By: Meaghan Anderson, Bob Hartzler & Michael D.K. Owen

While the registration of 2,4-D and dicamba products for over-the-top use in resistant-varieties has improved waterhemp control for many farmers, weed scientists warned that these herbicides would eventually select for resistant waterhemp populations. Bayer recently reported the discovery of two likely dicamba-resistant waterhemp populations in Iowa, which warrants a discussion on best management practices to slow the evolution of resistant waterhemp populations.

On Friday, September 1, Bayer released an external communication reporting that suspected dicamba-resistant waterhemp populations were sampled in Scott County in 2021 and Marshall County in 2022. After extensive screening the company states they are likely resistant to dicamba but will continue testing to positively confirm the resistance. This is the first case of plant growth regulator (HG 4) resistant waterhemp in Iowa, though other states have already documented HG 4 resistance in their own waterhemp populations. HG 4-resistant waterhemp were documented by university researchers in Nebraska in 2009, Illinois in 2016, and Missouri in 2018.

This discovery is not a cause for panic, but it is an important reality check for farmers who rely heavily on HG 4 herbicides or any other individual herbicide group for waterhemp control. Waterhemp is known for its ability to quickly adapt to herbicide management tactics; Iowa State University has already officially documented resistance to five HGs in Iowa waterhemp populations (Table 1).

While herbicides remain a primary tactic to manage many weed species, farmers can implement several best management practices to slow resistance development and better control weeds like waterhemp.

1. Choose an effective herbicide program for the weed spectrum present on a field-by-field basis.
   1. Use full rates of effective residual herbicides and plant into a weed-free seedbed.
   2. Include overlapping residual herbicides in postemergence applications to provide longer waterhemp control.
   3. Make timely applications and follow herbicide labels to choose appropriate adjuvants, nozzles, application volume, etc.
   4. Scout fields 7-10 days after postemergence herbicide applications to evaluate weed control.
2. Use a diversity of weed management tactics, including chemical, mechanical, and cultural options to better manage weeds. Narrow row spacing, cover crops, crop rotation, and tillage remain effective methods to suppress waterhemp.
3. Control weed escapes prior to seed set to reduce future weed populations and prevent resistant traits from spreading.
4. Reduce influx of weed seed into crop fields by managing field edges and cleaning equipment between movement from problematic fields to clean fields.

While many waterhemp have seed at this point in the growing season, farmers still can reduce the spread of any waterhemp seed yet this fall. Specifically, consider these management recommendations:

1. Remove seed-producing waterhemp plants by hand and dispose of them outside the crop field. While a labor-intensive option, this would be very effective at reducing seed inputs in fields.
2. Avoid harvesting through drowned out spots or other heavily weed-infested areas in fields.
3. Equip harvest equipment with harvest weed seed control (HWSC) tools.
4. Prioritize fields by waiting to harvest the weediest ones last, thus reducing the weed seed spread from problem fields.
5. Implement combine clean-out practices between fields to reduce seed spread.

Please report incidences of non-performance of products to their respective manufacturers. While Iowa State University doesn’t currently have capability to test resistant populations, please report suspected HG 4-resistant waterhemp populations to Meaghan Anderson, Iowa State University Extension and Outreach field agronomist.

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