Glufosinate-Resistant Waterhemp (Amaranthus tuberculatus) Confirmed in Illinois
Weed scientists at the University of Illinois and Southern Illinois University Carbondale recently announced the confirmation of glufosinate resistance in Illinois waterhemp populations. Glufosinate is a non-selective herbicide most commonly applied postemergence in Illinois corn and soybean crops engineered with resistance to the herbicide. Use of glufosinate has increased over the past decade as the efficacy of other postemergence herbicides against waterhemp has been compromised through the evolution of resistance. This confirmation marks the first time that herbicides from all site-of-action groups previously effective for postemergence control of Illinois waterhemp have instances of resistance; in other words, waterhemp now has evolved resistance to every postemergence herbicide previously effective for its control in Illinois corn and soybean crops.
Resistance to glufosinate was confirmed in three Illinois populations through greenhouse dose-response experiments and further supplemented with field research data for a northern Illinois population. Greenhouse dose-response experiments indicated the populations were between 2.2 – 3.4 fold resistant relative to a susceptible population based on plant survival, and 1.3 – 2.8 fold resistant based on dry biomass. These resistance ratios are relatively low compared with resistances to other herbicides but high enough to result in incomplete control. Twenty percent of treated plants survived the 1x glufosinate rate applied in the field experiment.
Additional research has suggested that glufosinate resistance in not mediated by a change in herbicide target site, but rather by enhanced glufosinate metabolism. Preliminary growth chamber research suggests resistance is “enhanced” under warmer air temperatures, possibly due to increased herbicide metabolism in warmer environments.
We do not know how extensive glufosinate resistance is in Illinois waterhemp populations, but we have no evidence to assume these are the only populations. Efficacy of glufosinate can be influenced by environmental conditions such as humidity, air temperature, solar radiation, etc., and previous research has demonstrated efficacy can be reduced when these and other environmental conditions are less than optimal. Not every instance of incomplete waterhemp control following glufosinate application is attributable to resistance, but applicators should be aware resistance has evolved and can be the primary (or contributing) factor of poor waterhemp control.
Applicators should do everything possible to ensure glufosinate efficacy remains high, including selecting the appropriate application rate, carrier volume, spray additive, and herbicide tankmix partner(s). Previous research has demonstrated improved waterhemp control when glufosinate is combined with 2,4-D choline on Enlist crops compared with either herbicide applied alone. There are NO new postemergence herbicides that will be commercially available within the next few growing seasons that provide effective waterhemp control. Weed management practitioners should strive to preserve glufosinate and 2,4-D choline as viable options for waterhemp control. Reducing or eliminating waterhemp seed production is the only effective method to ensure no change in resistance frequency occurs over time.
Dr. Patrick Tranel, molecular weed scientist at the University of Illinois, recently presented a podcast sponsored by the Illinois Soybean Association that addressed inconsistent performance of glufosinate for waterhemp control; the podcast is available here: https://fieldadvisor.org/understanding-inconsistent-performance-of-glufosinate-for-waterhemp-control/
A Few Final Thoughts
Three quotes from Dr. Tranel that bring clarity and urgency to the confirmation of glufosinate resistance are reproduced from a recent article written by Rhonda Brooks for AgWeb:
“You can’t manage chemical resistance with chemicals,” Tranel says. “We cannot exclusively rely on herbicides like we have been able to do in past decades.”
“At the end of the growing season, if you don’t have a weed going to seed, you’re not going to get evolution of resistance,” Tranel says. Achieving this goal, which requires ongoing effort, will reduce pressure on your herbicide program, he adds.
Tranel says he often ponders where the farming community would be today with regard to weeds if glyphosate had been stewarded better. It’s a lesson he hopes row crop growers take to heart. “Glufosinate might be the best thing we have for the next 10 years. How can we make sure we can keep using it for the next 10 years?” Tranel says.
You can read the entire article at: https://www.agweb.com/news/crops/crop-production/end-era-glufosinates-tight-grip-waterhemp-finally-breaks