Palmer Amaranth Can Contaminate Livestock Feed Ingredients

University of Minnesota Extension

October 2020

By Jared Goplen, U of MN Extension Educator, Crops

Keep an eye out for Palmer amaranth hitching a ride into Minnesota via contaminated feed ingredients. Minnesota has at least one infestation in the state that arrived via contaminated cattle feed. The potential for this to happen again is high, especially when sourcing feed ingredients from areas where Palmer amaranth and other problematic weeds are more common. 

Pay special attention to “unusual” weeds growing around:

  1. Livestock feeding and feed storage areas.
  2. Manure storage areas
  3. In fields with a history of manure application. 

These are areas where Palmer amaranth and other new weeds will likely show up first when arriving in contaminated feed ingredients.

Why the concern?

Palmer amaranth is a highly competitive pigweed that is closely related to waterhemp. Like waterhemp, weed control is often challenging as it has resistance to many different herbicides. Palmer amaranth is on Minnesota’s Prohibited Noxious Weed Eradicate list (in short, it’s bad news). Palmer amaranth looks very similar to waterhemp, but typically has very long petioles (longer than the leaf blade), and the seed head is typically longer than waterhemp (1-2 feet long).

Where are the weed seeds coming from?

While weeds can find their way to your farm via wildlife and other uncontrollable factors, many new weed seeds are brought onto the farm from normal farm activities. Most of the new infestations of Palmer amaranth have been brought in via equipment (especially used equipment purchased from an area where Palmer amaranth is more common), contaminated seed, or contaminated feedstuffs. The amount of risk for contamination with weed seeds depends on the type of feed and where it originates. Feed that has been ground, pelletized, or ensiled is less concerning compared to less-processed feed ingredients.

Feed ingredients most concerning:

  • Feedstuffs from southern states: Palmer amaranth is more common further south. Any feed ingredient that was produced where Palmer amaranth is present in fields is more likely to become contaminated. Cottonseed and sunflower screenings have been the source of several infestations in the Midwest. Other feed ingredients, including hay and animal bedding, produced where Palmer amaranth is more common are also concerns. Additionally, herbicide resistance is more common further south, meaning other weed seeds that may travel in with the feedstuffs, including waterhemp, may be herbicide resistant.   
  • Forages from weedy fields: Any forage produced in weedy fields will likely contain weed seeds, whether harvested as dry hay or ensiled. Ensiling forages will help reduce the viability of weed seed. Palmer amaranth seed viability can drop by over 50% after one month of ensiling alfalfa or corn silage.

Animal digestion doesn't kill all of the seeds

Feeding contaminated feed to livestock will reduce seed viability, but it will not eliminate all Palmer amaranth seed. In ruminants like cattle, nearly 30% of amaranth seed survives digestion. The gizzard digestive system of poultry is much more effective at destroying weed seeds. Over 95% of Palmer amaranth seeds can be destroyed during poultry digestion.

Manure management

Internal heat generated by properly composted manure will kill most weed seeds – even Palmer amaranth. Weed seeds are killed in composted manure due to warm temperatures (>140F) for several days. In typical on-farm composting sites, weed seed viability is typically reduced by >90%. This percentage is even higher in the best-managed compost sites. 

Even with ideal feed and manure management, weed seed from contaminated feed ingredients can survive the process and cause weed management issues. It is much easier to manage Palmer amaranth and other new weeds with prevention. Be on the lookout for Palmer amaranth and other new weeds around feed storage areas, feeding areas, and in fields with recent manure applications.

Key Palmer amaranth reporting contacts

If you suspect Palmer amaranth on your farm, please contact one of the numbers listed below. Early detection is key in preventing Palmer amaranth and other invasive species from becoming widely established.

  • Arrest the Pest: Webemail, or call 1-888-545-6684
  • Denise Theide: email or call 651-201-6531
  • Anthony Cortilet: email or call 651-201-6538
  • Shane Blair: email or call 507-884-2116
  • U of MN Extension Weed Scientist Debalin Sarangi: email
  • U of MN Extension IPM Specialist Bruce Potter: email

Contact any of the U of MN Extension agronomy educators.