Palmer amaranth and manure update

May 23, 2019 / by David Kee, MN Soybean director of research Categories: Minnesota Soybean Research & Promotion Council, Soybean News

In the fall and summer of 2018, Palmer amaranth was found in southern Minnesota soybean fields. Previous findings were linked to conservation planting, this time the infestation has been linked to livestock manure applications. In both Minnesota and North Dakota, the manure came from cattle fed sunflower screenings.

Screening sunflower seeds is part of the cleaning process separating foreign matter. This process, like other waste stream products (cotton gin trash, wheat screenings, soybean hulls, sugarbeet pulp, etc.) can be valuable livestock feed products. Generally, most of the weed seeds are removed in the first step of the cleaning process and are considerably reduced if not eliminated throughout the final processing steps. Minnesota Department of Agriculture collected 10 screening samples from feedlots this fall and winter. Palmer amaranth was found in three of those samples.

Past research, dating back over a century, states that it is not uncommon for there to be weed seed in livestock feed. The weed seed survival rate varies considerably depending on the animal, seed type and grain processing structure. The more intensive the grinding process, the lower the germination of the fecal recovered seed. Seed survival in chicken manure is extremely low (frequently less than 2 percent) while that from cattle is considerably higher (25-50 percent).

Researchers have found processing the feed or manure, can reduce weed seed survival. Increase feed grind fineness can decrease seed germination, too. With this, there are other factors to consider, but not limited to:

– Minor scratching of a hard seed coat may increase seed germination;
– Increasing temperature of wet manure compost has been found to decrease seed germination;
– Increasing temperature of a dry manure compost may have limited impact of certain weed seeds;
– Screenings used as bedding may be a potential source of Palmer amaranth.

A bit more disconcerting is the difference in genetics within weed species, specifically Palmer amaranth. Anthony Cortilet of the Minnesota Department of Agriculture (MDA) reports that Palmer amaranth found in conservation plantings in Minnesota has not, to date, been found to be herbicide resistant. Sharon Clay, SDSU weed scientist, reports Palmer amaranth from a swine (of southern USA origin) manure applied spot was tested and found to be herbicide resistant (glyphosate and PPO). How the manure is managed prior being applied to a field impacts its response to weed control effects.

Does this mean do not use manure in your operation? No, but it does mean you need to pay attention. Was the livestock feed Palmer amaranth free? Where did the feed originate? What ingredients are included in the ration? All, and more, are pertinent questions you need to consider.

In Minnesota, farmers and advisors have been the main discoverers of Palmer amaranth. Review your Palmer ID materials, communicate a plan to your operation and neighbors and stay connected. You, the farmer, is ultimately responsible for the Palmer amaranth control on your land. This weed has been proven to be persistent. Make sure your operation is ready if and when, it migrates to your land.

Future studies will be conducted by UMN, MDA and other groups. How it will be managed will be determined. However, MDA is considering its options to address this problem. You should, too.


David Kee is the director of research at the Minnesota Soybean Research & Promotion Council and can be reached at dkee@mnsoybean.com or 507-388-1635. 

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