As the 2019 harvest is getting underway, weed science researchers at Iowa State University are conducting a statewide survey to collect waterhemp seeds produced by plants in corn and soybean fields, as well in field edges.
The weed science team will revisit more than 200 field sites that were included in the last survey in 2012-13. The 2019 populations will be screened for possible multiple resistance to herbicide Groups 2, 5, 9, 14, 27 and others.
The escalating spread of herbicide-resistant weed populations has become a challenge in the corn and soybean cropping system of the Midwest. With the evolution of weed resistance to major herbicide groups used in corn and soybean production, farmers 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 herbicide with a new site of action coming to the marketplace in the next five to eight years would also be a rare event, say ISU Extension weed specialists Bob Hartzler and Prashant Jha.
Early detection needed
Early detection of herbicide-resistant waterhemp in a field and rapid response to control it are important to prevent further spread of resistance, the weed scientists note.
The weed science program in the department of agronomy at ISU has research facilities to screen herbicide resistance and detect the level of resistance that has evolved in weed populations. ISU weed scientists are working on the development of DNA-based molecular markers for rapid testing of live plant samples to disseminate results and provide recommendations within the growing season.
Waterhemp resistance to Group 15 herbicides (acetochlor, S-metolachlor, dimethenamid, and pyroxasulfone) has recently been found and documented in Illinois. ISU 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, weed resistance to 2,4-D was recently reported in a five-way resistant population from Illinois (2016) and a six-way resistant population from Missouri (2018).
With the increase in use of dicamba and 2,4-D to manage glyphosate-resistant waterhemp in Xtend and Enlist soybean varieties, the response of Iowa waterhemp populations to 2,4-D and dicamba needs to be evaluated to prevent failures.
Results from this fall’s survey by the ISU weed management team will help in developing proactive and reactive herbicide resistance management strategies in Iowa corn and soybean production.
Hartzler and Jha say based on specific cases, the ISU team will develop research-based 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 — all at a cropping systems level.
Get seed tested
The ISU weed science program is also encouraging growers, ISU Extension field agronomists, industry and commodity groups to be a part of this survey work and to send seed samples of suspected resistant weed populations to the ISU Lab in Ames.
To have the samples tested, you need to 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 and herbicide use history). Ship it to: ISU Weeds Lab, 3212 Agronomy Hall, 716 Farm House Lane, Ames, IA 50011.
Since waterhemp is a dioecious species, male and female plants are separate; hence, make sure to collect only seed-bearing female plants. If you have questions regarding seed collection or suspected new cases of resistance, contact the ISU weed science team at 515-294-7028 or email email@example.com.