Editor’s note: The following was written by Prashant Jha, Iowa State University associate professor, Bob Hartzler, professor of agronomy, and Meaghan Anderson, Extension field agronomist in Central Iowa, for the Extension Integrated Crop Management News website Sept. 20.

With the evolution of weed resistance to major herbicide groups used in corn and soybeans, we have a limited number of herbicide options left, which is even more disconcerting as no new herbicide site of action has been discovered in the past three decades. A new site of action coming to the marketplace in the next 5-8 years would also be a rare event.

Early detection and rapid response is key to prevent further spread of resistance. The Weed Science program in the Department of Agronomy at Iowa State University has research facilities to screen herbicide resistance.

As harvest approaches, we will conduct a statewide survey to collect waterhemp seeds produced by plants in corn/soybean fields as well in field edges. We will revisit more than 200 field sites (GPS tracked) that were included in the 2012-13 survey.

Waterhemp resistance to Group 15 herbicides (acetochlor, S-metolachlor, dimethenamid and pyroxasulfone) has recently been reported in Illinois. We will screen Iowa waterhemp populations for variable response to Group 15 herbicides since these are widely used both in corn and soybeans as a component of overlapping residual programs.

Furthermore, resistance to 2,4-D was recently reported in a five-way resistant population from Illinois in 2016 and a six-way resistant population from Missouri in 2018.

With the increase in use of dicamba and 2,4-D to manage glyphosate-resistant waterhemp in Xtend and Enlist soybean, respectively, the response of Iowa waterhemp populations to 2,4-D and dicamba needs to be evaluated.

Results from this survey will help in developing herbicide resistance management strategies in Iowa corn and soybean production. Based on specific cases, we will develop information on short- and long-term best management practices using a multi-tactic approach — cover crops, overlapping soil residuals (multiple effective sites of action herbicides), harvest weed seed control technologies, site-specific resistance management.

Our program is also encouraging growers, Extension field agronomists, industry and commodity groups to be a part of this survey work and send in seed samples of suspected-resistant weed populations.

To have the samples tested, collect a large portion of seed-bearing heads, put those in paper bags, label with details of the field site (GPS coordinates, address and crop/herbicide use history), and ship to our lab at: Weeds Lab, 3212 Agronomy Hall, 716 Farm House Lane, Ames, IA 50011.

Since waterhemp is a dioecious species, male and female plant are separate; hence, make sure to collect only seed-bearing female plants.

If you have any questions regarding seed collection or any suspected new cases of resistance, call 515-294-7028 or email: pjha@iastate.edu.

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