Scouting Michigan fields for Palmer amaranth

Scouting will be key to stopping the spread of glyphosate-resistant Palmer amaranth in Michigan.

It has been almost three years (fall 2010) since Palmer amaranth, resistant to glyphosate (Roundup) and ALS-inhibiting herbicides, was first reported in Michigan. Initially, populations of this weed appeared to be localized to parts of St. Joseph and Kalamazoo counties. However, last summer (2012) more populations of Palmer amaranth were confirmed in nine Michigan counties: St. Joseph, Kalamazoo, Cass, Barry, Ionia, Clinton, Shiawassee, Gratiot and Livingston. This weed is not native to Michigan and with resistance to glyphosate and other effective herbicides, this weed is undoubtedly the toughest that Michigan growers have ever faced. In fact, in many southern states where this weed is a problem it has been reported that the average increased cost to manage this weed ranges from $30 to $50 more per acre.

Some of the initial cases where glyphosate-resistant Palmer amaranth in Michigan was confirmed were in fields that had been spread with dairy manure. Michigan State University Extension specialists and educators have speculated that Palmer amaranth seed was brought in as a contaminant with cotton seed that was fed to dairy cattle. This may not be surprising when you consider the hundreds of thousands of cotton acres that are infested with glyphosate-resistant Palmer amaranth in the southern United States. While this may help establish the origins of some of these initial reports, once Palmer amaranth establishes itself, it is extremely difficult to control and seed can be moved from field to field with equipment and by other means.

It is essential for all growers to scout for Palmer amaranth in their fields. In areas where Palmer amaranth has not been confirmed, scouting efforts should be targeted in Roundup Ready fields that have been spread with manure in the past couple of years. If initial glyphosate applications are not controlling pigweed, it may be Palmer amaranth. It is important to get confirmation of this early to allow for potential management with herbicides or hand-weeding prior to seed production.

Remember: one female Palmer amaranth plant can produce an average of 400,000 seeds. In many cases, if Palmer amaranth is identified early in its first year of establishment, there may only be a few plants scattered throughout the field. Early identification and removal of this weed before it produces seed and spreads throughout the field is extremely important.

To help with the identification of glyphosate-resistant Palmer amaranth, we have developed two fact sheets “Keys to distinguishing Palmer amaranth from other species” and “Palmer amaranth in Michigan: Keys to Identification.” These fact sheets can be found on our website:

Dr. Sprague's work is funded in part by MSU's AgBioResearch.

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