Written by Bob Hartzler and Meaghan Anderson for Integrated Crop Management News and Iowa State University Extension and Outreach, Sept. 14, 2016
Most people are aware of Palmer amaranth seed contamination in native seed mixes. These findings have led to questions about whether cover crop seed might also be a source of Palmer amaranth. We are not aware of any situations of cover crop seed used in Iowa being a source of Palmer amaranth, and have not heard of this situation in other Midwest states.
We feel the risk should be very small for two reasons. The first is that the majority of cover crop species grown in Iowa are winter annuals (e.g. cereal rye, oats), whereas Palmer amaranth is a summer annual. Because of this difference in life cycle, cover crop species are harvested when there would be no mature Palmer amaranth present to directly contaminate the crop seed. The second reason is that most winter annual cover crop seed planted in Iowa is produced in Iowa, South Dakota, and North Dakota. None of these states currently have widespread Palmer amaranth (not considering the introduction of Palmer amaranth in conservation plantings this season), therefore reducing the likelihood of contamination.
There is no doubt that cover crop seed can be the source of weed seed. In most cases, the weed seed are winter annual species that are easily managed with tillage and/or herbicides. We cannot rule out the possibility of cover crop seed being contaminated with Palmer amaranth seed. If cover crop seed did contain this problematic weed, it would likely be due to seed being present in machinery used to harvest or clean seed, rather than plants growing in the cover crop seed field. In this situation, the number of weed seed in the cover crop seed would be much lower than has been observed in conservation plantings across the state.
To summarize, we are not aware of any cases where Palmer amaranth was introduced via seed of traditional cover crops. The risk of introducing this new species can be minimized by confirming that the source of the seed is from a state where Palmer amaranth is not widespread and purchasing Certified Seed. Since Palmer amaranth is not classified as Noxious in Iowa, Palmer amaranth seed is not restricted in Certified Seed, and its presence in the crop seed does not need to be listed. However, Certified Seed typically is produced under more stringent conditions, thus reducing the risk of contamination with weeds in general.