same populations have shown little if any glufosinate resistance in other trials. It is well known that environmental conditions, such as temperature and humidity, can strongly influence glufosinate efficacy. Therefore, we propose research to understand the interactions of multiple environmental factors with glufosinate efficacy on waterhemp, how these factors interact with the resistance phenotype, and the molecular mechanisms that contribute to decreased glufosinate sensitivity.

We will conduct glufosinate dose-response experiments with a sensitive and two putative resistant waterhemp populations using a multi-factor design to evaluate the effects of humidity, temperature, light intensity, and time of application on glufosinate efficacy. We will also sequence the transcriptome (all expressed genes) of each population to investigate the resistance mechanism.

This research will yield refined recommendations to not only optimize the performance of glufosinate but also to reduce the probability that incipient resistant individuals will survive and contribute to resistance evolution. For example, if we determine that resistance is only expressed when cool temperatures immediately follow application, we can recommend that applying glufosinate when such conditions are not forecasted can help in delaying evolution of glufosinate-resistant waterhemp. Additionally, transcriptome sequencing will provide leads for developing a rapid resistance diagnostic assay, similar to those we have developed previously for resistance to glyphosate and Group 14 herbicides. Such an assay would be particular useful for glufosinate resistance because it is difficult to determine if poor performance of this herbicide is due to environmental conditions or resistance. A better understanding of the resistance mechanism also will provide insight into the inconsistent expression of the resistance, further refining our management recommendations.

This research will yield refined recommendations to not only optimize the performance of glufosinate but also to reduce the probability that incipient resistant individuals will survive and contribute to resistance evolution.