Initial tests on weed seeds collected from Harrison County farm fields show resistance to common herbicides. “These results suggest that herbicide-resistant weeds are common in Harrison County and will continue to spread,” says Larry Buss, a farmer who leads the Harrison County Pest Resistance Management Project, in western Iowa.
The Harrison County project is one of several community-led resistance management projects implemented across the state as part of the Iowa Pest Resistance Management Program. IPRMP is an Iowa-specific effort to address pests (weeds, insects and diseases) that can adapt and become resistant to chemical, genetic and agronomic control practices. The program outlines various approaches for effective, integrated management solutions that will sustainably control pests.
Community effort to beat resistant weeds
The program is helping to form community teams of farmers, agronomists, crop advisers, researchers, bankers, agribusiness professionals and landowners to increase collaboration, spread awareness and find solutions to local resistance issues. By sharing information and ideas, they are better equipped to find workable solutions to manage and prevent increasing problems with weeds developing resistance to herbicides.
In Harrison County, seeds of three major weeds — waterhemp, giant ragweed and Palmer amaranth — were collected, grown in Iowa State University greenhouses in Ames and screened for resistance to some commonly used herbicides. A fourth weed, marestail, also was collected, but those seeds did not germinate in the tests.
All three of these weed species demonstrated resistance to Roundup (glyphosate, Group 9). The waterhemp also showed resistance to Cobra (lactofen, Group 14). Palmer amaranth and giant ragweed displayed resistance to Callisto (Mesotrione, Group 27).
Sample sizes for this screening were small, but additional sampling done last year will provide more insights into the threat of resistance in Harrison County. You can learn more about the Harrison weed screening results at Iowa State’s IPM web page.
A field day on the Harrison County Pesticide Management Project will be from 10 a.m. to 1 p.m. July 9, immediately northeast of the intersection of Niagara Trail and 262nd Street, which is about 1.5 miles southwest of downtown Logan.
Tips to avoid weed resistance
The Integrated Crop Management team of ISU Extension advises the following steps for farmers to take to avoid weed resistance:
- Scout fields in the fall for weed escapes and locations.
- Use scouting results to make a management plan for the following year.
- Make sure the herbicide you will use targets your weed.
- Ensure your plan targets multiple sites of action.
- Consider application rate options.
- Consider alternative methods.
For application rate options, note that premixed product of two herbicides may use less than the prescribed application rate for an individual herbicide, but this can make it less effective.
Alternative methods include narrow row spacing, increased soybean seeding rates, and cover crops to help outcompete weeds. Also, if you know a field has a weed that is difficult to manage (like Palmer amaranth) or has a resistant weed population, don’t let it become a management issue in your other fields. Plant and harvest that field last, and then clean your machine.