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All issues concerning Small Farms, Agriculture, Local Food Systems, and the Natural Resources.
Weed Height

Amaranth Has Defeated Two Herbicide Groups


If farming was baseball and Team Herbicides was at bat against the weed Team of Amaranths, then right now, there are two strikes against Herbicides. A new report from the University of Illinois Plant Clinic bears this out. This has weed scientists across the Midwest very concerned and it should have Illinois farmers and farmland owners concerned.

In a U of I Bulletin post, the Plant Clinic published a report of their findings for the 2016 season on herbicide resistance with regard to glyphosate and PPO inhibitor type herbicides. In the 378 samples from Illinois, 48 percent were resistant to both glyphosate and PPO inhibitors. In Illinois, we received samples from 52 counties that had at least one sampled field that had waterhemp or palmer amaranth plants that tested resistant to both glyphosate and PPO inhibitors.

"Fields with plants that are positive for both glyphosate and PPO inhibitor resistance are of particular concern, due to the limited possibilities for control of these weeds," says plant diagnostic outreach Extension specialist Diane Plewa.

Last week, I attended the Central Illinois Crop Management Conference where Dr. Aaron Hager, Extension Weed Specialist, spoke to the group about the seriousness of this problem. As he told us, one palmer amaranth plant will produce at least 1 million seeds and if only 10 percent of those seeds grow the following year, you will still have 100,000 weed plants to control. Combine this with the fact that Palmer amaranth is known to be resistant to six different herbicide site of action (SOA) groups. This is why, Dr. Hager recommends Illinois farmers to have a "Zero Tolerance" for palmer amaranth in their fields.

I mentioned to Dr. Hager that last year I noticed a number of fields that were sprayed with a PPO inhibitor herbicide to clean up an amaranth problem and later in the season, these same fields had amaranth that was not completely controlled. I asked Dr. Hager if these PPO treated fields with surviving amaranth weeds should be considered as having a PPO inhibitor resistance issue and the his reply was, "Yes".

If you do not understand herbicide sites of action and the reasons for rotating your SOA's to prevent development of herbicide resistant weeds in your fields, please go to the "Take Action on Weeds" website. Palmer amaranth has been described as a "superweed" weed and once it becomes established in your field, it takes a "super" effort to get it under control – row cultivators, weeding crews, multi-year crop rotations, and increased herbicide costs.

Here in central Illinois, we need to be proactive if we want to continue to have corn and soybeans as the foundation for our grain production system. Identify weeds early, and then control amaranth weeds at four inches or less. Remember no herbicide label shows control of palmer amaranth past four inches in height, period!


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