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Palmer amaranth in Michigan

Palmer amaranth was first discovered in Michigan in fall 2010 when a grower reported he was having difficulties controlling a “pigweed” in his soybean field with glyphosate, the active ingredient in Roundup Ready herbicide.

Michigan State University weed scientists confirmed this population of Palmer amaranth is glyphosate-resistant.

Distinguishing Palmer amaranth from other common pigweeds can be difficult unless the distinct characteristics are known. As a seedling, Palmer amaranth has egg-shaped, hairless leaves. A small point may be present on the leaf tip. An immature Palmer amaranth has a smooth surface with few to no hairs on the stem and leaf, and the petioles are often just as long, or longer, than the leaf blades. A mature Palmer amaranth has thick, non-branched flowering structures 1 to 2 feet long.

Key Points

• One Palmer amaranth per 10 feet of soybean row will reduce yields by 13%.

• Palmer amaranth can grow up to 2.5 inches per day.

• Limited options exist for controlling Palmer amaranth in soybeans.


Only a small number of acres in St. Joseph County have been confirmed as the glyphosate-resistant weed. However, since Palmer amaranth made its way up to Michigan, it could easily spread to other parts of the state if growers aren’t careful.

“It is very hard to prevent Palmer amaranth from spreading,” says David Powell, crop and soil sciences doctoral student at MSU. “If you have Palmer amaranth in some fields but not others, it is very important to thoroughly clean equipment.”

Yield drag

One Palmer amaranth per 10 feet of soybean row will cut yields 13%. The weed competes for the same water and nutrients as beans, reducing profits and increasing costs for herbicides.

Furthermore, Palmer amaranth has a rapid growth rate of up to 2.5 inches per day. With an average of 40,000 seeds per plant, it potentially hybridizes with other pigweeds and rapidly develops herbicide resistance. Because it’s dioecious, having separate male and female plants, it can have a high degree of genetic diversity.

Unfortunately, limited options for controlling Palmer amaranth in soybeans exist. “There are many more options to control it when the land is in corn production. The use of atrazine, as well as products like Callisto, will provide good control of Palmer amaranth for most of the growing season in corn,” says Powell. “As of right now, it is much harder to control Palmer amaranth in soybean production.”

MSU researchers recommend using a strong preherbicide followed by a post-herbicide with a residual activity, as it may be the best option to manage Palmer amaranth in soybeans. From their 2011 trials, products containing Valor (flumioxazin) provided the greatest level of control.

For control of Palmer amaranth post-emergence, MSU recommends growers apply Cobra (lactofen) or Flexstar (fomesafen) before the weed gets to be 6 inches tall. Control of Palmer amaranth from these products is dramatically reduced the taller the plants get.

“Because these products have little residual activity, we would like growers to also apply a product with residual when they apply their postherbicide,” says Powell. Products such as Dual Magnum (s-metolachlor) and Warrant (acetochlor) will provide residual activity, but will not control weeds that are already growing.

Fournier is the communications specialist for the MSU Integrated Pest Management Program.

This article published in the April, 2012 edition of MICHIGAN FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.