Palmer Pigweed: Where Your Cows Go, The Weed May Follow

Young palmer amaranth plant.

Young palmer amaranth plant.

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 attention to weeds growing around livestock feeding areas, near manure storage areas, or in fields with a history of manure application.

These are likely the areas where Palmer amaranth and other new weeds will show up first.

Why the concern?

Palmer amaranth is a highly competitive pigweed that is closely related to waterhemp. Like waterhemp, Palmer amaranth emerges throughout the growing season, and can grow 2-3 inches per day, causing large yield reductions if it goes uncontrolled.

Control is often challenging, as Palmer amaranth has resistance to many different herbicides. Palmer amaranth is on Minnesota’s Prohibited Noxious Weed Eradicate list. For more information and tips on identifying this weed, click here.

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. This is especially the case for grass weed seeds. Even hard-coated seeds like pigweed can be degraded during ensiling. Palmer amaranth seed viability can drop by 40-60% after one month of ensiling alfalfa or corn silage.

Don’t assume animal digestion will kill all of the seeds

Though it will reduce weed seed viability, feeding contaminated feed to livestock will not eliminate all Palmer amaranth seed. Grass and soft-coated broadleaf seeds are more easily destroyed in digestion than hard-coated seeds like Palmer amaranth. In ruminants like cattle, nearly 30% of amaranth seed survives digestion.

The gizzard digestive system of poultry is highly effective at destroying weed seeds. For instance, only 3.5% of Palmer amaranth seeds fed to ducks were recovered and found viable.

Manure management

Internal heat generated by properly composted manure will kill most weed seeds – even Palmer amaranth. Aged manure is not composted manure. 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.

Source URL: https://blog-crop-news.extension.umn.edu/2020/09/palmer-amaranth-in-feed-ingredients.html